Paleozoic magmatism and crustal recycling along the ancient
Pacific margin of North America, northern Cordillera1, 2
Stephen J. Piercey
Mineral Exploration Research Centre, Department of Earth Sciences, Laurentian University, 933 Ramsey Lake Road, Sudbury, Ontario, P3E 6B5, Canada, [email protected]
JoAnne L. Nelson
B.C. Geological Survey, P.O. Box 9333, Stn Prov Govt, Victoria, British Columbia, V8W 9N3, Canada
Maurice Colpron
Yukon Geological Survey, P.O. Box 2703 (K-10), Whitehorse, Yukon, Y1A 2C6, Canada
Cynthia Dusel-Bacon
U.S. Geological Survey, Mineral Resources Program, 345 Middlefield Road, Menlo Park, California, 94025, USA
Renée-Luce Simard
Department of Geology, Brandon University, Brandon, Manitoba, R7A 6A9, Canada
Charlie F. Roots
Geological Survey of Canada, P.O. Box 2703 (K-10), Whitehorse, Yukon, Y1A 2C6, Canada
Piercey, S.J., Nelson, J.L., Colpron, M., Dusel-Bacon, C., Simard, R.-L. and Roots, C.F., 2006, Paleozoic magmatism
and crustal recycling along the ancient Pacific margin of North America, northern Cordillera, in Colpron, M. and
Nelson, J.L., eds., Paleozoic Evolution and Metallogeny of Pericratonic Terranes at the Ancient Pacific Margin of
North America, Canadian and Alaskan Cordillera: Geological Association of Canada, Special Paper 45, p. 281-322.
Abstract
Devonian to Permian igneous rocks in the Yukon-Tanana terrane (YTT) record six cycles of arc, arc-rift, continental
rift and back-arc basin magmatism, each set apart from the others by changes in the locus and/or character of igneous
activity, as well as deformational episodes and unconformities. The first four cycles, from mid-Devonian to Late
Mississippian, record largely bimodal arc magmatism above a west-facing (east-dipping) subduction zone, with or
without accompanying back-arc basin magmatism and continental margin rifting. The fifth, Pennsylvanian-Early
Permian cycle, involved more primitive, mafic to intermediate volcanism in a west-facing arc with a corresponding
marginal back-arc basin to the east. The sixth, Late Permian cycle reflects subduction reversal, and continental-arc
Data Repository items Piercey_DR1.xls (Table DR1), Piercey_DR2.xls (Table DR2) are available on the CD-ROM in pocket.
1
2
GSC Contribution 2004058
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Piercey et al .
magmatism associated with an east-facing (west-dipping) subduction zone. Mafic rocks in all cycles of both arc and nonarc character were derived from variably enriched sources, with contributions from depleted mantle wedge or back-arc
asthenosphere, and enriched lithospheric mantle, with or without a subducted slab component. Felsic rocks in arc, arc-rift
and back-arc geodynamic settings were derived predominantly from melting and recycling of upper continental crustal
material (UCC; La/SmUCN ≈ 1). Arc felsic rocks have calc-alkalic and tholeiitic signatures, whereas non-arc rocks are
enriched in high field strength elements and rare earth elements (A-type or peralkaline signatures). Notably, throughout
the late Paleozoic magmatic history of the YTT that spanned over 150 m.y., there are no systematic temporal variations in
the composition of most mafic and felsic rocks. Igneous source regions and igneous processes were essentially unchanged
throughout the tectonic history of the YTT.
An important aspect of many mafic rocks from YTT intra-arc rifts and back-arc basins is their high Nb/Thmn and Nb/Lamn
> 1 (mn = primitive mantle normalized), implying excess Nb relative to Th and La compared to primitive mantle ratios.
Excess Nb in these rocks implies a recycled oceanic crustal component in their genesis, and is a common feature of plumederived magmas and magmatic rocks from large igneous provinces. The recurrence of this recycled component over the
extended magmatic history in YTT, and the common occurrence as low volume eruptions, argues against a direct plume
or large-igneous province origin; however, it is possible that this signature reflects the reactivation of recycled plume or
large igneous province components in the YTT lithospheric mantle that were originally derived via lithospheric fertilization
by magmatism associated with Neoproterozoic breakup of Rodinia.
Résumé
Les roches ignées dévoniennes à permiennes du terrane de Yukon-Tanana (YTT) témoignent de la présence de six cycles
de formation d’arc, d’ouverture d’un fossé tectonique d’arc, d’ouverture d’un fossé tectonique continental et d’un magmatisme de bassin d’arrière-arc, chacun se distinguant des autres par des changements dans l’emplacement ou le caractère
de l’activité ignée, ainsi que par des épisodes de déformation et des discordances. Les quatre premiers cycles, du Dévonien
moyen au Mississipien supérieur, témoignent en gros d’un magmatisme d’arc bimodal au-dessus d’une zone de subduction
orientée à pendage vers l’est, avec ou sans magmatisme d’arrière-arc et ouverture d’un fossé tectonique continental. Le
cinquième cycle, du Pennsylvanien au Permien inférieur, affiche un volcanisme de composition mafique à intermédiaire,
plus primitif, dans un arc orienté vers l’ouest avec un arrière-arc correspondant vers l’est. Le sixième cycle, au Permien
supérieur, reflète un renversement de la subduction, avec un magmatisme d’arc continental associé à une zone de subduction à pendage vers l’ouest. Les roches mafiques de tous les cycles, qu’elles soient de caractère d’arc ou non-arc provenaient de sources diversement enrichies, avec des apports provenant d’un prisme de matériau mantélique appauvri ou de
l’asthénosphère d’arrière-arc, et d’un manteau lithosphérique enrichi, avec ou sans composante de plaque subductée. Les
roches felsiques de contexte géodynamique d’arc, d’ouverture de fossé tectonique d’arc et d’arrière-arc proviennent
principalement de la fusion et du recyclage de matériau de la partie supérieure de la croûte continentale (UCC; La/SmUCN
≈ 1). Les roches felsiques de contexte d’arc ont des signatures calco-alcalines et tholéiitiques, alors que les roches qui ne
sont pas de contexte d’arc sont enrichies en éléments à forts effets de champ et d’éléments de la famille des terres rares
(signatures de type A ou hyperalcalines). Il est remarquable qu’il n’y ait pas eu de variations systématiques temporelles
dans la composition de la plupart des roches mafiques et felsiques, durant l’ensemble de l’histoire magmatique du YTT au
Paléozoïque tardif, s’étendant sur plus de 150 millions d’années. Pour l’essentiel, les régions sources et les processus ignées
sont demeurés inchangés pendant toute la durée de l’histoire tectonique du YTT.
Une caractéristique importante de nombreuses roches mafiques des fossés intra-arc et des bassins d’arrière-arc du YTT,
est la valeur élevée des ratios Nb/Thmn et Nb/Lamn > 1 (mn = normalisées selon le manteau primitif), ce qui implique
l’existence d’un excès de Nb par rapport au Th et au La, comparé aux ratios des matériaux d’un manteau primitif. L’excès
de Nb dans ces roches implique l’existence d’une composante de croûte océanique recyclée dans leur genèse, et c’est une
caractéristique commune des magmas et des roches dérivées de panaches des grandes provinces ignées. La récurrence
d’une telle composante recyclée sur une longue période de l’histoire magmatique du YTT, et l’occurrence commune dans
des éruptions de petits volumes, plaident contre l’hypothèse d’une origine découlant directement d’un panache ou d’une
grande province ignée; cependant, il est possible que cette signature soit le reflet d’une réactivation d’un panache ou d’une
composante d’une grande province ignée recyclé du manteau lithosphérique du YTT, et dont l’origine serait une fertilisation lithosphérique par un magmatisme associé à la fragmentation de Rodinia au Néoprotérozoïque.
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Paleozoic magmatism and crustal recycling
INTRODUCTION
The Yukon-Tanana terrane (YTT) is a vast pericratonic province
exposed in the central Cordillera of northern British Columbia,
Yukon and eastern Alaska. The terrane lies between rocks of the
cratonic margin of North America, to the east, and oceanic and arc
terranes that were accreted to the YTT and cratonic margin in the
Mesozoic, to the west (Fig. 1). It contains metamorphosed and
polydeformed mid- to late Paleozoic volcanic, plutonic and sedimentary rocks whose stratigraphic record differs significantly from coeval strata of the North American miogeocline (Colpron et al., this
volume-a). However, the rocks of the YTT have some geochemical,
isotopic, metallogenic and geochronological characteristics that
suggest that its older parts may have originated as part of the North
American craton (Mortensen, 1992a; Creaser et al., 1997; Paradis
et al., 1998; Nelson et al., 2002; Villeneuve et al., 2003; Mortensen
et al., this volume). Both allochthonous and parautochthonous elements, with respect to the North American craton, make up the YTT
(Fig. 1). Yukon-Tanana terrane locally hosts important volcanogenic
massive sulphide deposits.
Regional studies undertaken under the auspices of the Ancient
Pacific Margin NATMAP (National Mapping Program) project have
generated a wealth of new stratigraphic, geochronological and biochronological data from Yukon-Tanana and affiliated terranes in
northern British Columbia, Yukon and eastern Alaska (e.g., Colpron
et al., this volume-a, and references therein). These data provide a
stratigraphic framework that is critical to elucidating the tectonic
evolution of the terrane and onto which detailed metallogenic, geochemical and isotopic studies can be based. In particular, geochemical and isotopic data allow for determining the relationship between
the timing of magmatism, magma composition, and, ultimately, the
tectonic setting in which ancient terranes have formed (e.g., Stern
et al., 1995; Swinden et al., 1997). These data give insights into the
relative roles that the mantle, continental crust and subducted slab
play in the genesis of magmatic rocks. They provide important information on the processes governing crustal growth and assembly
of continents (e.g., Pearce, 1983; Pearce and Peate, 1995; Pearce and
Parkinson, 1993), and therefore on the Paleozoic evolution of YTT
and the ancient Pacific margin of North America.
In this paper we present an overview and synthesis of the petrologic, geochemical and (in some cases) isotopic attributes of over
500 volcanic and intrusive rocks from YTT and related terranes
(Fig. 1). Our presentation follows a series of time slices that correspond to documented major tectonic and magmatic events within
the terrane. In spite of significant dynamothermal metamorphism
throughout much of the terrane, primary igneous textures are preserved in many areas, and immobile element signatures have not
been changed appreciably (e.g., Creaser et al., 1999; Dusel-Bacon
and Cooper, 1999; Piercey et al., 2001a, b, 2002a, b, 2003).
Accordingly, where initial compositions can be determined, the
protolith names are used to emphasize the pre-metamorphic igneous
evolution of these rocks. Many of the data sets analyzed in this paper
are published either in previous papers (e.g., Creaser et al., 1997,
1999; Piercey et al., 2001a, b, 2002a, b, 2003, 2004; Colpron, 2001;
Simard et al., 2003; Dusel-Bacon et al., 2004; Nelson and Friedman,
2004) or elsewhere in this volume (e.g., Dusel-Bacon et al., this
volume); the reader is referred to these original studies for more detailed treatment of the data.
Given the breadth of new stratigraphic information, geochronologic and biostratigraphic age constraints, and associated geochemical and isotopic data, YTT may well become one of the best understood ancient continental margin geodynamic systems in the
world.
METHODOLOGY AND APPROACH
This paper synthesizes over 500 high precision major, trace and rare
earth element analyses, and, where available, Nd-isotopic data, from
various locations throughout YTT; where applicable, data from the
Stikine, Quesnel and Slide Mountain terranes are also discussed.
Representative analyses are presented in Tables DR1 and DR2 (see
footnote 1) and were compiled from published and unpublished
sources. Most samples were analyzed by a combination of X-ray
fluorescence (XRF), inductively-coupled plasma emission spectroscopy (ICP-ES), and inductively-coupled plasma mass spectroscopy
(ICP-MS; see Piercey et al., 2001b for an example). Descriptions of
instrumentation, analytical techniques, precision and accuracy are
found in the original studies cited in Tables DR1 and DR2.
The following sub-sections explain the trace element diagrams
used in this paper and the rationale behind their usage. We will discuss, where available, Nd isotopic signatures, because they highlight
the relative importance of continental crust and mantle sources in
the genesis of YTT magmatic rocks.
Petrology and Geochemistry
Trace element and radiogenic isotope geochemistry are useful in
establishing the tectonic setting and petrogenesis of rocks in both
modern (e.g., Pearce, 1983; Pearce and Peate, 1995; Pearce and
Parkinson, 1993) and ancient (e.g., Stern et al., 1995; Swinden et al.,
1997) geodynamic environments. Trace elements are more sensitive
to petrological processes than major elements, and provide a process-oriented fingerprint or signature suggestive of a given tectonic
environment (e.g., Pearce and Cann, 1973; Pearce and Norry, 1979;
Shervais, 1982; Wood, 1980). Furthermore, most major elements
(except Al2O3, TiO2, P2O5) are highly mobile during hydrothermal
alteration and metamorphism (e.g., Gibson et al., 1983; MacLean,
1990). In contrast, the high field strength elements (HFSE: Zr, Hf,
Nb, Ta, Y), rare earth elements (REE: La-Lu, except Eu), transition
elements (TE: Cr, Ni, Sc, V), and the low field strength element
(LFSE) Th are considered immobile during metamorphism (up to
mid-amphibolite facies) and hydrothermal alteration at low waterto-rock ratios (e.g., Campbell et al., 1984; Whitford et al., 1988; You
et al., 1996; Jenner, 1996; Swinden et al., 1997; Johnson and Plank,
1999). Previous geochemical studies of YTT also have confirmed
that these elements are immobile during regional metamorphism
and alteration (e.g., Creaser et al., 1997; Dusel-Bacon and Cooper,
1999; Piercey, 2001; Piercey et al., 2001a, b, c, 2002a, b, 2003, 2004;
Dusel-Bacon et al., 2004).
In addition to fingerprinting tectonic setting, trace element data
can provide insights into the nature and type of mantle (e.g., enriched,
283
Piercey et al .
AA
Ambler
0°W
SD
16
AG
140°W
68
°N
Ko
tze
So bue
un
d
CO
Beaufort
Sea
AK
AA
TZ
E
3
NA
Oceanic assemblage
li
WM
BS
YT
Ak
YT
Devonian - Triassic
Slide Mountain - Seventymile
assemblages
YN
B
7
Continent margin assemblages
Neoproterozoic? - Miss.
Alaska Range and
Yukon-Tanana Upland
Neoproterozoic - Devonian
Selwyn basin
Neoproterozoic - Paleozoic
Other continent margin
assemblages
WL
8
59°N
10
Ocean
Tulsequah
YT
SYMBOLS
YT
60°N
B.C.
SM
9
NA
ST
Gataga
Blueschist / eclogite
occurrence
Devonian magmatic
rocks in North American
miogeocline
Devonian - Mississippian
mineral districts
56°N
0
Scale
km
200
128°W
Proterozoic - Devonian
North American
platformal facies
CA
Ak
B.C.
Yukon-Tanana
Devonian - Mississippian
Snowcap and Finlayson
assemblages
Pacific
Finlayson
Lake
5
YT
n
Pennsylvanian - Permian
Klinkit assemblage
IN
YT
Devonian - Permian
Stikine - Boswell assemblage
Permian
Klondike assemblage
Macmillan
Pass
AS
6
Wh
Arc assemblages
LW
deformatio
Paleozoic - Jurassic
Tozitna - Angayucham
assemblages
SE
a
na
YT
ran
4
De
le
Cordil
T
Delta
D
in
nt
Ti
148°
W
limit
YT
Bonnifield
MY
NA
of
2
67°N
124°W
MN
1
DL
TZ
Fb
BC
NW T
NX
WS
YT
RB
WS
IN
Area of
map
eastern
TZ
TZ
PC
TZ
RB
Kaltag
YT
11
Figure 1. Paleozoic lithotectonic terranes and assemblages of the northern Cordillera: AA – Arctic Alaska (includes Endicott Mountains,
North Slope and Skajit allochthon); AG – Angayucham; CA – Cassiar; CO – Coldfoot (schist belt of southern Brooks Range); DL – Dillinger;
IN – Innoko; MN – Minchumina; MY – Mystic; NA – North American miogeocline; NX – Nixon Fork; PC – Porcupine; RB – Ruby;
SD – Seward; SM – Slide Mountain - Seventymile (includes Chatanika); ST – Stikine (Asitka); TZ – Tozitna; WM – Windy-McKinley;
WS – Wickersham (includes Chena River, Fairbanks schist); YT – Yukon-Tanana. Areas discussed in this paper: (1) Alaska Range; (2) YukonTanana upland; (3) Fortymile River; (4) Stewart River (including Dawson); (5) Finlayson Lake; (6) Glenlyon; (7) Teslin; (8) Wolf Lake –
Jennings River (including Big Salmon Complex); (9) Sylvester allochthon; (10) Tulsequah; (11) Lay Range. Other abbreviations: Ak – Alaska;
B.C. – British Columbia; D – Dawson; E – Eagle; Fb – Fairbanks; NWT – Northwest Territories; Wh – Whitehorse; WL – Watson Lake;
T – Tok; YT – Yukon Territory. Blueschists and eclogite occurrences are from Dusel-Bacon (1994) and Erdmer et al. (1998).
284
Paleozoic magmatism and crustal recycling
Arc and Non-Arc Geochemical Signatures
Mafic Geochemical Signatures
Mafic rocks from non-arc settings (e.g., mid-ocean ridges, ocean
islands, etc.) are characterized by very smooth PM-normalized trace
element patterns and have flat to positive anomalies of Nb relative
to Th and La (Fig. 2A). In the non-arc mafic group, there are three
broad subdivisions, including normal mid-ocean ridge basalts
(N-MORB), enriched mid-ocean ridge basalts (E-MORB) and ocean
island (or oceanic intraplate) basalts (OIB).
Normal-MORB (N-MORB) magmas are typified by flat PMnormalized signatures with depleted light rare earth elements
(LREE), Nb and Th contents relative to PM (Fig. 2A). They are derived from incompatible element depleted mantle source regions
(McKenzie and Bickle, 1988; McKenzie and O’Nions, 1991; Sun and
McDonough, 1989) and are commonly found at spreading ridges,
but can also be found in back-arc basin settings (e.g., Gribble et al.,
1996; Sun and McDonough, 1989).
So-called ocean island basalt (OIB or within-plate) signatures,
are characterized by steep PM-normalized patterns with LREE-
enrichment, high HFSE contents, and, in particular, positive Nb
anomalies relative to Th and La (Fig. 2A). Rocks with OIB signatures
can be derived from a variety of sources, but it is generally assumed
that they represent melts from incompatible element-enriched mantle
1000
Th La Pr Sm Hf Ti Tb Y Yb Al Sc
Nb Ce Nd Zr Eu Gd Dy Er Lu V
A)
OIB
Rock / Primitive Mantle
E-MORB
N-MORB
100
10
1
1000/.1
Rock / Primitive Mantle
B)
100
CAB
L-IAT
IAT
BON
10
1
1000/0.1
Rock / Primitive Mantle
depleted), crust (e.g., continental, oceanic) and subducted slab components involved in mafic and felsic rock genesis. In this paper, trace
element data have been plotted on diagrams that reflect the tectonic
setting of these rocks, and diagrams that provide insight into the
nature of mantle, crustal and subducted slab components in YTT
igneous rocks.
Rocks of mafic to intermediate and felsic to intermediate compositions are treated separately. For mafic to intermediate rocks, data
are presented on primitive mantle (PM)-normalized multi-element
diagrams that depict elemental abundances for a suite of elements
from different chemical groups, including REE, HFSE and transition
elements (Fig. 2). In contrast with mafic rocks, the significance of
geochemical signatures for felsic volcanic and intrusive rocks can
be problematic in continental margin environments, as they may
merely reflect the continental crust itself (e.g., Piercey et al., 2001b).
Nevertheless, there are subtle differences between granitoids from
different tectonic environments (e.g., Piercey et al., 2001b), and these
differences, used in conjunction with the compositional characteristics of coeval mafic magmatism, and the nature of volcanic and
sedimentary facies (e.g., Piercey and Murphy, 2000), can provide
significant insight into the origin of the felsic rocks. Upper continental crust (UCC)-normalized trace element plots (Fig. 3) and the
Nb-Y diagram of Pearce et al. (1984; Fig. 4) are used to portray the
chemical characteristics of felsic rocks. The Nb-Y diagram is particularly useful in deciphering the relative HFSE enrichment (nonarc) or HFSE depletion (arc) in felsic rocks, which can be used to
differentiate arc from non-arc rocks (e.g., Piercey et al., 2001b, 2003;
Fig. 4). This diagram, however, cannot separate rocks that have a
true arc signature from those that have inherited an arc signature
from interaction or melting of pre-existing arc crust; thus, when we
describe “arc” signatures in felsic rocks we have relied on a combination of field relationships and the lithogeochemical signatures of
both mafic and felsic magmatic rocks.
C)
T-NEB
BABB
100
10
1
.1
Th La Pr Sm Hf Ti Tb Y Yb Al Sc
Nb Ce Nd Zr Eu Gd Dy Er Lu V
Figure 2. Primitive mantle-normalized plots for representative examples of arc and non-arc mafic rocks. (A) normal-mid-ocean ridge
basalt (N-MORB), enriched-mid-ocean ridge basalt (E-MORB) and
ocean island basalt (OIB; Sun and McDonough, 1989); (B) boninite
(BON; Jenner, 1981), island arc tholeiite (IAT; Piercey, 2001; Piercey
et al., 2004), LREE-enriched IAT (L-IAT; Shinjo et al., 2000) and
calc-alkaline basalt (CAB; Stoltz et al., 1990); (C) back-arc basin
basalt (BABB; Ewart et al., 1994) and Th-enriched OIB (T-NEB;
Shinjo et al., 2000). Primitive mantle values in this diagram, and all
other primitive mantle normalized plots in this paper, are from Sun
and McDonough (1989).
285
Piercey et al .
Rock / Upper Continental Crust
100
A)
B)
YTT - A-type
YTT - Peralkaline
10
1
Ethiopia, Peralkaline
Yellowstone, A-type
.1
.01
Th La Pr Sm Hf Ti Tb Y Yb Al
V
Nb Ce Nd Zr Eu Gd Dy Er Lu Sc
Th La Pr Sm Hf Ti Tb Y Yb Al
V
Nb Ce Nd Zr Eu Gd Dy Er Lu Sc
C)
D)
Rock / Upper Continental Crust
10
1
Andes, CAR
YTT - ThR
Crater Lake
YTT - CAR
.1
Th La Pr Sm Hf Ti Tb Y Yb Al
V
Nb Ce Nd Zr Eu Gd Dy Er Lu Sc
Th La Pr Sm Hf Ti Tb Y Yb Al
V
Nb Ce Nd Zr Eu Gd Dy Er Lu Sc
Figure 3. Upper continental crust (UCC)-normalized plots for felsic rocks from YTT and recent analogues (Cenozoic and younger). Typical
UCC-normalized signatures for non-arc felsic rocks: (A) A-type and peralkaline rhyolites from YTT (Piercey et al., 2001b; Dusel-Bacon
et al., 2004); (B) within-plate (A-type) felsic rocks from the Yellowstone Plateau (Hildreth et al., 1991) and Quaternary peralkaline rhyolites
from Ethiopia (Peccerillo et al., 2003). Typical UCC-normalized signatures for arc felsic rocks: (C) tholeiitic rhyolite (ThR) and calc-alkaline
rhyolite from YTT (CAR; Piercey et al., 2001b); (D) calc-alkaline rhyolite built on mafic crust (similar to tholeiitic rhyolites in YTT?) from
Crater Lake, Oregon (Bacon and Druitt, 1988) and calc-alkalic rhyolites from the central Andes, Chile (Lindsay et al., 2001). Upper continental crust values for this diagram, and all other continental crust normalized plots in this paper, are from McLennan (2001).
sources, such as mantle plumes, in within-plate environments such
as ocean island (e.g., Hawaii), oceanic plateau (e.g., Ontong-Java)
and continental flood basalt environments (e.g., Lassiter and DePaolo,
1997; Sun and McDonough, 1989). These signatures can also be
found in basalts derived from enriched lithospheric mantle source
melts formed in plate-margin environments during the rifting of
continental arcs or continent margins (e.g., van Staal et al., 1991;
Piercey et al., 2002a, b; Dusel-Bacon and Cooper, 1999; Shinjo et al.,
1999; Shinjo and Kato 2000). These types of basalts also occur in
modern arcs associated with slab windows (e.g., Kamchatka) and
Archean greenstone belts, and have been termed Nb-enriched basalts
(Kepezhinskas et al., 1997; Wyman et al., 2000). In this paper, rocks
with OIB signatures may be described as “within-plate” in places.
This reflects their positions on discrimination diagrams and does
286
1000
Within-Plate
(A-type)
100
& dge
i
ate
-Pl ean R
n
i
th Oc
i
W us
alo
om
Syncollisional
Nb
An
Volcanic Arc
10
Ocean Ridge
(OR-type)
M-type
1
1
10
100
Y
1000
Figure 4. The Nb-Y discrimination diagram for felsic rocks from
Pearce et al. (1984). Symbols and data sources are as in Figure 3.
Paleozoic magmatism and crustal recycling
not imply that they formed in a within-plate setting. Most, if not all,
of these rocks formed along plate margins due to continental or arc
rifting.
Enriched-MORB (E-MORB) signatures are hybrid signatures
between N-MORB and OIB (Fig. 2A) due to mixtures of magmas
from depleted and enriched mantle sources (Sun and McDonough,
1989). Enriched-MORB magmas can be found at so-called plumecentered spreading ridges (e.g., Iceland; Sun and McDonough, 1989),
spreading centres with heterogeneous plum pudding-type mantle
(e.g., East Pacific Rise; Niu et al., 1999), where there is mixing of
enriched (OIB) “plums” within a depleted (N-MORB-type) mantle
matrix, and can also be found in rocks derived from the mixing of
enriched lithospheric/asthenospheric sources during the evolution
of continental rifts and back-arc basins (e.g., Piercey, 2001; Piercey
et al., 2002a).
Mafic to intermediate arc rocks, unlike non-arc rocks, do not
have smooth PM-normalized signatures. They generally (although
not universally; e.g., Piercey et al., 2002a, b) show a distinctive
negative Nb anomaly relative to Th and La, the so-called “arc signature” (Fig. 2; Swinden et al., 1997). Arc volcanic rocks, like nonarc rocks, are derived from similar variably enriched mantle sources;
however, unlike non-arc rocks, they have an additional component,
the slab component, superimposed upon the mantle wedge (Pearce
and Peate, 1995, and references therein). During the subduction of
oceanic lithosphere, dehydration of hydrous silicate minerals within
the slab (and from sedimentary rocks atop it) results in the transfer
of fluid-mobile elements to metasomatize the sub-arc mantle wedge
(e.g., Pearce, 1983; Pearce and Parkinson, 1993; Pearce and Peate,
1995; You et al., 1996; Johnson and Plank, 1999). Due to the high
mobility of the LFSE in fluids, arc rocks are typically enriched in
these elements; Th, the relatively immobile LFSE, also involved in
slab metasomatism, is used here as the indicator of the slab-derived
fluid flux. Thorium, however, can also be enriched in mafic rocks
contaminated by continental crust. In these instances, Th often shows
systematic relationships with other indicators of crustal contamination (i.e., εNdt, SiO2, Zr), and these geochemical relationships were
used to screen our data in order to identify samples that may have
been influenced by crustal contamination versus samples that have
a subducted slab signature (see Piercey et al., 2002a, 2004, for examples). In addition to Th-enrichment, arc rocks are typically characterized by HFSE depletions, in particular Nb, that result from retention of HFSE in accessory minerals within the subducted slab
(e.g., rutile; Foley et al., 2000), which can be intensified by HFSEdepletions due to previous melting events in the sub-arc mantle
wedge (e.g., Pearce et al., 1992; Woodhead et al., 1992). The elevated
LFSE and depleted HFSE signatures lead to high LFSE/HFSE
(Th/Nb) ratios in arc rocks (Fig. 2B). Within the arc group of YTT
mafic rocks, there are broadly four common signatures: island arc
tholeiites (IAT), LREE-enriched island arc tholeiites (L-IAT), calcalkaline basalts (CAB) and rare boninites (BON; Fig. 2B).
Island arc tholeiitic rocks represent melts of a source similar to
N-MORB but include a subduction zone metasomatic component.
The IAT suites have flat PM-normalized patterns and are characterized by a negative Nb anomaly relative to Th and La, HFSE depletion
(Fig. 2B), and follow a tholeiitic (Fe-enrichment) differentiation
trend (Swinden et al., 1997); in some cases they have very low Ti
contents (Brown and Jenner, 1989). These suites of rocks are commonly associated with the early construction of island arcs, usually
before the arc edifice accumulated up to a significant thickness.
Calc-alkaline basalts and andesites typically reflect the mature
stages of arc development coincident with arc edifice growth to a
significant thickness. They are also the most common mafic-intermediate rocks in arcs built on or near continental crust (Swinden
et al., 1997). The CAB suite is characterized by PM-normalized
patterns with LREE-enrichment, very strongly developed negative
Nb and Ti anomalies (Fig. 2B), and calc-alkaline fractionation trends
(e.g., early Fe-depletion). Calc-alkaline suites can also be formed
when tholeiitic magmas, and possibly N-MORB magmas, become
contaminated by continental crust early in their petrogenetic history
(e.g., Swinden et al., 1997). Calc-alkaline magmatic suites can also
be differentiated from tholeiitic suites by high Zr/Y, La/Yb and
Th/Yb values (see Barrett and MacLean, 1999, and references
therein), all of which can be observed on a standard primitive mantle
normalized plot (Fig. 2).
The L-IAT suite of magmas is believed to be a hybrid and part
of a continuum between the IAT and CAB suites, and likely represents either derivation from an E-MORB source with a subduction
component (Shinjo and Kato, 2000; Shinjo et al., 1999), or a weakly
crustally contaminated IAT (Piercey, 2001; Piercey et al., 2004).
Boninitic rocks are unique magmatic rocks that reflect derivation from high temperature melting of ultra-depleted mantle sources
(i.e., more depleted than N-MORB). They commonly form during
the initiation of island arcs or back-arc basins (Pearce et al., 1992;
Crawford et al., 1989; and references therein), where they are commonly spatially associated with IAT. Although generally occurring
in fore-arc environments within intra-oceanic arcs, in YTT boninites
are associated with the initiation of intracontinental (ensialic) backarc basin magmatism (Piercey et al., 2001a). Boninitic rocks are
characterized by U-shaped PM-normalized trace element patterns
with negative Nb anomalies, middle-REE depletions, very low TiO2,
HFSE and REE contents, high compatible element contents (Cr, Ni,
Co, Sc, V), and often, but not always, they have positive Zr and Hf
anomalies relative to Sm (Fig. 2B).
In addition, YTT also contains mafic to intermediate magmatic
rocks that have geochemical signatures transitional between arc and
non-arc. Typically they have non-arc affinities but have a weak
subduction signature, these include: back-arc basin basalts (BABB)
and Th-enriched, Nb-enriched basalts (T-NEB). The BABB suites
are essentially similar to N-MORB but have a weak negative Nb
anomaly (Fig. 2C). Similarly, the T-NEB suite has a signature very
similar to OIB but with a flat to weakly negative Nb anomaly
(Fig. 2C). The BABB suite commonly form during back-arc basin
development and represent MORB-type magmatism with minor
subduction zone fluid influence (Hawkins, 1995; Gribble et al., 1996;
Piercey et al., 2004). The T-NEB suite is interpreted to represent arc
rift rocks with a minor subduction signature (Kepezhinskas et al.,
1997; Piercey et al., 2004).
287
Piercey et al .
Sm-Nd Isotopic Signatures: Crust vs. Mantle
Components in Magmas
Samarium-neodymium isotope geochemistry is commonly used in
ancient belts to decipher the relative contributions of crust and mantle
in igneous rocks, because the Sm-Nd system is very resistant to al-
288
teration and metamorphism, and, in most cases, is insensitive to
fractionation during magma crystallization or partial melting
(e.g., DePaolo, 1988). This technique is based on the radiogenic decay
of 147Sm to 143Nd; both of these isotopes are commonly presented as
a ratio to the nonradiogenic 144Nd (e.g., 147Sm/144Nd, 143Nd/144Nd). The
model isotopic evolution of the Sm-Nd system involves the separation of continental crustal (CC) and depleted mantle (DM) reservoirs
from a chondritic uniform reservoir (CHUR). In the case of the DM
reservoir, the separation from a CHUR reservoir resulted in a higher
Sm/Nd ratio in DM than in CHUR reservoir, which leads to a timeintegrated increase in 143Nd/144Nd relative to CHUR (Fig. 5). In
contrast, continental crust evolved with a lower Sm/Nd ratio, slower
increase in 147Sm and, by association, resulted in a time-integrated
143
Nd/144Nd ratio lower than CHUR (Fig. 5A). Neodymium-isotopic
data are commonly presented in the shorthand form, epsilon notation,
which represents the 147Sm/144Nd variation of a rock relative to CHUR
at a given point in the past (i.e., εNdt; Fig. 5B). Rocks derived from
crustal reservoirs, or with recycled crustal components, have εNdt <0,
whereas rocks derived from depleted mantle-type reservoirs have
εNdt >0; values from chondritic reservoirs have εNdt ~ 0 (Fig. 5B).
A)
Depleted Mantle
La Nd Sm Lu
144
Nd/ Nd
Delineating arc and non-arc signatures in felsic rocks is very difficult.
Commonly felsic rocks have non-distinctive geochemical signatures
that may be derived in whole or in part from melting pre-existing
continental crust (Piercey et al., 2001b). In order to understand the
origin and setting of felsic rocks in YTT, an integrated approach is
required. This approach includes a detailed documentation of volcanic and sedimentary facies, and using the geochemical signatures
of associated mafic rocks. This approach has led to establishing
empirical relationships for arc and non-arc felsic assemblages within
YTT (e.g., Piercey et al., 2001b). In general, non-arc felsic rocks from
YTT have PM- and UCC-normalized REE patterns that are somewhat
different than that of arc rocks (Fig. 3). Non-arc rocks have higher
total REE and HFSE contents and lower compatible element contents
(Sc, V, Ti, Ni, Cr) and are similar to crustally-derived A-type and
peralkaline felsic rocks (Figs. 3, 4; Piercey et al., 2001b, 2003; DuselBacon et al., 2004). Shown for comparison on Figure 3B are withinplate (A-type) felsic rocks from the Yellowstone caldera (Hildreth
et al., 1991) and Quaternary peralkaline rhyolitic rocks from Ethiopia
(Peccerillo et al., 2003). Although some of the felsic rocks within
this paper are described as having “within-plate” signatures, this
reflects their position on a discrimination diagram, and does not
imply that they formed in a within-plate setting. Most, if not all, of
these rocks occur along plate margins due to continental or arc
rifting.
Arc felsic rocks are present in many parts of the terrane throughout its evolution (Mortensen, 1992a; Colpron et al., this volume-a,
and references therein). They typically are of crustally-derived, calcalkaline affinity (e.g., Mortensen, 1992a; Piercey et al., 2003; Grant,
1997). Their UCC-normalized patterns are relatively flat, often with
depletions in Ti, Sc, V, and in some cases Th ± Nb (Fig. 3), features
consistent with derivation from upper crustal sources accompanied
by oxide and/or accessory mineral fractionation (Piercey et al., 2001b,
2003). Compared to the non-arc suites in YTT, the calc-alkalic arc
suites have lower HFSE and REE contents with volcanic-arc (I-type)
signatures (Figs. 3, 4). Tholeiitic arc felsic rocks are rare within YTT
(Grant, 1997; Piercey et al., 2001b, 2003). They have LREE- and
HFSE-depleted UCC-normalized patterns when compared to calcalkalic arc and A-type felsic rocks (Figs. 3, 4; Piercey et al., 2001b).
The tholeiitic arc felsic rocks are interpreted to have been derived
from melting of primarily mafic to andesitic crust more juvenile than
the source for the A-type and calc-alkaline suites (Piercey et al.,
2001b, 2003). Illustrated for comparison are arc felsic rocks built
upon a thick crust of accreted oceanic crustal material from Mount
Mazama, Crater Lake, Oregon (Bacon and Druitt, 1988), and arc
felsic rocks erupted in a continental arc setting with thick crust
(~calc-alkalic) from the Central Andes, Chile (Fig. 3; Lindsay et al.,
2001). Notably, there are no transitional felsic suites in YTT.
143
Felsic Geochemical Signatures
Continental Crust
La Nd Sm Lu
CHUR
La Nd Sm Lu
1.0
2.0
3.0
Time (b.y.)
4.0
B)
Depleted Mantle-Sm/Nd~0.36
+
ENd
0
CHUR-Sm/Nd=0.325
Continental Crust-Sm/Nd~0.18
1.0
2.0
3.0
Time (b.y.)
4.0
Figure 5. Isotope evolution diagrams for the Sm-Nd isotope system.
Modified from Swinden et al. (1997) based on concepts outlined in
DePaolo (1988; and references therein). See text for discussion.
CHUR = chondrite uniform reservoir.
Paleozoic magmatism and crustal recycling
The magmatic and tectonic evolution of Yukon-Tanana and related
terranes in the northern Cordillera is recorded in six Paleozoic
magmatic cycles (Tables 1 and 2; see also Colpron et al., this volume-a; Nelson et al., this volume). The magmatic cycles include two
pulses of felsic (plus mafic and intermediate) magmatism, separated
by Pennsylvanian – Early Permian mainly intermediate to mafic
activity (Fig. 6). The first pulse of felsic magmatism in YTT corresponds primarily to voluminous Late Devonian to Mississippian
igneous rocks of the Finlayson and lower Klinkit assemblages
(Colpron et al., this volume-a). It contains four distinct cycles (I-IV;
Fig. 6) that are punctuated by at least two episodes of deformation
and erosion. The Pennsylvanian to Early Permian lull in felsic magmatism corresponds to a cycle dominated by intermediate to mafic
magmatism and basinal sedimentation in the Slide Mountain and
upper Klinkit assemblages (Cycle V; Fig. 6). The second pulse of
felsic magmatism, and the last Paleozoic magmatic cycle, is represented by more localized Middle to Late Permian magmatism of the
Klondike assemblage (Cycle VI; Figs. 1, 6).
Ecstall Cycle (Cycle I - Middle to Late Devonian 390-365 Ma)
The first magmatic cycle is most widespread in the Alaska Range
and Yukon-Tanana Upland of eastern Alaska (Fig. 1). Coeval felsic
volcanism of probable within-plate affinity is locally present in miogeoclinal strata of Selwyn basin in central Yukon (Hunt, 2002), and
magmatism of probable arc affinity is documented in the Coast
Mountains of British Columbia and southeastern Alaska, including
parts of the Tracy Arm and Endicott Arm assemblages (Gehrels
et al., 1992) and the Ecstall belt (Gareau and Woodsworth, 2000;
Alldrick and Gallagher, 2000; Alldrick, 2001; Figs. 1, 7). Massive
sulphide occurrences and deposits of the Ecstall belt, Selwyn basin
and the Bonnifield district and Delta mineral belt formed during this
cycle.
Limited trace element data presented by Dashevsky et al. (2003)
for the Delta mineral belt of the Alaska Range suggest that these
rocks formed in a continental arc setting. In the Coast Mountains,
the predominance of tonalitic plutons, and limited isotopic data from
the Ecstall belt, also suggest a continental arc setting for these rocks
(Table 1; Gareau and Woodsworth, 2000). However, because the
dataset from the Delta mineral belt is limited, and because highprecision geochemical data are not available for magmatic rocks of
the Ecstall belt, Tracy Arm and Endicott Arm assemblages, these
rocks will not be discussed further here.
Cycle I magmatism in the Alaska Range and Yukon-Tanana
Upland likely occurred in a continental rift setting (Dusel-Bacon
et al., this volume; Figs. 1, 7). In the Bonnifield mining district of
the Alaska Range, the Late Devonian to earliest Mississippian
stratigraphic sequence consists of a varying succession (Healy schist,
Keevy Peak Formation, Totatlanika Schist and Wood River assemblage) of felsic and mafic metavolcanic and shallow intrusive rocks
associated with variably carbonaceous metasedimentary rocks
(Wahrhaftig, 1968; Dusel-Bacon et al., 2004, this volume). Mafic
VI
V
IV
III
II
I
Probability density
MAGMATIC AND PETROLOGIC
EVOLUTION
TRIASSIC
240
PERMIAN
260
280
PENN.
300
MISSISSIPPIAN
320
Age (Ma)
340
DEVONIAN
360
380
400
n = 129 U/Pb ages
n = 252 fossil ages
Figure 6. Probability density plot for 129 U-Pb zircon ages (solid
curve) and 252 fossil age determinations (dotted curve) for the
pericratonic terranes of the northern Cordillera. The peaks in probability density of U-Pb ages outline the two main pulses of felsic
magmatism in YTT. The probability density curve for fossil ages
(mainly from conodonts) is shown to illustrate biostratigraphic
control during the Pennsylvanian-Permian lull in felsic magmatism
(Cycle V). Peaks in probability density of fossil ages in Mississippian
and Pennsylvanian times correspond to main episodes of carbonate
deposition in YTT. Fossil age determinations whose range exceeded
± 25 Ma were excluded from this computation. Magmatic cycles
(I-VI) discussed in the text are labelled at top of the diagram.
metavolcanic rocks within the Totatlanika Schist (Moose Creek and
Chute Creek members) have alkalic, OIB-like PM-normalized signatures, with the one Moose Creek sample exhibiting a minor negative Nb and Ti anomaly (Fig. 8A). These features are consistent with
derivation from a moderately enriched mantle source region; the
Moose Creek sample exhibits minor crustal contamination. Felsic
rocks in the Bonnifield mining district occur throughout the succession from the Healy Schist through the Totatlanika Schist (DuselBacon et al., 2004, this volume). Most felsic rocks, with the exception
of the Mystic Creek Member of the Totatlanika Schist and some
Wood River assemblage samples, have very similar calc-alkalic
patterns on UCC-normalized plots but with differing HFSE and REE
abundances (Fig. 9A). In contrast, the Mystic Creek member contains
a very distinctive suite of peralkaline rhyolites with very high HFSE
and REE contents and positive Nb anomalies (Fig. 9A).
In the Yukon-Tanana Upland, Cycle I comprises bimodal mafic
(amphibolites) and felsic (augen gneiss, felsic schist) meta-igneous
rocks associated with variable amounts of metasedimentary rocks
(quartzite, pelite, marble, phyllite of the Lake George assemblage;
Weber et al., 1978; Smith et al., 1994; Dusel-Bacon et al., 2004).
Mafic rocks in the Yukon-Tanana Upland show mostly OIB-like
signatures (Fig. 8B), with some samples (unit Dag) exhibiting weak
negative Nb anomalies that are likely due to crustal contamination
of an OIB-like magma (Dusel-Bacon and Cooper, 1999; Dusel-Bacon
et al., 2004, this volume). Felsic rocks in the Yukon-Tanana Upland
exhibit greater variability, but generally have flat UCC-normalized
289
Piercey et al .
1000
N
A)
(Eifelian - Famennian)
390-365 Ma
RIFT
SYMBOLS
Volcanism
63
357368
Chute Creek Mbr
Th La Pr Sm Hf Ti Tb Y Yb Al Sc
Nb Ce Nd Zr Eu Gd Dy Er Lu V
1000
°N
59
YT
RIFT
?
Rock / Primitive Mantle
?
Pacific
Ocean
12
4°W
57°
N
1
pre-362
12
8°
W
382
Ak
B.
C.
374
367
0
Butte assembl.
Nasina assembl.
Dag
Pzsq
MDgn
Th La Pr Sm Hf Ti Tb Y Yb Al Sc
Nb Ce Nd Zr Eu Gd Dy Er Lu V
Figure 8. Primitive mantle normalized plots for Cycle I mafic rocks.
(A) Non-arc mafic rocks from the Totatlanika Schist, Alaska Range
(data from Dusel-Bacon et al., 2004); (B) Non-arc mafic rocks from
the Yukon-Tanana Upland (data from Dusel-Bacon et al., 2004).
Tintina
ARC
370
Yukon-Tanana Upland - Non-Arc Rocks
10
.1
375
B)
100
CA
YT
B.
C.
366
367
Moose Creek Mbr
1
?
YT
B.
C.
56
°N
Scale
300
km
Figure 7. Distribution of tectonic and magmatic events during the
Middle to Late Devonian Ecstall cycle (Cycle I: 390-365 Ma). Events
are located on a base map of Yukon-Tanana and related terranes
prior to ~430 km of dextral displacement along Tintina fault. Dashed
red line marks approximate limit between geodynamic environments.
Dashed blue line represents relative position of subduction zone;
teeth indicate dip of subducting slab. See Figure 1 for tectonic assemblage legend. Relative position of Cassiar terrane (CA) and
Selwyn basin (SB) are also shown.
290
10
SB
?
W
0°
14
100
°N
Compressional
deformation
Ak
Alaska Range - Non-Arc Rocks
(Totatlanika Schist)
.1
ARC
Denali
Massive sulphide
deposit
357 375
360 375
Delta
Plutonism
RIFT
n
io
W
8°
14
373 381
ns
te
ex
357 375
Marg
13
6°
W
Bonnifield
360 375
Rock / Primitive Mantle
64°
Cycle I - Ecstall
Middle to Late Devonian
multi-element patterns with depletions in Ti, Al, Sc, V (±Zr, Hf and
Eu) that are consistent with derivation from melting of continental
crust accompanied by feldspar and oxide fractionation (Fig. 9). Data
for these rocks cluster close to the boundary between within-plate
and volcanic arc fields, with the exception of two felsic samples from
the Nasina assemblage, which have peralkaline affinities and withinplate signatures (Fig. 9). On UCC-normalized plots, some felsic rocks
from the Yukon-Tanana Upland (units Pzsq, MDmg and Butte assemblage of Dusel-Bacon et al., 2004, this volume) all have similar
relatively flat calc-alkalic patterns, with depletions in Ti, Al, Sc, V
(±Zr, Hf and Eu) consistent with derivation from melting of continental crust accompanied by feldspar and oxide fractionation (Fig. 9).
In contrast, two samples of felsic rocks from the Nasina assemblage
are much more depleted in LREE, but are highly enriched in Nb, Zr,
Hf and HREE contents (Figs. 9D, E).
The distinctive occurrence of peralkaline rocks in the Bonnifield
mining district, the dominance of OIB-like mafic rocks, the lack of
mafic rocks with arc signatures, and the dominance of metasedimen-
Paleozoic magmatism and crustal recycling
tary rocks has led Dusel-Bacon et al. (2004, this volume) to suggest
that Cycle I magmatism in the Alaska Range and the Yukon-Tanana
Upland was the result of crustal attenuation and rifting of a continental-margin sequence that was parautochthonous with respect to
the craton, coeval with extension in Selwyn basin and arc magmatism
in the outboard parts of YTT (Fig. 7; Dusel-Bacon et al., 2004, this
volume; Nelson et al., this volume).
Finlayson Cycle (Cycle II - Late Devonian to Early
Mississippian - 365-357 Ma)
The second magmatic cycle marks the onset of felsic magmatism in
many parts of YTT and the formation of massive sulphide deposits
in the Finlayson Lake district (Figs. 6, 10). Magmatic activity, which
began during Cycle I in the Alaska Range and Yukon-Tanana Upland
persisted during Cycle II (Fig. 10). Localized felsic magmatism of
probable within-plate affinity also occurs in miogeoclinal strata of
the Selwyn basin and Cassiar terrane (Figs. 1, 10). In YTT, this cycle
Table 1. Summary of geochemical attributes of arc assemblages for Paleozoic magmatic cycles in YTT.
Cycle
I
II
III
IV
V
VI
Age Range
Arc Setting
Sources of Data
(Figure)
Locations*
Geochemical Signatures
Isotopic Signatures
390-365 Ma
Ecstall belt, B.C.
No geochemical data; mafic to intermediate
(minor felsic) metavolcanic rocks; metatonalite plutons.
Felsic: εNdt = -4.0.
Alldrick and Gallagher (2000);
Alldrick (2001); Gareau and
Woodsworth (2000).
Coast Mountains, SE Alaska
N/A
Uncertain. Proterozoic-Archean inheritance in
some zircon populations.
Gehrels et al. (1992).
Delta mineral belt,
Alaska Range
Felsic: Limited trace element data suggest
calc-alkalic arc.
Uncertain. Proterozoic-Archean inheritance in
zircon populations.
Dashevsky et al. (2003).
Finlayson Lake (5)
(Cleaver Lake, Waters Creek
and Fire Lake formations)
Felsic: calc-alkalic arc; Mafic: IAT, L-IAT, MORB, NEB and BON.
Felsic: εNdt = +0.1 to -4.8; Mafic: εNdt = -5.1 to +7.0.
Piercey et al. (2001a, b, 2002a,
2003, 2004); Grant (1997).
Stewart River (3)
Mafic (amphibolite): IAT (MORB).
Uncertain. Proterozoic-Archean inheritance in
zircon populations.
S. Piercey and J. Ryan
(unpublished data).
Finlayson Lake (5)
(Simpson Range suite)
Felsic: calc-alkalic metaluminous intrusions
Felsic: εNdt = -7.4 to -12.9.
Piercey et al. (2001a, b, 2002b,
2003); Grant (1997).
Stewart River (3)
Felsic: Calc-alkalic intrusions.
Uncertain. Proterozoic-Archean inheritance in
zircon populations.
J. Ryan and M. Villeneuve
(personal communication); S. Piercey and J. Ryan
(unpublished data).
Fortymile River (3)
Felsic: calc-alkaline intrusions; Mafic: MORB.
N/A
Mortensen (1990), J. Mortensen
(personal communication).
Glenlyon (6)
(Little Kalzas suite)
Felsic: calc-alkaline andesites, rhyolites, and
intrusive rocks; Mafic: OIB-like mafic volcanic rocks.
Uncertain. Proterozoic inheritance in zircon
populations.
Colpron (2001)
Teslin (7)
Felsic: calc-alkalic metaluminous intrusions; Mafic: rift-like mafic rocks?
Felsic: εNdt = -2.5 to -6.2; Mafic: εNdt = +4.1 to +6.4.
Stevens et al. (1995); Creaser
et al. (1997).
Wolf-Jennings (8)
(upper Dorsey and Ram Creek
complexes; Swift River Group)
Felsic: calc-alkaline intrusions; Mafic: L-IAT to CAB; minor N-MORB to
OIB.
Uncertain. Proterozoic inheritance in zircon
populations.
Nelson and Friedman (2004).
Glenlyon (6)
(Little Salmon fm. and
Tatlmain suite)
Felsic: calc-alkalic andesite-dacite-rhyolite;
calc-alkaline intrusions;
Mafic: OIB-like.
Uncertain. Proterozoic-Archean inheritance in
zircon populations.
Colpron (2001), Colpron et al.
(this volume-b).
Wolf-Jennings (8)
(Ram Creek and Big Salmon
complexes)
Felsic: calc-alkaline schist;
Mafic: L-IAT, CAA, E-MORB/OIB.
Uncertain. Proterozoic-Archean inheritance in
zircon populations.
Nelson and Friedman (2004).
M. Mihalynuk (unpublished data).
Tulsequah (10)
Felsic: calc-alkalic rhyolites;
Mafic: N-MORB to E-MORB with
subduction signature.
Uncertain.
Sebert and Barrett (1996).
Wolf-Jennings (8)
(Klinkit Group)
Mafic: calc-alkalic fragmental volcanic and
volcaniclastic rocks; OIB.
Mafic: εNdt = +6.7 to +7.4.
Simard et al. (2003).
Sylvester allochthon (9)
(Fourmile succession)
L-IAT mafic and calc-alkalic felsic
Lay Range, B.C. (11)
Mafic: L-IAT to MORB.
Stewart River (3)
(Klondike schist; Sulphur
Creek orthogneiss)
Felsic: calc-alkalic volcanic and intrusive
rocks.
Uncertain. Proterozoic-Archean inheritance in
zircon populations. Radiogenic Pb in associated
syngenetic occurrences. εNdt - crustal.
S. Piercey and J. Mortensen
(unpublished data); Mortensen
(1990); J. Mortensen (unpublished
data).
Wolf-Jennings (8)
(Meek pluton)
Felsic: calc-alkaline intrusion.
N/A
Nelson and Friedman (2004).
Fortymile River (3)
Felsic: calc-alkalic schist.
N/A
J. Mortensen and C. Dusel-Bacon
(unpublished data).
365-357 Ma
(Fig. 10)
357-342 Ma
(Fig. 13)
342-314 Ma
(Fig. 17)
314-269 Ma
(Fig. 21)
269-253 Ma
(Fig. 24)
Nelson (1993).
Ferri (1997).
Note: * Bold characters indicate areas for which data are summarized in this paper; numbers refer to locations shown in Figure 1.
291
Piercey et al .
Rock / Upper Continental Crust
100
A) Alaska Range: Arc/Non-Arc Felsic Rocks
Mystic Creek
California Creek
Moose Creek
Wood River
Keevy Peak
Healy Schist
10
B)
1000
100
Nb
e
&
te
idg
pla an r
n
i
e
h
c
wit us o
alo
om
within plate
(A-type)
syn-collisional
(S-type)
an
1
volcanic arc
(I-type)
10
ocean ridge
(OR-type)
M-type
.1
1
Th La Pr Sm Hf Ti Tb Y Yb Al
V
Nb Ce Nd Zr Eu Gd Dy Er Lu Sc
10
100
Yukon-Tanana Upland:
Arc/Non-Arc Felsic Rocks
Rock / Upper Continental Crust
Rock / Upper Continental Crust
C)
1
.1
Butte assem.
Dag
MDmg
Pzsq
Th La Pr Sm Hf Ti Tb Y Yb Al
V
Nb Ce Nd Zr Eu Gd Dy Er Lu Sc
E)
1000
100
Nb
e
&
te
idg
pla n r
in cea
h
t
wi us o
lo
ma
no
within plate
(A-type)
syn-collisional
(S-type)
10
1
10
D)
Y
100
1000
Yukon-Tanana Upland:
Non-Arc Felsic Rocks
(Nasina assemblage)
1
.1
Group 1
Group 2
.01
Th La Pr Sm Hf Ti Tb Y Yb Al
V
Nb Ce Nd Zr Eu Gd Dy Er Lu Sc
Figure 9. Upper continental crust (UCC)-normalized and Nb-Y
discrimination plots for Cycle I felsic rocks. (A, B) Alaska Range;
(C, D, E) Yukon-Tanana Upland (data from Dusel-Bacon et al.,
2004).
a
volcanic arc
(I-type)
10
ocean ridge
(OR-type)
M-type
1
1
10
100
1000
Y
ended with a deformational event recorded in rocks of the Finlayson
Lake district (Murphy et al., this volume). This deformational event
is recorded by fabric development, uplift and erosion, and is interpreted to represent contractional deformation in an intra-arc setting
(Murphy et al., this volume).
The Finlayson cycle is characterized by arc, arc-rift and backarc basin magmatism in most of YTT, with the exception of the
Bonnifield district and Yukon-Tanana Upland, where magmatic activity is interpreted to have occurred in an extended continental
margin setting (Dusel-Bacon et al., 2004, this volume; Fig. 10). Arc
magmatism predominated in the western parts of the terrane in this
292
cycle, and includes rocks in the western Finlayson Lake district
(Murphy et al., this volume), the Stewart River-Dawson area (J. Ryan,
personal communication), and possibly also the Delta mineral belt
of the Alaska Range (Dashevsky et al., 2003; Figs. 1, 10). Coeval
back-arc basin magmatism occurred predominantly in the eastern
Finlayson Lake district and in the Slide Mountain terrane in the
Sylvester allochthon (Fig. 10).
The Finlayson Lake district is divided into two contrasting
Devonian-Mississippian magmatic regions, which are juxtaposed
along the Permian Money Creek thrust and associated faults (see
Murphy et al., this volume). Rocks of arc affinity occur primarily to
Paleozoic magmatism and crustal recycling
Table 2. Summary of geochemical attributes of non-arc assemblages for Paleozoic magmatic cycles in YTT.
Cycle
I
Age Range
Back-Arc or Non-Arc Setting (Rift?)
Sources of Data
(Figure)
Locations*
Geochemical Signatures
Isotopic Signatures
390-365 Ma (Fig. 7)
Alaska Range (1)
Felsic: within-plate and peralkaline; some
low HFSE-REE crustal melts;
Mafic: OIB (+ contaminated OIB).
Uncertain, Proterozoic-Archean inheritance in
zircon populations. Dusel-Bacon et al. (2004, this volume).
Yukon-Tanana Upland (2)
Felsic: within-plate and peralkaline felsic;
low HFSE-REE crustal melts; Mafic: OIB (+ contaminated OIB).
Uncertain, Proterozoic-Archean inheritance in
zircon populations. Dusel-Bacon et al. (2004, this volume).
Felsic: within-plate (A-type);
Mafic: OIB-like; NEB, T-NEB, BABB, and E-MORB in Fire Lake formation. Felsic: εNdt = -7.8 to -9.5 (Kudz Ze Kayah); Mafic: εNdt = -2.8 to +1.1 (Wind Lake); εNdt = -1.6 to +8.5 (Fire Lake).
Piercey et al. (2001a, b, 2002a,
2003, 2004); Grant (1997).
II
365-357 Ma
(Fig. 10)
Finlayson Lake (5)
(Fire Lake, Kudz Ze Kayah
and Wind Lake formations)
Sylvester allochthon (9)
Mafic: MORB
N/A
Nelson (1993); Ferri (1997)
III
357-342 Ma
(Fig. 13)
Finlayson Lake (5)
(Wolverine Lake group)
Felsic: within-plate (A-type) volcanic rocks; Mafic: N-MORB to BABB volcanic rocks.
Felsic: εNdt = -7.1 to -8.2; Mafic: εNdt = +6.9.
Piercey et al. (2001a, b, 2002b, 2003); Grant (1997).
Sylvester allochthon (9)
Mafic: MORB
IV
342-314 Ma
(Fig. 17)
No back-arc yet discovered.
Paucity of basalts
in Slide Mountain.
N/A
V
314-269 Ma
(Fig. 21)
Finlayson Lake (5); (Campbell Range fm.)
Permian eclogites
Mafic: N-MORB, E-MORB, OIB, and BABB;
MORB-signatures in eclogites derived from
Campbell Range. VI
269-253 Ma
(Fig. 24)
Nelson (1993).
Campbell Range equivalent eclogites: εNdt = +5.4 to +9.3. Juvenile Pb-isotopes in VMS associated with
Campbell Range. Creaser et al. (1999); Mann and Mortensen (2000);
S. Piercey and D. Murphy
(unpublished data); Mortensen et al. (this volume).
Sylvester allochthon (9)
Mafic: N-MORB
Nelson (1993).
Lay Range, B.C. (11)
(Nina Creek Group)
Mafic: N-MORB
Ferri (1997).
No back-arc yet identified.
N/A
Note: *Bold characters indicate areas for which data are summarized in this paper; numbers refer to locations shown in Figure 1.
the southwest, in the hanging wall of the thrusts; coeval back-arc
facies occur exclusively to the east, in the footwall. Cycle II arc
magmatism is represented by the ~365-360 Ma Waters Creek and
Cleaver Lake formations, which comprise mafic volcanic, volcaniclastic and intrusive rocks, with lesser felsic volcanic and sedimentary rocks (Mortensen, 1992a, b; Murphy, 1998, 2001; Murphy and
Piercey, 1999; Murphy et al., 2002, this volume). Mafic arc rocks in
the Waters Creek and Cleaver Lake formations are geochemically
diverse, with signatures that include BON, IAT, L-IAT and CAB
(Fig. 11A). Their source magmas were derived from a variably enriched mantle wedge, with components from subducted slab and/or
crustal contamination (Fig. 11; Piercey, 2001; Piercey et al., 2001a,
2004). They have εNdt values that range from –5.0 to +7.1; however,
the bulk of the samples (6 of 8) have εNdt >0, reflecting derivation
from chondritic to juvenile sources (Piercey, 2001; Piercey et al.,
2004). Volumetrically minor felsic rocks of the Cleaver Lake formation have tholeiitic and calc-alkalic affinities, consistent with an arc
parentage (Figs. 12A, B; Piercey et al., 2001b, 2003). Neodymium
isotopic data for the felsic rocks is limited. A tholeiitic rhyolite
sample has εNdt = +0.1, and a calc-alkaline rhyolite sample has
εNdt = –4.8 (Piercey et al., 2003). These variations in Nd isotopic
attributes, coupled with various trace element ratios, suggest that
the YTT in this region was built upon a heterogeneous basement of
both oceanic and continental basement (Grant, 1997; Piercey et al.,
2001a, 2003; see also Murphy et al., this volume).
In the Finlayson Lake district (Figs. 1, 10), bimodal back-arc
magmatic activity is represented by non-arc and transitional volcanic
and intrusive rocks of the Grass Lakes group (Fire Lake, Kudz Ze
Kayah and Wind Lake formations; Murphy et al., this volume). The
stratigraphically lower Fire Lake formation is dominated by mafic
volcanic and plutonic rocks with non-arc to transitional signatures
including E-MORB, back-arc basin basalts, OIB and Th-rich OIB
which are likely crustally contaminated or slab-influenced OIB
(Fig. 11B; Piercey, 2001; Piercey et al., 2004). Neodymium isotopic
data for these rocks suggest derivation from mantle sources with
near-chondritic to depleted affinities (εNdt = -1.6 to +8.5), with the
lower εNdt values associated with the most enriched rocks, suggesting the influence of lithospheric mantle within these suites (Piercey,
2001; Piercey et al., 2004).
Back-arc magmatic rocks in the slightly younger Kudz Ze Kayah
formation include felsic volcanic, volcaniclastic and volcanosedimentary rocks (Murphy et al., 2002, this volume; Piercey et al.,
2001b, 2002a, 2003). Felsic rocks of the Kudz Ze Kayah formation
and coeval intrusions of the Grass Lakes plutonic suite are characterized by within-plate (A-type) geochemical signatures, with very
high HFSE and REE contents and low compatible element contents
(Figs. 12C, D; Piercey et al., 2001b, 2003). The flat UCC-normalized
patterns, coupled with negative Ti, Eu, Al, Sc and V anomalies
(Fig. 12C) are consistent with their derivation from partial melting
of continental crustal sources coupled with the fractionation of
feldspar and oxide minerals. An origin by partial melting of continental crust is also supported by the Nb/Ta, Ti/Sc and La/Yb ratios
in these rocks (Piercey et al., 2001b, 2003), neodymium isotopic data
(εNdt = -7.8 to -9.5; Piercey et al., 2003), ubiquitous Proterozoic-
293
Piercey et al .
N
64°
Cycle II - Finlayson
Late Devonian - Early Mississippian
360 375
357 375
13
Bonnifield
6°
W
(Famennian - Early Tournaisian)
365-357 Ma
n
io
ns
te
ex
SB
RIFT
357 375
W
8°
14
358
360 375
Delta
RIFT
358 361
SYMBOLS
Volcanism
357368
Denali
Plutonism
Massive sulphide
deposit
BAB
ARC
Fossil
F
361 365
YT
?
Kzk
357 365
360
Compressional
deformation
Ak
63
°N
357
- 360
Fyre
Lk
?
°N
59
YT
B.
C.
W
0°
14
358
359
363
RIFT
CA
Cirque
SEDEX
Ocean
Tintina
12
4°W
Pacific
?
YT
B.
C.
57°
N
Famennian -
56
°N
ARC
F Tournaisian
MORB
362
359
12
8°
W
360
Ak
B.
C.
357
0
Scale
300
km
Figure 10. Distribution of tectonic and magmatic events during
the Late Devonian – Early Mississippian Finlayson cycle (Cycle II:
365-357 Ma). Events are located on a base map of Yukon-Tanana
and related terranes prior to ~430 km of dextral displacement
along Tintina fault. Dashed red line marks approximate limit between geodynamic environments. Dashed blue line represents
relative position of subduction zone; teeth indicate dip of subducting slab. See Figure 1 for tectonic assemblage legend. Relative
position of Cassiar terrane (CA) and Selwyn basin (SB) are also
shown.
294
Archean zircon inheritance in geochronological samples (Mortensen,
1992a; Piercey, 2001), and very radiogenic (high-µ ~12) Pb isotopic
systematics in sulphides associated with felsic volcanic-hosted massive sulphide deposits (Mortensen et al., this volume).
The culmination of back-arc magmatism during Cycle II is
represented by the metamorphosed mafic volcanic rocks of the
stratigraphically highest Wind Lake formation and accompanying
high level intrusions (Murphy et al., this volume; Piercey et al.,
2002a). This formation is characterized by OIB signatures, with a
subsuite of rocks that have been contaminated by continental crust
(CC-OIB; Fig. 11C; Piercey et al., 2002a). Both suites have high TiO2,
P2O5, HFSE and REE contents and LREE-enriched signatures. The
uncontaminated suite shows positive Nb-anomalies (and εNdt = +1.1),
and the contaminated suite has slightly negative Nb anomalies and
higher Th/Nb ratios (eNdt = -2.8; Fig. 11C; Piercey et al., 2002a).
The low volume of mafic magmatism in the upper parts of the Grass
Lakes group, a ~357-360 Ma intra-arc deformation event, and an
unconformity overlying the succession at ~357 Ma (Murphy et al.,
this volume), suggest that the Grass Lakes back-arc became an
aborted rift and did not evolve to full seafloor spreading.
In the Stewart River area, Cycle II mafic magmatism is represented by amphibolitic rocks that are interlayered with a lower
metasedimentary package (Ryan and Gordey, 2001, 2002; Ryan et al.,
2003). Preliminary geochemical data (S.J. Piercey and J.J. Ryan,
unpublished data) for amphibolitic rocks are characterized by IAT
signatures, with one sample having a N-MORB signature (Fig. 11D),
suggesting that these rocks were derived from depleted mantle source
Figure 11. (facing page, top) Primitive mantle normalized plots for
Cycle II mafic rocks. (A) Average values for arc rocks of the Waters
Creek and Cleaver Lake formations, Finlayson Lake district (BON
= boninite; IAT= island arc tholeiite; L-IAT = LREE-enriched island
arc tholeiite; CAB = calc-alkaline basalt; data from Piercey, 2001;
Piercey et al., 2004); (B) Average values for non-arc rocks of the
Fire Lake and Cleaver Lake formations, Finlayson Lake district
(OIB-1 = Nb-enriched basalt with low La/Yb; T-OIB = Th-rich,
Nb-enriched basalt; BABB = back-arc basin basalt); (C) Average
values for non-arc basaltic rocks from the Wind Lake formation,
Finlayson Lake district (data from Piercey et al., 2002a); (D) Arc
and non-arc rocks from amphibolitic rocks from the Stewart River
area (S. Piercey and J. Ryan, unpublished data).
Figure 12. (facing page, bottom) Upper continental crust (UCC)
normalized and Nb-Y discrimination plots for Cycle II felsic rocks.
(A, B) Cleaver Lake formation (CAR = calc-alkaline rhyolite, ThR
= tholeiitic rhyolite; data from Piercey et al., 2001b); (C, D) Grass
Lakes group (KZK-Rhy = Kudz Ze Kayah formation rhyolite; KZKFPI = Kudz Ze Kayah formation feldspar porphyritic intrusion;
KZK-FT = Kudz Ze Kayah formation felsic tuff; GLS-Gr = Grass
Lakes suite granitoid; data from Piercey et al., 2001b).
Paleozoic magmatism and crustal recycling
A)
1000
100
10
CAB
1
10
10
CC-OIB
OIB
1
10
Rock / Upper Continental Crust
Figure 12.
Caption on
facing page.
10
1
Th La Pr Sm Hf Ti Tb Y Yb Al Sc
Nb Ce Nd Zr Eu Gd Dy Er Lu V
B)
1000
e
&
te
idg
pla an r
hin oce
t
i
w us
lo
ma
no
within plate
(A-type)
syn-collisional
(S-type)
Nb
1
a
volcanic arc
(I-type)
10
CAR
ocean ridge
(OR-type)
ThR
M-type
10
Rock / Upper Continental Crust
Finlayson Lake - Arc Felsic Rocks
(Cleaver Lake formation)
Stewart River - Arc and Non-Arc Rocks
(lower amphibolites)
100
100
.1
1
Th La Pr Sm Hf Ti Tb Y Yb Al
V
Nb Ce Nd Zr Eu Gd Dy Er Lu Sc
C)
1
Finlayson Lake - Non-Arc Felsic Rocks
1000
&
e
te
dg
pla n ri
hin ocea
t
i
w us
alo
om
an
within plate
(A-type)
syncollsional
(S-type)
Nb
KZK - Rhy
KZK - FPI
KZK - FT
GLS-Gr
100
D)
1000
100
1
10
Y
(Kudz Ze Kayah formation and
Grass Lakes suite granitoids)
volcanic arc
(I-type)
10
ocean ridge
(OR-type)
M-type
.1
A)
D)
.1
Th La Pr Sm Hf Ti Tb Y Yb Al Sc
Nb Ce Nd Zr Eu Gd Dy Er Lu V
BABB
T-OIB
Th La Pr Sm Hf Ti Tb Y Yb Al Sc
Nb Ce Nd Zr Eu Gd Dy Er Lu V
1000
Finlayson Lake - Non-Arc Rocks
(Wind Lake formation)
100
.1
.1
Th La Pr Sm Hf Ti Tb Y Yb Al Sc
Nb Ce Nd Zr Eu Gd Dy Er Lu V
C)
OIB-1
OIB
E-MORB
1
Rock / Primitive Mantle
Rock / Primitive Mantle
1000
Finlayson Lake - Non-Arc Rocks
(Fire Lake and Cleaver Lake formations)
100
L-IAT
IAT
BON
.1
B)
Finlayson Lake - Arc Rocks
(Waters Creek and Cleaver Lake formations)
Rock / Primitive Mantle
1000
Rock / Primitive Mantle
Figure 11.
Caption on
facing page.
Th La Pr Sm Hf Ti Tb Y Yb Al
V
Nb Ce Nd Zr Eu Gd Dy Er Lu Sc
1
1
10
100
1000
Y
295
Piercey et al .
13
6°
W
353 355
357
RIFT
MORB
W
8°
14
Tournaisian
F - Viséan
SYMBOLS
347 356
Volcanism
Massive sulphide
deposit
ARC
356
Wolv
355
Blueschist / eclogite
occurrence
Ak
BAB
345
Fossil
F
63
°N
342 357
350 357
Denali
Plutonism
345 350
352
353
346
ARC
344 347
356
YT
346 349
343
YT
B.
C.
350
W
0°
14
347
355
Pacific
348
?
350
12
4°W
YT
B.
C.
ARC
354
356
340 350
56
°N
ARC
57°
N
MORB
F
345
Tintina
Ocean
296
(Tournaisian)
357-342 Ma
351
°N
59
?
Famennian Tournaisian
12
8°
W
355
357
354
Ak
B.
C.
The third magmatic cycle corresponds to the climax of felsic magmatism in YTT (Fig. 6). It begins at a marked angular unconformity
at the base of the Wolverine Lake group in the Finlayson Lake district
(Murphy et al., this volume; Colpron et al., this volume-a), and is
therefore named after this succession. The base of the Wolverine
Lake group comprises quartz- and feldspar-pebble conglomerate,
grit and sandstone derived from the underlying Late Devonian –
Early Mississippian volcanic and plutonic rocks (Murphy et al., this
volume). This unconformity represents a hiatus of approximately
3-4 m.y. The Wolverine massive sulphide deposit formed during this
cycle. This cycle ends with another deformational event, best defined
in the Glenlyon area of central Yukon (Colpron et al., this volume-b),
but also inferred in many parts of YTT (Colpron et al., this volumea). This deformational event is recorded by fabric development in
~347-343 Ma volcanic and plutonic rocks that are intruded by an undeformed ca. 340 Ma pluton, and by foliated quartzite clasts in basal
conglomerate of the ca. 340 Ma and younger Little Salmon formation
(Colpron et al., this volume-b).
Cycle III magmatism was of arc, arc-rift and back-arc types.
Arc magmatism occurred in the western Finlayson Lake district,
Stewart River, Fortymile River, Glenlyon, Teslin and Wolf LakeJennings River areas (Fig. 13). Magmatism of probable arc affinity
is also documented in the Stikine assemblage of northwestern British
Columbia (Logan et al., 2000; Gunning et al., this volume) and the
Endicott Arm assemblage of southeastern Alaska (Fig. 13;
McClelland et al., 1991; Gehrels, 2001). Back-arc magmatic activity
was restricted to the eastern Finlayson Lake area, in the footwall of
the Money Creek thrust (Fig. 13; Piercey et al., 2001b, 2002b,
2003).
Arc magmatism in the Finlayson Lake district is manifested
primarily by hornblende- and biotite-bearing granitoids of the
Simpson Range plutonic suite. They range in age from ~357-345 Ma,
but are mostly in the range of ~350-345 Ma (Mor tensen,
1992a; J.K. Mortensen and D.C. Murphy, unpublished data). The
UCC-normalized plots for Simpson Range granitoids show slight
LREE-enrichment, weakly negative Nb anomalies (Fig. 14A) and
low Y, consistent with an arc affinity (Fig. 15A; Piercey et al., 2003;
Grant, 1997). These patterns are also consistent with derivation from
upper crustal sources. This is further supported by their Nd isotopic
signatures (εNdt = -7.4 to -12.9; Grant, 1997; Piercey et al., 2003),
upper crust-like Pb-isotope systematics (Grant, 1997) and inherited
Proterozoic (±Archean) zircon (Mortensen, 1992a, b; Grant, 1997).
Cycle III back-arc magmatic rocks occur in the Wolverine Lake
group in the eastern Finlayson Lake district (Fig. 13). Wolverine
N
Wolverine Cycle (Cycle III - Early Mississippian 357-342 Ma)
Cycle III - Wolverine
Early Mississippian
64°
regions. Furthermore, the spatial, temporal and geochemical similarities to rocks of the Fire Lake, Cleaver Lake and Waters Creek
formations in the Finlayson Lake district suggest that the Stewart
River area represents a continuation of the arc front recorded in the
Cleaver Lake and Waters Creek formations, and initial back-arc basin
development recorded in the Finlayson Lake district during Cycle II
(Fig. 10).
0
Scale
300
km
Figure 13. Distribution of tectonic and magmatic events during the
Early Mississippian Wolverine cycle (Cycle III: 357-342 Ma). Events
are located on a base map of Yukon-Tanana and related terranes
prior to ~430 km of dextral displacement along Tintina fault. Dashed
red line marks approximate limit between geodynamic environments.
Dashed blue line represents relative position of subduction zone;
teeth indicate dip of subducting slab. See Figure 1 for tectonic assemblage legend.
Paleozoic magmatism and crustal recycling
10
Finlayson Lake - Arc Felsic Rocks
(Simpson Range plutonic suite)
1
SRPS-P
SRPS-G
.1
10
Rock / Upper Continental Crust
10
Glenlyon area - Arc Felsic/Intermediate Rocks
(Little Kalzas)
1
LK-And
LK-Rhy
LKS-Gr
Th La Pr Sm Hf Ti Tb Y Yb Al
V
Nb Ce Nd Zr Eu Gd Dy Er Lu Sc
G)
Rock/Upper Continental Crust
Rock / Upper Continental Crust
Eastern Alaska: Arc Felsic Orthogneiss
(Fortymile River assemblage)
1
Calc-Alkalic (>70% SiO2)
Transitional (~64% SiO2)
Th La Pr Sm Hf Ti Tb Y Yb Al
V
Nb Ce Nd Zr Eu Gd Dy Er Lu Sc
10
E)
10
Th La Pr Sm Hf Ti Tb Y Yb Al
V
Nb Ce Nd Zr Eu Gd Dy Er Lu Sc
D)
.1
Th La Pr Sm Hf Ti Tb Y Yb Al
V
Nb Ce Nd Zr Eu Gd Dy Er Lu Sc
Teslin: Arc Felsic Rocks
(Simpson Range plutonic suite)
1
Th La Pr Sm Hf Ti Tb Y Yb Al
V
Nb Ce Nd Zr Eu Gd Dy Er Lu Sc
Rock / Upper Continental Crust
Rock / Upper Continental Crust
Stewart River - Arc Felsic Rocks
(tonalitic-dioritic orthogneiss)
1
.1
WV-5f
WV-6f-fw
WV-6f-hw
10
C)
.1
Finlayson Lake: Non-Arc Felsic Rocks
(Wolverine Lake group)
1
.1
Th La Pr Sm Hf Ti Tb Y Yb Al
V
Nb Ce Nd Zr Eu Gd Dy Er Lu Sc
10
.1
B)
Rock / Upper Continental Crust
Rock / Upper Continental Crust
A)
F)
Wolf Lake / Jennings River: Arc Felsic Rocks
(Ram Creek intrusions, upper Dorsey complex rhyolites)
1
.1
Upper Dorsey - rhyolites
Ram Creek - tonalites
Th La Pr Sm Hf Ti Tb Y Yb Al
V
Nb Ce Nd Zr Eu Gd Dy Er Lu Sc
Figure 14. Upper continental crust (UCC) normalized plots for Cycle III felsic
rocks. (A) Plutonic rocks from the Simpson Range plutonic suite (SRPS), Finlayson
Lake district (SRPS-P from Piercey et al., 2003; SRPS-G from Grant, 1997);
(B) Non-arc rocks from the Finlayson Lake district (WV-5f - Wolverine Lake group
felsic volcanic rocks; WV-6f-FW - Wolverine Lake group - felsic volcanic rocks
in footwall of Wolverine deposit; WV-6f-HW - Wolverine Lake group - aphyric
rhyolites in hanging wall of Wolverine deposit; data from Piercey et al., 2001b);
(C) Tonalitic-dioritic orthogneiss from Stewart River area (S. Piercey and J. Ryan,
unpublished data); (D) Orthogneiss from the Fortymile River area (Dusel-Bacon
et al., this volume); (E) Felsic plutonic and volcanic rocks from the Glenlyon region
(LK-And = Little Kalzas andesite; LK-Rhy - Little Kalzas rhyolite; LKS-Gr - Little
Kalzas suite granite; data from Colpron, 2001); (F) Felsic plutonic and volcanic
rocks from the Wolf Lake-Jennings River area (data from Nelson and Friedman,
2004); and (G) Simpson Range plutonic suite equivalents from the Teslin area
(data from Stevens et al., 1995).
297
Piercey et al .
A)
1000
within-plate
(A-type)
syncollisional
(S-type)
100
Nb
&
e
late ridg
n-p ean
i
h
t
c
wi u s o
lo
ma
o
n
a
volcanic arc
(I-type)
10
B)
1000
Nb
an
volcanic arc
(I-type)
10
ocean-ridge
(OR-type)
M-type
1
1
within plate
(A-type)
syncollisional
(S-type)
100
ocean ridge
(OR-type)
M-type
10
100
1
1000
1
10
100
C)
100
Nb
D)
1000
within-plate
(A-type)
syncollisional
(S-type)
a
10
&
e
ate n ridg
-pl
hin ocea
t
i
w us
lo
ma
no
100
Nb
a
10
volcanic arc
(I-type)
1
within-plate
(A-type)
syncollisional
(S-type)
volcanic arc
(I-type)
ocean ridge
(OR-type)
M-type
1
10
100
1000
1000
&
ge
late rid
n p cean
i
h
t
wi us o
alo
om
within plate
(A-type)
(S-type)
Nb
1
10
100
volcanic arc
(I-type)
Nb
1
a
volcanic arc
(I-type)
10
&
e
te
dg
pla n ri
hin ocea
t
i
w us
lo
ma
no
within plate
(A-type)
syncollisional
(S-type)
100
ocean ridge
(OR-type)
ocean ridge
(OR-type)
M-type
M-type
1
1000
F)
an
10
ocean ridge
(OR-type)
Y
E)
100 syncollisional
&
e
ate n ridg
-pl
hin ocea
t
i
w us
lo
ma
no
M-type
1
Y
1000
1000
Y
Y
1000
&
e
te
dg
pla n ri
hin ocea
t
i
w us
alo
om
10
100
1000
1
1
10
100
1000
Y
Y
G)
1000
&
e
te
dg
pla n ri
hin ocea
t
i
w us
alo
om
within plate
(A-type)
syncollisional
(S-type)
100
Nb
an
volcanic arc
(I-type)
10
ocean ridge
(OR-type)
M-type
1
1
10
100
Y
298
1000
Figure 15. Nb-Y discrimination plots for Cycle III felsic rocks. Data
sources and symbology as in Figure 14.
Paleozoic magmatism and crustal recycling
Lake group strata below the Wolverine VHMS deposit contain felsic
tuffs and flows. Above the deposit, silicified, aphyric rhyolites are
overlain by massive mafic lava flows (Murphy and Piercey, 1999;
Piercey et al., 2001b, c, 2002b). The felsic rocks in the footwall of
the Wolverine deposit are characterized by HFSE- and REE-enriched
signatures, which are virtually identical to the rocks of the Kudz Ze
Kayah formation, and are consistent with formation within an ensialic back-arc basin environment (Figs. 14B, 15B; Piercey et al.,
2001b, 2002b). Above the Wolverine deposit, the rhyolites become
less HFSE- and REE-enriched exhibiting more arc-like patterns and
Nb-Y contents (Figs. 14B, 15B; Piercey et al., 2001b, 2002b). The
position of these thin, silicified, aphyric rhyolite flows, above non-arc
felsic rocks and below MORB-type basalts (see below), would require
an unacceptably transient arc location. The relatively depleted, arclike signatures of the aphyric rhyolites could result from silicification,
which could have diluted their trace element abundances (Fig. 15);
however, this would not appreciably change the trace element profiles
of these rocks. Alternatively, Piercey et al. (2001b) suggested that
trace element depletions in the aphyric rhyolites may be due either
to mixing of HFSE- and REE-depleted mafic magmas with continental crust, or to lower temperature melting of continental crust.
In either case, a continental crustal influence is evident in both the
hanging wall and footwall felsic rocks of the Wolverine deposit, as
both have distinctly negative εNdt values (εNdt = -7.1 to -8.2; Piercey
et al., 2003).
Capping the entire felsic back-arc sequence in the Wolverine
Lake group are massive basalts (Murphy et al., this volume). They
have MORB-like signatures that have been derived from a depleted
to weakly enriched mantle source with a weak subduction signature
(Fig. 16A; Piercey et al., 2002b); a single Wolverine basalt sample
yielded εNdt = +6.9, consistent with derivation from depleted mantle
sources (Piercey, 2001). The presence of N-MORB to E-MORB like
signatures in the Wolverine basalts suggests that they were derived
by melting in response to crustal thinning during back-arc basin
formation and seafloor spreading (Piercey et al., 2002b).
In the Stewart River area, Cycle III magmatism is dominated
by an arc-like assemblage of tonalitic-dioritic-granodioritic orthogneiss, which intrudes a lowermost pre-Late Devonian metasedimentary package and Late Devonian amphibolites (Ryan and Gordey,
2001, 2002; Ryan et al., 2003). The orthogneissic rocks have preliminary U-Pb ages (Villeneuve et al., 2003) and petrological attributes that are similar to the Simpson Range plutonic suite in the
Finlayson Lake district. Preliminary geochemical data for the tonalitic orthogneiss show relatively flat UCC-normalized patterns with
flat REE but weak positive anomalies in Nb, Eu, Ti and Al, and arclike Nb-Y systematics (Figs. 14C, 15C; S. Piercey and J. Ryan, unpublished data). There are no Sm-Nd data for felsic rocks of the
Stewart River area, but their higher Ti, Nb and Al relative to UCC
suggest they may have been derived from a source more depleted
than UCC, or as a result of mantle-UCC mixing (Fig. 14C).
Nevertheless, samples of the orthogneiss in the Stewart River area
contain zircons with Proterozoic inheritance (Mortensen, 1990;
Villeneuve et al., 2003), and have flat REE relative to UCC (Fig. 14C),
suggesting that a component of continental material was involved
in their genesis.
In the eastern Yukon-Tanana Upland, just northwest of the
Stewart River area, Cycle III magmatism is represented by
~357-341 Ma (J. Mortensen and C. Dusel-Bacon, unpublished data;
Day et al., 2002) mafic amphibolites (gabbro/diorite?), intermediatecomposition quartz diorite/tonalite orthogneiss, and subordinate
coeval granodiorite and felsic metatuff of the Fortymile River assemblage. These igneous rocks are interlayered with metasedimentary rocks (Dusel-Bacon et al., this volume, and references therein).
Amphibolitic rocks have trace element signatures that have LREEenriched island arc and, to a lesser extent, MORB affinities (Fig. 16B;
Dusel-Bacon and Cooper, 1999; Day et al., 2002). Plutonic orthogneiss samples are subdivided into those with intermediate compositions (~64% SiO2) and more felsic affinities (>70% SiO2). On UCCnormalized diagrams, the lower SiO2 group are depleted in LREE
and Th relative to UCC and enriched in Al, Sc and V (Fig. 14D)
suggesting derivation from sources more primitive than UCC. In
addition, these samples likely have an affinity transitional between
tholeiitic and calc-alkaline (see Barrett and MacLean, 1999) and
were most likely intruded within an arc setting (Fig. 15D). The higher
SiO2 group have calc-alkalic patterns with flat UCC normalized
patterns and minor depletions in Nb, Sc and V, consistent with derivation from UCC-like sources within an arc setting (Figs. 14D, 15D).
Derivation from UCC-like sources is supported by Precambrian
inheritance in some zircons from this region (J. Mortensen and
C. Dusel-Bacon, unpublished data), as is the case with nearby, and
likely related, magmatic rocks of the Stewart River area. The occurrence of several amphibolites with MORB characteristics may be
evidence for local intra-arc or back-arc magmatism.
Cycle III magmatism in the Glenlyon area is represented by
volcanic and sedimentary rocks of the 347-344 Ma Little Kalzas
formation and coeval granitoids of the Little Kalzas plutonic suite
(Colpron et al., this volume-b). Volcanic rocks in the Little Kalzas
formation consist of andesites with lesser rhyolites and basalts,
interbedded with epiclastic and basinal sedimentary strata (Colpron,
2001; Colpron et al., this volume-b). Granitoids of the Little Kalzas
suite are metaluminous, in part K-feldspar megacrystic, diorite and
tonalite, and resemble the Simpson Range plutonic suite (Colpron,
2001).
Little Kalzas formation andesites have relatively flat REE relative to the UCC but have lower Th, and higher Eu, Ti, Al and V relative to UCC (Fig. 14E), yet have Zr/Y ratios (~4-9) that are transitional to calc-alkalic in affinity (Barrett and MacLean, 1999). These
features suggest possible derivation via mixing between mafic
(i.e., Eu, Ti, V-enriched) material and continental crust, within a
continental arc environment. Little Kalzas rhyolites have flat UCCnormalized patterns consistent with derivation from a UCC-like
source (Fig. 14E). Depletions in Ti, V and to a lesser extent Nb
(Figs. 14E, 15E) are consistent with oxide fractionation at high levels
in the crust (cf. Lentz, 1998; Piercey et al., 2001b). Plutonic rocks of
the Little Kalzas suite have a similar, but less erratic UCC-normalized pattern, likely reflecting derivation from continental crust-mantle mixing within a continental arc setting (Fig. 14E). Both intrusive
and extrusive rocks have relative low Nb and Y, and plot within the
volcanic arc field in Nb-Y space (Fig. 15E). Tracer isotopic data are
not available for the Little Kalzas formation and Little Kalzas suite
299
Piercey et al .
rocks; however, most U-Pb geochronological samples from this region have inherited Proterozoic-Archean zircon (Colpron et al., this
volume-b), suggesting that there is a significant older crustal component in these rocks. Mafic rocks associated with the Little Kalzas
formation, and the underlying Pelmac formation, have smooth PMnormalized patterns with OIB-like signatures consistent with derivation from enriched mantle sources (Fig. 16C); no isotopic data are
presently available for these rocks. These mafic rocks likely reflect
rifting of the Little Kalzas arc (Colpron, 2001), which may have been
related to coeval arc-rifting and ensialic back-arc magmatic activity
in the Finlayson Lake district (Piercey et al., 2001b, 2002b).
In the Wolf Lake – Jennings River area, Cycle III is represented
by intrusive rocks structurally associated with, and possibly basement to the Ram Creek Complex, metavolcanic rocks in the upper
1000
100
10
1
.1
B)
Rock / Primitive Mantle
Rock / Primitive Mantle
A)
1000
Finlayson Lake: Non-Arc Mafic Rocks
(Wolverine basalts)
10
L-IAT?
1
.1
Rock / Primitive Mantle
10
1
Wolf Lake / Jennings River: Arc Mafic Rocks
(upper Dorsey Complex; Swift River Group)
100
10
Swift River - L-IAT/CAB
1
Upper Dorsey - L-IAT/CAB
.1
Th La Pr Sm Hf Ti Tb Y Yb Al Sc
Nb Ce Nd Zr Eu Gd Dy Er Lu V
Th La Pr Sm Hf Ti Tb Y Yb Al Sc
Nb Ce Nd Zr Eu Gd Dy Er Lu V
1000
1000
E) Wolf Lake / Jennings River: Non-Arc Mafic Rocks
F)
100
10
OIB
Rock / Primitive Mantle
(Swift River Group)
Rock / Primitive Mantle
Th La Pr Sm Hf Ti Tb Y Yb Al Sc
Nb Ce Nd Zr Eu Gd Dy Er Lu V
D)
Glenlyon Region: Arc-Rift Mafic Rocks
(Pelmac and Little Kalzas formations)
100
1
MORB
1000
C)
Rock / Primitive Mantle
100
Th La Pr Sm Hf Ti Tb Y Yb Al Sc
Nb Ce Nd Zr Eu Gd Dy Er Lu V
1000
.1
Eastern Alaska: Arc and Non-Arc Mafic Rocks
(Fortymile River assemblage)
Teslin Region: Arc Mafic Rocks
100
10
1
N-MORB
.1
Th La Pr Sm Hf Ti Tb Y Yb Al Sc
Nb Ce Nd Zr Eu Gd Dy Er Lu V
.1
Th La Pr Sm Hf Ti Tb Y Yb Al Sc
Nb Ce Nd Zr Eu Gd Dy Er Lu V
Figure 16. Primitive mantle normalized plots for Cycle III mafic rocks. (A) Mafic rocks from the uppermost part of the Wolverine Lake
group (data from Piercey et al., 2002b); (B) Amphibolites from the Fortymile River assemblage (Dusel-Bacon and Cooper, 1999); (C) Mafic
rocks from the Glenlyon region (data from Colpron, 2001); (D) Arc and (E) non-arc mafic rocks from Wolf Lake – Jennings river area (data
from Nelson and Friedman, 2004); (F) mafic rocks from the Teslin region (data from Creaser et al., 1997).
300
Paleozoic magmatism and crustal recycling
Dorsey Complex, and minor mafic-intermediate rocks in the Swift
River Group (Roots et al., this volume; Nelson, 1999, 2000; Roots
and Heaman, 2001). The Dorsey Complex, which structurally overlies the Ram Creek complex, consists of a lower unit of quartzofeldspathic metasedimentary rocks and metabasites (basalt/gabbro
flows/sills), and an upper unit of siliciclastic rocks, marble and minor
felsic tuffs (Roots et al., this volume; Nelson and Friedman, 2004).
Early Mississippian ages have been obtained for felsic metavolcanic
rocks from the upper Dorsey Complex (Roots and Heaman, 2001).
The Swift River Group lies in structurally modified depositional
contact with Early Mississippian rocks of the Dorsey Complex, and
is overlain by the mid-Mississippian (Viséan) Screw Creek Limestone
(Nelson, 2000; Roots et al., 2000). The Swift River Group consists
predominantly of basinal sedimentary rocks, with minor volcaniclastic debris and tuffaceous rocks (Nelson, 2000; Roots et al.,
2000).
Early Mississippian tonalitic intrusions associated with the
Ram Creek Complex have relatively flat UCC-normalized patterns
but lower Th, and higher Ti, Al and V relative to UCC (Fig. 14F).
These features are consistent with potential derivation from a more
mafic source, or from crust-mantle mixing within an arc environment
(Figs. 14F, 15F; Nelson and Friedman, 2004). Early Mississippian
UCC-normalized patterns for coeval rhyolitic metavolcanic rocks
from the upper Dorsey Complex have UCC-normalized patterns,
with relatively flat REE but with elevated Th, Zr and Hf relative to
UCC, and Nb-Y systematics that are transitional between arc and
non-arc (Figs. 14F, 15F). One sample of mafic rock from the Upper
Dorsey complex has an L-IAT to CAB affinity (Fig. 16D). Mafic
tuffs from the Swift River Group are dominated by L-IAT to CAB
signatures consistent with derivation from variably enriched mantle
sources coupled with a subduction zone component with or without
minor crustal contamination. In addition, two samples have non-arc
affinities with N-MORB and OIB signatures (Fig. 16E; Nelson and
Friedman, 2004). Collectively, these geochemical data are consistent
with arc magmatism that was interrupted by intra-arc rifting.
In the Teslin area, Cycle III magmatism is represented by
Simpson Range plutonic suite equivalents (Stevens et al., 1995) and
possibly by mafic metavolcanic rocks (unit PMgr of Stevens, 1994;
“Anvil assemblage” of Creaser et al., 1997). The mafic rocks have
uncertain age and stratigraphic relationships and are considered to
be part of Cycle III due to their spatial association with intrusions
of that age; however, they could be part of any cycle in YTT. The
Simpson Range plutonic suite equivalents in the Teslin region consist
of variably deformed hornblende-bearing tonalite to quartz-diorite
(Figs. 14G, 15G; Stevens et al., 1995). These granitoids have very
erratic signatures that are broadly flat with positive Eu, Ti and Al
anomalies, low La and Th relative to Nb (Fig. 14G) and low Nb-Y
(Fig. 15G) that are consistent with formation within an arc environment. Their relative LREE-depletions and high Nb, Eu, Ti and Al
(Fig. 14G) suggest derivation from a more primitive source than the
UCC; however, εNdt values for these Simpson Range plutonic suite
equivalents (εNdt = -2.5 to -6.2; Stevens et al., 1995) clearly point
to an ancient crustal component. Mafic rocks from the Teslin region
have been interpreted to reflect calc-alkaline basaltic protoliths
(Creaser et al., 1997); however, a re-evaluation of the geochemical
data suggest that these rocks more closely resemble E-MORB with
a weak subducted slab component (Fig. 16F). Their signatures are
consistent with derivation from moderately enriched sources during
arc development or arc-rifting, as they are transitional between arc
and back-arc magmatism. Furthermore, the Nd-isotopic data for the
greenstones of the Teslin area are consistent with derivation from a
moderately enriched mantle source, as they exhibit εNdt = +4.1 to
+6.4 (Creaser et al., 1997), which is less than the depleted mantle at
350 Ma (εNdt = +9.5; Goldstein et al., 1984).
Little Salmon Cycle (Cycle IV - Late Mississippian 342-314 Ma)
The fourth magmatic cycle is mostly represented in the southern
part of YTT (Fig. 17). It begins at an unconformity beneath the Little
Salmon formation of central Yukon and is thus named after this
succession. The sub-Little Salmon unconformity is locally marked
by a basal conglomerate containing foliated quartzite clasts and
Early Mississippian detrital zircons (Colpron et al., this volume-b).
Cycle IV magmatism was dominated by arc, and lesser intra-arc rift
magmatism; no corresponding back-arc magmatism has yet been
identified for this cycle. It is represented by the Little Salmon formation and associated plutons in the Glenlyon area (Colpron et al., this
volume-b), the Ram Creek and Big Salmon complexes in the Wolf
Lake-Jennings River area (Roots et al., this volume; Nelson and
Friedman, 2004), and bimodal magmatism in Stikine terrane (Sebert
and Barrett, 1996; Logan et al., 2000; Gunning et al., this volume;
Fig. 17). The Tulsequah Chief VMS deposit in northwestern British
Columbia formed during this cycle (Fig. 17).
Coeval successions of YTT in the Finlayson Lake district and
parts of Teslin and Wolf Lake areas are characterized by deposition
of carbonates in the arc marginal regions. Basinal sedimentary rocks
are predominant in back-arc areas rather than igneous rocks (Fig. 17;
Slide Mountain/Seventymile assemblages; Nelson, 1993; Murphy
et al., this volume; Dusel-Bacon et al., this volume).
In the Glenlyon area, volcanic rocks of the Little Salmon formation consist of basalt, andesite, and lesser dacite, tuff and quartzfeldspar porphyry, interbedded with volcano-sedimentary rocks
(Colpron, 2001). Plutons of the Tatlmain suite are coeval and cogenetic with them. Little Salmon andesites and volcaniclastic rocks
have LREE-depleted UCC-normalized patterns with higher Eu, Ti,
V and HREE relative to UCC (Fig. 18A). These features are consistent with derivation from a source more depleted than UCC, or with
mixing between mantle-derived magma and crustal material. In
contrast, the dacites and quartz-feldspar porphyries from the Little
Salmon formation have broadly flat UCC-normalized patterns with
relatively weak depletions in Nb and V, and higher LREE and total
REE (Fig. 18A), all of which are consistent with derivation from
melting of continental crust accompanied by high-level oxide fractionation. The Tatlmain suite plutons have fairly flat, slightly LREEdepleted signatures (Fig. 18A), reflecting derivation from mixed
mantle-crust sources. All of the felsic to intermediate rocks of the
Little Salmon formation and the Tatlmain suite intrusions have low
Nb-Y, typical of arc settings (Fig. 19A). Mafic rocks from the Little
301
Piercey et al .
64°
Cycle IV - Little Salmon
Late Mississippian
N
13
6°
W
(Viséan - Serpukhovian)
342-314 Ma
Tournaisian
W
8°
14
F - Viséan
SYMBOLS
F
342
Viséan - 63°
SerpukhovianN
MORB
Volcanism
F
Viséan F
Asselian
Denali
Plutonism
Fossil
Serpukhovian
F
Compressional
deformation
Ak
342
ARC
340
°
59
YT
N
340 344
Viséan Moscovian
Paci fic
Ocean
327
YT
B.
C. ARC
?
F
335
Viséan Serpukhovian
325
322
334
340
56
°N
Tintina
Tulsequah
RIFT
332
323
335
338 340
12
4°W
W
0°
14
F
YT
B.
C.
57°
N
Viséan -
F Serpukhovian
MORB
F
Serpukhovian
- Bashkirian
340
Ak
B.
C.
12
8
F
°W
?
Viséan Gzhelian
0
Scale
300
km
Figure 17. Distribution of tectonic and magmatic events during the
Late Mississippian Little Salmon cycle (Cycle IV: 342-314 Ma). Events
are located on a base map of Yukon-Tanana and related terranes
prior to ~430 km of dextral displacement along Tintina fault. Dashed
red line marks approximate limit between geodynamic environments.
Dashed blue line represents relative position of subduction zone;
teeth indicate dip of subducting slab. See Figure 1 for tectonic assemblage legend.
302
Salmon formation are dominated by OIB-like alkalic signatures,
which indicate derivation from enriched mantle source regions
(Fig. 20A; Colpron, 2001). This geochemical signature likely reflects
intra-arc rifting episodes, which are also indicated by the presence
of spatially associated Mn-rich exhalite horizons (Colpron, 2001).
Only limited geochemical data are available for mafic volcanic
rocks of the Big Salmon Complex (M. Mihalynuk, unpublished data;
Nelson and Friedman, 2004). In the Wolf Lake-Jennings River area,
the Big Salmon Complex contains a stratigraphic succession of
greenstone overlain by marble, Mn-rich siliceous exhalite and siliciclastic sedimentary rocks with minor felsic tuff (Mihalynuk et al.,
1998, 2000, this volume). Mafic volcanic rocks from the Big Salmon
Complex are dominated by LREE-enriched signatures with negative
Nb anomalies, typical of L-IAT suite magmas (Fig. 20B); andesite
with a calc-alkaline signature is locally present (Fig. 20B). The Big
Salmon Complex also contains a few samples that exhibit E-MORB
to OIB affinities with positive Nb anomalies (Fig. 20B). These nonarc magmas probably reflect intra-arc rift events. As in the Little
Salmon formation, the presence of Mn-rich exhalative horizons,
interpreted to have formed during volcanic hiatuses, within the Big
Salmon Complex also supports this interpretation (Mihalynuk and
Peter, 2001).
The Late Mississippian Ram Creek Complex in the Wolf LakeJennings River area comprises intermediate to felsic tuff, interbedded with basinal and siliciclastic sedimentary rocks and rare marble
(Roots et al., this volume; Nelson and Friedman, 2004). Quartzsericite schist from the Ram Creek Complex exhibits a calc-alkalic
arc affinity (Figs. 18B, 19B), consistent with formation within an
arc environment.
The Tulsequah Chief VMS deposit in northwestern British
Columbia provides the most comprehensive geochemical dataset for
the Stikine portion of Cycle IV magmatism. The Tulsequah Chief
deposit is hosted by a Mississippian (327 ± 1 Ma and 330 +10/-1 Ma,
U-Pb zircon on rhyolites; Childe, 1997) bimodal assemblage of basaltic and rhyolitic volcanic and volcaniclastic rocks (Sebert and
Barrett, 1996). The footwall succession to the Tulsequah Chief deposit is bimodal: calc-alkalic arc rhyolitic rocks are interbedded with
basalts that display E-MORB signatures, and in some cases include
a subordinate subduction zone component (Figs. 18C, 19C); hanging
wall basaltic rocks exhibit similar signatures (Fig. 20C). Collectively,
these rocks probably represent derivation from a weakly enriched
mantle source, coupled with crustal melting to form a bimodal volcanic assemblage in response to arc rifting (e.g., Sebert and Barrett,
1996).
Klinkit Cycle (Cycle V - Pennsylvanian to Early
Permian - 314-269 Ma)
The fifth cycle is primarily represented by arc magmatism of the
Klinkit Group and Fourmile succession in southern Yukon and
northern British Columbia (Roots et al., 2002, this volume; Simard
et al., 2003; Nelson and Friedman, 2004; Fig. 21) and the Lay Range
assemblage of central British Columbia (Fig. 1; Ferri, 1997). Arc
magmatism is also inferred in the Semenof Hills of south-central
Yukon (Fig. 1; Tempelman-Kluit, 1984; Simard and Devine, 2003;
Paleozoic magmatism and crustal recycling
A)
10
Glenlyon - Arc Intermediate/Felsic Rocks
(Tatlmain suite)
1
LS - And
LSS-Gr
LS - QFP
Tatlmain
LS - Dac
.1
C)
Th La Pr Sm Hf Ti Tb Y Yb Al
V
Nb Ce Nd Zr Eu Gd Dy Er Lu Sc
Nb
within-plate
(A-type)
te & dge
-pla n ri
hin ocea
t
i
w us
lo
ma
ano
volcanic arc
(I-type)
10
B)
1000
syncollisional
(S-type)
100
Th La Pr Sm Hf Ti Tb Y Yb Al
V
Nb Ce Nd Zr Eu Gd Dy Er Lu Sc
Figure 18. Upper continental crust (UCC) normalized plots for
Cycle IV felsic rocks. (A) Tatlmain suite arc felsic rocks from the
Glenlyon region (data from Colpron, 2001); (B) Arc felsic rocks from
the Wolf Lake – Jennings River area (data from Nelson and Friedman,
2004); and (C) Arc felsic rocks from the footwall of the Tulsequah
Chief VMS deposit, Stikine terrane (data from Sebert and Barrett,
1996).
A)
1000
ocean-ridge
(OR-type)
1
Nb
a
volcanic arc
(I-type)
10
te & dge
-pla an ri
e
hin
wit us oc
lo
a
nom
ocean-ridge
(OR-type)
M-type
10
100
1000
Y
1
1
10
100
1000
Y
C)
1000
within-plate
(A-type)
syncollisional
(S-type)
100
Nb
te & dge
-pla n ri
hin ocea
t
i
w us
lo
ma
o
n
a
volcanic arc
(I-type)
10
ocean-ridge
(OR-type)
Figure 19. Nb-Y discrimination plots for Cycle IV felsic rocks. Data
sources and symbology as in Figure 18.
M-type
1
within-plate
(A-type)
syncollisional
(S-type)
100
M-type
1
Ram Creek Complex - Arc Felsic
(Quartz-sericite schist)
Tulsequah Chief: Arc Felsic Rocks
(footwall rhyolites)
1
.1
B)
1
.1
Th La Pr Sm Hf Ti Tb Y Yb Al
V
Nb Ce Nd Zr Eu Gd Dy Er Lu Sc
10
Rock / Upper Continental Crust
Rock / Upper Continental Crust
Rock / Upper Continental Crust
10
1
10
100
1000
Y
303
Piercey et al .
1000
1000
Glenlyon: Non-Arc Basalt
(Little Salmon formation)
100
10
1
.1
B)
Rock / Primitive Mantle
Rock / Primitive Mantle
A)
Th La Pr Sm Hf Ti Tb Y Yb Al Sc
Nb Ce Nd Zr Eu Gd Dy Er Lu V
Wolf-Jennings: Arc/Non-Arc Rocks
(Big Salmon Complex)
100
10
1
.1
Avg. L-IAT
CAA
Avg. E-MORB/OIB
Th La Pr Sm Hf Ti Tb Y Yb Al Sc
Nb Ce Nd Zr Eu Gd Dy Er Lu V
1000
Rock / Primitive Mantle
C)
Tulsequah Chief: Non-Arc Rocks
(footwall and hanging-wall basalts)
100
10
1
.1
FW-E-MORB
FW-SZ-E-MORB
HW-SZ-E-MORB
Th La Pr Sm Hf Ti Tb Y Yb Al Sc
Nb Ce Nd Zr Eu Gd Dy Er Lu V
M. Colpron and J.K. Mortensen, unpublished data; Figs. 1, 21). This
cycle corresponds to a lull in felsic magmatism in YTT (Fig. 6) and
important deposition of carbonate and basinal sedimentary rocks in
the Finlayson Lake district and Stikine terrane (Colpron et al., this
volume-a).
Coeval, back-arc magmatism is present in the Campbell Range
formation of the Finlayson Lake district (Murphy et al., this volume;
Plint and Gordon 1997; Piercey et al., 1999) and Slide Mountain
terrane in the Sylvester allochthon and Nina Creek Group (Nelson,
1993; Ferri, 1997; Figs. 1, 21). The beginning of this cycle is arbitrarily set at the Mississippian-Pennsylvanian boundary, a time marking
the end of felsic magmatism, and the maximum preservation of fossil
faunas in sedimentary strata of YTT (Fig. 6; Colpron et al., this
volume-a).
The Klinkit Group outcrops discontinuously from northern
British Columbia into southern Yukon, a strike length of over 250 km
(Figs. 1, 21). It is characterized by the predominance of Pennsylvanian
to Permian volcaniclastic rocks overlying carbonates of Viséan to
Bashkirian age (Roots et al., this volume). The Klinkit Group unconformably overlies the rocks of the Swift River Group (Nelson,
2002; Roots et al., this volume). Within the Klinkit Group, the volcaniclastic members of the Butsih and Mount McCleary formations
are composed of crystal and lithic tuff. They are mainly basaltic in
composition, with calc-alkaline arc PM-normalized signatures
304
Figure 20. Primitive mantle normalized plots for Cycle IV mafic
rocks. (A) Non-arc basaltic rocks from the Glenlyon region (data
from Colpron, 2001); (B) Arc and non-arc rocks from the Wolf Lake
– Jennings River area (data from Nelson and Friedman, 2004; and
M. Mihalynuk, unpublished data); (C) Non-arc basaltic rocks from
the Tulsequah Chief VMS deposit, Stikine terrane (data from Sebert
and Barrett, 1996).
(Fig. 22A), but TiO2 and Ti/V ratios akin to modern island-arc tholeiitic basalts (Gamble et al., 1995), suggesting that the rocks have
transitional signatures between tholeiitic and calc-alkaline (Simard
et al., 2003). Their εNdt values (+6.7 to +7.4) and Th/La ratios (0.13
to 0.17) indicate that these rocks were derived from depleted mantle
sources with minimal crustal contamination (Simard et al., 2003).
Based on geochemical and geological evidence, the volcaniclastic
members of the Klinkit Group are interpreted as primitive arc lavas
erupted either through relatively young crust that consists of slightly
older arc basement, or rapidly emplaced through coated conduits
and/or relatively thin continental crust without significant crustal
contamination (Simard et al., 2003). The alkali-basalt member of
the Mount McCleary Formation comprises scarce discontinuous
lenses within the volcaniclastic member (Roots et al., this volume).
These mafic rocks have OIB-like alkalic signatures with high TiO2,
HFSE and LREE (Fig. 22A). They are interpreted to have been derived from an enriched mantle source associated with episodic intraarc rifting of the Klinkit arc (Simard et al., 2003).
In the Sylvester allochthon, Cycle V magmatic rocks occur
within the Fourmile succession. It occurs as thrust panels, in which
undeformed volcanic and epiclastic units, minor limestone, overlie
polydeformed black phyllite, siltstone and argillite (Nelson, 2002;
Nelson and Friedman, 2004). It forms part of Division III, the structural division of the allochthon assigned to the Harper Ranch (late
Paleozoic magmatism and crustal recycling
64°
Cycle V - Klinkit
Pennsylvanian - Early Permian
N
13
6°
W
(Bashkirian - Kungurian)
314-269 Ma
Asselian Sakmarian
F
W
8°
14
F Asselian -
SYMBOLS
Volcanism
Massive sulphide
deposit
N
MORB
Ice
Denali
Plutonism
274
E Permian
Fossil
F
63
°
Sakmarian
F
Giant parafusulina
Bashkirian Kungurian
Compressional
deformation k
A
307
N
°
59
YT
YT
B.
C.
Moscovian
314
F Viséan -
W
0°
14
Bashkirian - F
Moscovian
ARC
Bashkirian -
F Asselian
281
Ocean
56
°N
? 270
Artinskian
- Kazanian F
ARC
EMORB
57°
N
F
ARC
Gzhelian Artinskian
Bashkirian Artinskian
MORB
262 270
?
F
307
Ak
B.
C.
12
8
°W
312
12
4°W
Tintina
Pacific
YT
B.
270 C.
0
Scale
300
km
Figure 21. Distribution of tectonic and magmatic events during the
Pennsylvanian – Early Permian Klinkit cycle (Cycle V: 314-269 Ma).
Events are located on a base map of Yukon-Tanana and related terranes prior to ~430 km of dextral displacement along Tintina fault.
Dashed red line marks approximate limit between geodynamic environments. Dashed blue line represents relative position of subduction zone; teeth indicate dip of subducting slab. See Figure 1 for
tectonic assemblage legend.
Paleozoic arc) subterrane of Quesnellia (Nelson, 1993). Basaltic to
andesitic rocks of the Fourmile succession have predominantly LIAT affinities, consistent with derivation from weakly-enriched
mantle sources within a subduction zone environment (Fig. 22B).
Andesitic to dacitic volcanic rocks of the Fourmile succession have
calc-alkalic arc signatures, slightly depleted relative to the UCC
(Fig. 23).
Pennsylvanian-Early Permian arc magmatism within the Lay
Range assemblage of the Quesnel terrane (Ferri, 1997) is coeval
with, and inferred to be correlative to, the Klinkit Group (Simard
et al., 2003). In the Lay Range, Ferri (1997) has described both L-IAT
(Fig. 22C) and MORB signatures in basaltic rocks from the Upper
Mafic Tuff division (Fig. 22D). These signatures are very similar to
magmatism in the Klinkit Group (Simard et al., 2003) and Fourmile
succession (Nelson, 2002; Fig. 20). Simard et al. (2003) suggested
that by the late Paleozoic, YTT was the basement to the Quesnel arc.
Furthermore, they argued that the Klinkit Group represented distal
turbiditic sedimentation in response to arc magmatic activity within
a large YTT arc system. The presence of MORB-type magmatism,
although minor, in the Lay Range assemblage suggests that the Lay
Range, Klinkit Group and Fourmile succession reflect an arc system
that was subsequently rifted during renewed back-arc extension, as
recorded in the Campbell Range formation of the Finlayson Lake
district (Fig. 21).
In the Campbell Range, Pennsylvanian-Permian massive and
pillowed lava flows, chert and carbonates unconformably overlie
older rocks of the Money Creek formation, the Wolverine Lake and
Grass Lakes groups, and the Fortin Creek group east of the Jules
Creek fault (Murphy et al., 2002, this volume). Diabase, gabbro and
ultramafic rocks intrude both the basalt succession and its basement.
Basalts of the Campbell Range formation have an array of non-arc
signatures, including N-MORB, E-MORB and OIB, representing
derivation from a variably enriched mantle wedge (Fig. 22E;
S. Piercey and D. Murphy, unpublished data). Additional data from
basalts of the Campbell Range formation (Plint and Gordon, 1997;
Piercey et al., 1999), and from eclogitic rocks which are interpreted
to be recycled Campbell Range basalts (Creaser et al., 1999), provide
additional evidence in support of derivation from variably enriched
mantle in a back-arc basin environment. At present, limited isotopic
data exist to elucidate the nature of the crust-mantle interaction in
the Campbell Range formation. Nd isotopic data for the MORB-like
Permian eclogites show εNdt = +5.4 to +9.3, suggesting derivation
from depleted mantle to weakly enriched sources (Creaser et al.,
1999). Furthermore, Pb-isotopic signatures on sulphides from VMS
occurrences hosted by the Campbell Range basalts in the Finlayson
Lake district are non-radiogenic and suggestive of mantle sources
(Mann and Mortensen, 2000). These primitive isotopic signatures,
coupled with the prevalence of N-MORB and E-MORB lavas, suggest that full seafloor spreading occurred in the Campbell Range
back-arc basin.
In north-central British Columbia, magmatism coeval with the
Campbell Range formation is recorded within the Slide Mountain
terrane (Nina Creek Group and Sylvester allochthon; Ferri, 1997;
Nelson, 1993, 2002). Overall, Slide Mountain basalts range from
305
Piercey et al .
1000
1000
A)
B)
10
1
Rock / Primitive Mantle
1000
CAB
CAA
OIB
Lay Range Assemblage: Arc Rocks
(upper mafic tuff)
100
10
1
.1
1000
Finlayson Lake: Non-Arc Rocks
(Campbell Range formation)
10
CRB-OIB
CRB-E-MORB
CRB-N-MORB
Th La Pr Sm Hf Ti Tb Y Yb Al Sc
Nb Ce Nd Zr Eu Gd Dy Er Lu V
D)
Lay Range Assemblage: Non-Arc Mafic Rocks
(upper mafic tuff)
100
10
1
1000
E)
1
1
.1
Th La Pr Sm Hf Ti Tb Y Yb Al Sc
Nb Ce Nd Zr Eu Gd Dy Er Lu V
100
10
1000
C)
Sylvester Allochthon: Arc Mafic Rocks
(Fourmile succession)
100
.1
Th La Pr Sm Hf Ti Tb Y Yb Al Sc
Nb Ce Nd Zr Eu Gd Dy Er Lu V
Rock / Primitive Mantle
.1
Rock / Primitive Mantle
Rock / Primitive Mantle
100
Rock / Primitive Mantle
Rock / Primitive Mantle
Klinkit Group - Arc and Non Arc Mafic Rocks
Th La Pr Sm Hf Ti Tb Y Yb Al Sc
Nb Ce Nd Zr Eu Gd Dy Er Lu V
F)
Nina Creek Group: Non-Arc Mafic Rocks
100
10
Pillow Ridge succession
Mount Howell succession
1
Mafic-Ultramafic unit
Rock / Primitive Mantle
1000
Figure 22.
Caption on
facing page,
306
100
1000
G)
Sylvester Allocthon: Non-Arc Mafic Rocks
(Slide Mountain terrane)
10
1
.1
.1
Th La Pr Sm Hf Ti Tb Y Yb Al Sc
Nb Ce Nd Zr Eu Gd Dy Er Lu V
Th La Pr Sm Hf Ti Tb Y Yb Al Sc
Nb Ce Nd Zr Eu Gd Dy Er Lu V
Rock / Primitive Mantle
.1
100
Th La Pr Sm Hf Ti Tb Y Yb Al Sc
Nb Ce Nd Zr Eu Gd Dy Er Lu V
H)
Glenlyon area: Mafic Rocks
(Slide Mountain terrane)
10
1
.1
Th La Pr Sm Hf Ti Tb Y Yb Al Sc
Nb Ce Nd Zr Eu Gd Dy Er Lu V
Paleozoic magmatism and crustal recycling
Rock / Upper Continental Crust
10
A)
B)
1000
Nb
1
Sylvester Allochthon: Arc Felsic Rocks
(Fourmile succession)
Th La Pr Sm Hf Ti Tb Y Yb Al
V
Nb Ce Nd Zr Eu Gd Dy Er Lu Sc
te & dge
-pla n ri
hin ocea
t
i
w us
lo
ma
ano
volcanic arc
(I-type)
10
.1
within-plate
(A-type)
syncollisional
(S-type)
100
ocean-ridge
(OR-type)
M-type
1
1
10
100
1000
Y
Figure 23. (A) Upper continental crust normalized and (B) Nb-Y discrimination plots for felsic rocks in the Sylvester allochthon (data from
Nelson and Friedman, 2004).
latest Devonian through mid-Permian in age, corresponding to
Cycles II-V in YTT. Pennsylvanian-Permian units include the Pillow
Ridge succession in the Nina Creek Group, and PennsylvanianPermian units in Division II of the Sylvester allochthon. Basalts from
the Pillow Ridge succession all show N-MORB signatures (Fig. 22F),
as do rocks of the older Mount Howell and Mafic-Ultramafic units.
N-MORB signatures are also ubiquitous in the Slide Mountain terrane in the Sylvester allochthon (Fig. 22G; Nelson 1993). These data
are similar to the Campbell Range N-MORB; all are consistent with
deriviation from depleted mantle sources within an oceanic or backarc basin environment (Nelson, 1993, 2002; Ferri, 1997). Back-arc
magmatism in the Campbell Range, however, also included more
enriched-mantle products (OIB and E-MORB).
The predominant N-MORB signatures and the juvenile nature
of the isotopic systematics of Pennsylvanian – Early Permian mafic
rocks in the Slide Mountain terrane suggest that magmatism occurred
in a back-arc basin setting with no interaction with continental crust.
However, local interbedding of terrigenous clastic rocks (e.g., Nelson,
1993), and evolved isotopic systematics from sedimentary rocks of
Slide Mountain terrane (Patchett and Gehrels, 1998), indicate a
continental source for this detritus. This implies that the Slide
Figure 22. (facing page) Primitive mantle normalized plots for
Cycle V mafic rocks. (A) Arc and non-arc mafic lavas from the Klinkit
Group (data from Simard et al., 2003); (B) Arc rocks from the
Sylvester allochthon (data from Nelson and Friedman, 2004); (C) Arc
and (D) non-arc rocks from the Lay Range area (data from Ferri,
1997); (E) Non-arc rocks from the Campbell Range formation
(S. Piercey and D. Murphy, unpublished data); (F) Non-arc rocks
from the Nina Creek Group (data from Ferri, 1997); and (G) Non-arc
rocks from the Sylvester allochthon (data from Nelson, 1993; Nelson
and Friedman, 2004); (H) Cycle VI greenstones from the Slide
Mountain terrane in Glenlyon area (data from Colpron et al.,
2005).
Mountain ocean was a marginal sea that developed in proximity to
a cratonic region; this situation is similar to the Japan Sea relative
to the island of Japan and the Sino-Korean craton (Pouclet et al.,
1995; Creaser et al., 1999).
Klondike Cycle (Cycle VI - Middle to Late Permian
- 269-253 Ma)
The last Paleozoic magmatic cycle is primarily represented in the
Klondike region of western Yukon, and is therefore named after this
area (Fig. 22). It corresponds to the second pulse of felsic magmatism
in the pericratonic terranes (Fig. 6), and its beginning is marked by
the formation of eclogites in Yukon (Erdmer et al., 1998). The
Klondike cycle ends with the cessation of magmatism in YTT, and
a period of depositional and igneous hiatus in the Early Triassic
(Fig. 6). Subsequent deposition of Middle to Upper Triassic clastic
sedimentary rocks overlap Yukon-Tanana and Slide Mountain terranes, and also the North American miogeocline (Colpron et al., this
volume-a; Nelson et al., this volume).
Cycle VI magmatism is represented by the mid- to Late Permian
Klondike Schist (~263-253 Ma), a sequence of felsic volcanic and
volcaniclastic rocks with lesser interlayered mafic rocks, and coeval
and probably cogenetic monzonitic to quartz-monzonitic granitoids
of the Sulphur Creek orthogneiss (Mortensen, 1990). Late Permian
felsic schist layers occur also within carbonaceous rocks assigned
to the Nasina assemblage in the Fortymile River area of eastern
Alaska (Figs. 1, 24; J. Mortensen and C. Dusel-Bacon, unpublished
data; Dusel-Bacon et al., this volume). Biotite-bearing granitic orthogneiss of Late Permian age also intrudes Devono-Mississippian
metasedimentary rocks of the Nasina assemblage (Ryan et al., 2003;
J. Mortensen, unpublished data). Although limited, geochemical
data for felsic rocks of the Klondike Schist consistently exhibit flat
calc-alkalic signatures, with low Nb, Eu, Ti, Sc and V relative to
UCC (Fig. 25), indicative of an arc setting (e.g., Mortensen, 1990;
S. Piercey and J. Mortensen, unpublished data). The UCC-profiles
for the Sulphur Creek orthogneiss exhibit less systematic behaviour
than the Klondike Schist (Fig. 25A), perhaps due to mobility of some
307
Piercey et al .
N
64°
Cycle VI - Klondike
Middle Permian - Early Triassic
13
6°
W
(Guadalupian - Olenekian)
269-253 Ma
MORB
W
8°
14
F
252 269
SYMBOLS
273
256
Denali
Plutonism
Fossil
63
°N
261
260
Volcanism
ARC
254 259
261 263
253 260
Compressional
deformation
253
259 262
Blueschist / eclogite
occurrence
254
Permo-Triassic synorogenic clastics
Ak
F
Permian
°N
59
YT
264
260
269
14
W
0°
MORB
269
Pacific
~266
YT
B.
C.
258262
Tintina
259
ARC
Ocean
56
YT
B.
C.
12
4°W
F
Artinskian
- Kazanian
57°
N
F Artinskian -
°N
262 270
Guadalupian
MORB
Ak
B.
C.
Artinskian
0
12
8°
W
F Asselian -
DISCUSSION
Scale
300
km
Figure 24. Distribution of tectonic and magmatic events during the
Middle Permian – Early Triassic Klondike cycle (Cycle VI:
269-253 Ma). Events are located on a base map of Yukon-Tanana
and related terranes prior to ~430 km of dextral displacement along
Tintina fault. Dashed red line marks approximate limit between
geodynamic environments. Dashed blue line represents relative
position of subduction zone; teeth indicate dip of subducting slab.
See Figure 1 for tectonic assemblage legend.
308
elements due to alteration or metamor phism, or cumulate
processes.
In the Fortymile River area, felsic schist layers within carbonaceous rocks of the “Nasina assemblage” have UCC-normalized
profiles that can be broken into two groups: a group with relatively
flat patterns with depletions in Eu, Zr, Hf and compatible elements,
and a second group with similar characteristics but with additional
LREE-depletion (Fig. 25B). The first group is consistent with an arc
environment; however metarhyolite with LREE-depletion and
slightly higher Nb (and Ta) contents suggest a possible within-plate
origin for those samples (Fig. 25D). Alternatively, these may represent residual enrichment of Nb and Ta at the expense of LREE, due
to loss of LREE during hydrothermal alteration leading to closurerelated gains in Nb and Ta (see Stanley and Madeisky, 1994).
Permian magmatic activity is recorded by a suite of plutons
that intrude YTT and related allochthons in the Wolf Lake-Jennings
River area and the Sylvester allochthon (Fig. 24). A suite of midPermian (270-262 Ma) intrusions, including the Ram stock, and Nizi
and Meek plutons, intrudes the Dorsey Complex and panels of
Cycle V arc rocks in far northern British Columbia (Nelson and
Friedman, 2004). One body in this suite stiches a post-Early Permian
thrust fault in the Sylvester allochthon. Compositionally, the suite
ranges from gabbro to granite; tonalite is the dominant phase. A
sample of granite from the Meek pluton exhibits a calc-alkalic arc
signature, like the rocks of the Klondike region (Fig. 25C); this
scenario is consistent with a southerly continuation of arc magmatism
during the Klondike cycle throughout eastern YTT. Carbonate
deposition dominates the mid- to late Permian record of the Stikine
terrane (Gunning et al., this volume).
Mafic volcanism of back-arc basin affinity (N-MORB) persisted
into mid-Permian time in Slide Mountain terrane in the Sylvester
allochthon and Seventymile terrane (Figs. 1, 24; Dusel-Bacon et al.,
this volume; Nelson, 1993). In the Glenlyon area, the Slide Mountain
terrane is represented by a narrow belt of chert, argillite, greenstone
and serpentinite of Middle Permian age (Colpron et al., 2005).
Greenstones from Slide Mountain terrane in Glenlyon area have
mixed N-MORB and calc-alkaline signatures indicative of crustal
contamination of an N-MORB parent magma (Fig. 22H; Colpron
et al., 2005).
Petrochemical Constraints on the Regional Tectonic
Evolution of Yukon-Tanana and Related Terranes
Models for the tectonic evolution of Yukon-Tanana and Slide
Mountain terranes have three common themes: (1) rifting of part of
YTT from the western margin of Laurentia in mid-Paleozoic time
(~390-365 Ma); (2) mid- to late Paleozoic arc activity, back-arc extension and opening of the Slide Mountain ocean (~365-269 Ma); and
(3) subduction of Slide Mountain crust and lithosphere (~269-253 Ma),
and the subsequent accretion of YTT back onto the western margin
of North America (Tempelman-Kluit, 1979; Mortensen, 1992a;
Hansen and Dusel-Bacon, 1998; Nelson, 1993; Nelson et al., this
volume). The geochemical and isotopic data reviewed in this paper
Paleozoic magmatism and crustal recycling
10
A)
Klondike Region: Arc Felsic Rocks
1
.1
Rock / Upper Continental Crust
10
Rock / Upper Continental Crust
Rock / Upper Continental Crust
10
Fortymile River Area
Arc/Non-Arc Rocks
1
.1
Th La Pr Sm Hf Ti Tb Y Yb Al
V
Nb Ce Nd Zr Eu Gd Dy Er Lu Sc
C)
B)
Th La Pr Sm Hf Ti Tb Y Yb Al
V
Nb Ce Nd Zr Eu Gd Dy Er Lu Sc
D)
1000
Klond. Sch.
SCO
Wolf Lake/Jennings River: Arc Felsic Rocks
(Meek pluton)
BioOrth
1
Nb
Nasina 1
Nasina 2
within-plate
(A-type)
syncollisional
(S-type)
100
Meek Pl.
te & dge
-pla an ri
e
hin
wit us oc
alo
m
o
n
a
volcanic arc
(I-type)
10
ocean-ridge
(OR-type)
M-type
.1
Th La Pr Sm Hf Ti Tb Y Yb Al
V
Nb Ce Nd Zr Eu Gd Dy Er Lu Sc
1
1
10
100
1000
Y
Figure 25. Upper continental crust normalized plots for Cycle VI felsic rocks. (A) Felsic rocks from the Klondike region (S. Piercey and
J. Mortensen, unpublished data); (B) Felsic schist in Nasina assemblage, Fortymile River area (Dusel-Bacon et al., this volume); (C) Meek
pluton (data from Nelson and Friedman, 2004); and (D) Nb-Y plot for Klondike and Meek pluton; SCO = Sulphur Creek orthogneiss.
provide further constraints that help refine our understanding of the
mid- to late Paleozoic evolution of Yukon-Tanana and related
terranes.
It is important to note that the present configuration of YTT
(Fig. 1) and the distribution of magmatic belts discussed in this paper
are the result of the protracted deformational history of YukonTanana, Slide Mountain and the North American miogeocline between late Paleozoic and early Cenozoic times (e.g., Murphy et al.,
2002; Roddick, 1967). On Figures 7, 10, 13, 17, 21 and 24 we show
the distribution of the main Paleozoic magmatic and tectonic events
on a series of maps of Yukon-Tanana and related terranes prior to
~430 km of early Cenozoic dextral displacement along Tintina fault
(Murphy and Mortensen, 2003; Gabrielse et al., in press). These
maps essentially show the Late Cretaceous paleogeography of YukonTanana and related terranes. The effects and magnitude of offset
associated with late Paleozoic to Mesozoic faulting within YTT are
for the most part poorly constrained, and have not been removed
from the maps. It should also be noted that displacement along the
Denali and Border Ranges faults have not been restored. In the following discussion we assume that the Late Cretaceous distribution
of magmatic and tectonic events in YTT approximates their relative
original distribution in the Paleozoic.
Alaska Range cycle (I – 390-365 Ma) magmatic activity involved crustal extension and attenuation of the North American
continental margin in the Alaska Range and Yukon-Tanana Upland
(Dusel-Bacon et al., this volume, 2004), and continental arc magmatism in the Coast Mountains of southeastern Alaska and British
Columbia (Fig. 7). Cycle I magmatic rocks in the Bonnifield mineral
belt and correlative assemblages (Lake George assemblage and siliceous, carbonaceous and volcanic assemblage) have diagnostic
crustally-derived peralkaline rhyolites and associated OIB-like mafic
rocks (Fig. 8). These geochemical characteristics, and the predominantly older ages of these Alaskan rocks, resemble coeval volcanic
rocks of the North American miogeocline (Selwyn basin and Cassiar
terrane, Fig. 1; Mortensen, 1982; Mortensen and Godwin, 1982;
Goodfellow et al., 1995). Mid-Paleozoic alkalic volcanism and extension of the continental margin is likely associated with coeval arc
magmatism in YTT in the Coast Mountains, and initiation of the
Slide Mountain rift (Paradis et al., 1998; Nelson et al., 2002).
309
Piercey et al .
Finlayson cycle (II – 365-357 Ma) magmatism was characterized by arc and back-arc environments in the Finlayson Lake district
and Stewart River area. Rift-related magmatism in the Alaska Range
and Yukon-Tanana Upland persisted during this cycle. Arc magmatism occurred predominantly in a western belt, now preserved primarily in the Stewart River region and in the southwestern Finlayson
Lake district (Table 1; Fig. 10). Arc sequences are dominated by
felsic magmatic rocks with calc-alkalic signatures and mafic rocks
with arc signatures (Table 1; Figs. 11-12). This magmatic arc belt is
coeval with back-arc magmatism in the eastern Finlayson Lake
district and MORB-dominated marginal basin facies of the Slide
Mountain assemblage in the Sylvester allochthon (Fig. 10; Nelson,
1993), implying a west-facing, east-dipping subduction zone
(Mortensen, 1992a; Nelson, 1993; Nelson et al., this volume).
Wolverine cycle (III – 357-342 Ma) magmatism is most widespread in the central and southern parts of the YTT (Fig. 13). Its
configuration is similar to that of Cycle II, with arc sequences located
in the western portions of the terrane, in front of the more easterly
back-arc region in eastern Finlayson Lake, and even more distal
back-arc of the Slide Mountain assemblage in the Sylvester allochthon (Fig. 13). There is little Cycle III arc magmatic activity recorded
in the Yukon-Tanana Upland of eastern Alaska. The intermediate
to mafic volcanic rocks of the Fortymile River assemblage span the
Alaska-Yukon border and have arc and MORB signatures (Fig. 16B).
They may represent an arc and back-arc sequence, although the
tectonic implications of these rocks have yet to be resolved. It is
important to note the paucity of magmatism in the Alaska Range
from Cycle III onward (Fig. 13). On the other hand, arc magmatic
activity was voluminous and is well represented in the Glenlyon and
Teslin regions (Fig. 13; Table 1). These latter arc sequences also
contain alkalic mafic rocks (Fig. 16; Table 1), suggesting that the arc
or arcs underwent a period of extension during Cycle III. Furthermore,
the presence of arc-dominated successions in the central and western
portions of YTT, and the occurrence of seafloor spreading recorded
by MORB-type and BABB-type basaltic rocks in the easternmost
part of the terrane in the Finlayson Lake district (e.g., Wolverine
basalt; Piercey et al., 2002b) and in the Sylvester allochthon
(e.g., Slide Mountain basalts; Nelson, 1993), imply the likelihood of
westerly rollback of the slab and the induction of back-arc seafloor
spreading.
The Little Salmon cycle (IV – 342-314 Ma) marks a fundamental
shift in the configuration of the arc system. Both arc and back-arc
magmatism essentially ceased in the Finlayson Lake and Stewart
River areas, and the locus of arc activity shifted southwards to the
Glenlyon and Wolf Lake-Jennings River areas (Fig. 17). Coeval arc
magmatism also occurs in the Stikine assemblage of northwestern
British Columbia (Fig. 17). Cycle IV sequences are dominated by
crustally-derived, calc-alkalic magmatic rocks with OIB-like effusions of mafic material (Figs. 18-20), implying continental arc
magmatism with intra-arc rift episodes. The lack of documented
Late Mississippian magmatic activity in the Slide Mountain assemblage of the Sylvester allochthon has led Nelson and Bradford (1993)
to suggest that there was a lull in volcanic activity in the back-arc
basin. Alternatively, this may be a function of poor preservation of
310
Late Mississippian Slide Mountain crust. Nevertheless, Cycle III
magmatism most likely occurred above an east-dipping subduction
zone, as with previous magmatic cycles (Fig. 17).
Klinkit cycle (V – 314-269 Ma) magmatism represents another
shift in the style and nature of magmatism in YTT. Arc magmatism
was widespread in the Wolf Lake-Jennings River areas, as represented by the Klinkit Group (Simard et al., 2003), and within the
Fourmile succession in the Sylvester allochthon (Nelson and
Friedman, 2004; Fig. 21). Arc volcanic rocks within the Lay Range
assemblage of Quesnellia in north-central British Columbia are also
correlated with this magmatic cycle (Fig. 1; Ferri, 1997; Simard et al.,
2003). Coeval back-arc magmatism is present in the Campbell Range
formation in the Finlayson Lake district and in the Slide Mountain
assemblage and the Nina Creek Group, in the Sylvester allochthon
and the Lay Range area respectively (Figs. 1, 21; Nelson, 1993; Ferri,
1997; Plint and Gordon, 1997; Piercey et al., 1999; Creaser et al.,
1999). The location of these back-arc rocks to the northeast and north
of the dominantly arc assemblages suggests that subduction polarity
was east- to northeast-dipping (Fig. 21). This is likely a continued
evolution of the east-dipping subduction zone that characterized all
previous magmatic cycles.
An interesting feature of this phase of magmatism is the dominance of mafic to intermediate material with very little felsic magmatism compared to earlier YTT episodes (e.g., Nelson, 1993; Ferri,
1997; Plint and Gordon, 1997; Murphy and Piercey, 1999; Piercey
et al., 1999; Simard et al., 2003; Fig. 6). Furthermore, Cycle V mafic
magmatism is characterized by very juvenile isotopic characteristics.
In the Klinkit Group, arc-related samples have juvenile εNdt values
of +6.7 to +7.4 (Simard et al., 2003). Permian eclogites near Faro and
Ross River, north of the Finlayson Lake district (Figs. 1, 24), which
have back-arc-related protoliths and are inferred to be Campbell
Range equivalents, have εNdt = +5.4 to +9.3 (Creaser et al., 1999).
Back-arc-related VMS occurrences in the Campbell Range belt have
juvenile Pb-isotopic systematics (Mann and Mortensen, 2000).
Collectively, these features suggest that both arc and back-arc related
rocks had minimal interaction with continental crustal materials.
Two potential mechanisms can be invoked to explain this: (1) both
arc and back-arc regions were built (at least in part) upon juvenile
substrates; and/or (2) that the rate of extension was rapid, and coupled
with rapid effusion rates and/or conduit armouring that prevented
any substantial interaction with continental crustal material. Nd
isotopic and trace element geochemical evidence suggests that both
mechanisms were operative in YTT (Piercey et al., 2001a; Simard
et al., 2003); and it is likely that both were important in explaining
the dominance of juvenile material during Cycle V magmatism.
Klondike cycle (VI – 269-253 Ma) tectonic/magmatic patterns
mark a significant departure from that of all previous cycles in YTT.
Cycle VI is characterized by a change in subduction polarity such
that the arc now faced east above a west-dipping subduction zone
(Fig. 24; Mortensen, 1990). This east-facing subduction geometry
is indicated by pairing of a belt of mid-Permian eclogite and blueschists along the eastern edge of the terrane (Dusel-Bacon, 1994;
Erdmer et al., 1998) with mid- to Late Permian arc rocks that occur
primarily in the Klondike district to the west and southwest
Paleozoic magmatism and crustal recycling
(Mortensen, 1990, 1992a; Fig. 24). This shift from east- to west-dipping subduction marks the onset of the closure of the Slide Mountain
marginal ocean (Mortensen, 1992a; Nelson, 1993). The occurrence
of calc-alkalic felsic rocks in both the Klondike district and in
northern British Columbia suggest that this magmatism was of
continental arc affinity (Fig. 25). However, geochemical data for
these rocks are sparse, and further work is required to fully document their character. Cycle VI culminated with cessation of volcanism in YTT and the deposition of synorogenic conglomerate, which
continued into Late Triassic time and represents an overlap assemblage that links Yukon-Tanana and Slide Mountain terranes to
western North America (Fig. 24; Nelson et al., this volume).
The combination of geological, geochronological and geochemical data reviewed here indicates that the middle to late Paleozoic
evolution of YTT (~390-269 Ma) is characterized by extension of a
continental margin behind a west-facing arc system (Figs. 7, 10, 13,
17 and 21). Development of this arc system was punctuated by episodic arc rifts and formation of a back-arc basin, which eventually
led to the opening of the Slide Mountain marginal ocean. The final
stage of the Paleozoic evolution of the terrane is marked by a fundamental shift in geodynamic setting from an east-dipping to a
west-dipping subduction zone. During this last cycle (269-253 Ma),
the Slide Mountain ocean was ultimately consumed and rocks of the
Klondike arc formed above the west-dipping subduction zone
(Fig. 24).
Mantle Sources and Mixing During YTT Evolution
Mafic magmatic rocks along convergent margins often have complex
petrological histories involving the interaction of subducted slab,
mantle wedge, lithospheric mantle and continental crust (e.g., Gill,
1981; Pearce, 1983; Rogers and Hawkesworth, 1989; Pearce and Peate,
1995; Shinjo and Kato, 2000; Shinjo et al., 1999). In order to evaluate
the mantle sources of mafic rocks, discriminants that specifically
characterize mantle, as opposed to slab-metasomatic and crustal
components must be used. In Figure 26 the element ratio Zr/Yb is
plotted against Nb/Yb for mafic rocks from each magmatic cycle in
YTT. This diagram is particularly useful in discriminating the relative incompatible element enrichment of the mantle source region
for basaltic rocks, because Zr, Nb and Yb are all moderate to highly
incompatible elements and ratios of these elements to one another
are largely insensitive to slab metasomatism, crustal contamination
and fractionation.
The broad range of Nb/Yb and Zr/Yb in Figure 26 shows that,
regardless of age, mafic rocks in YTT were derived from heterogeneous mantle sources that ranged from depleted to enriched. In YTT,
a depleted end-member mantle source played a role in the genesis
of arc, and to a lesser extent, non-arc rocks. On Figure 26, most arc
rocks cluster near or slightly below the N-MORB end-member and
extend toward E-MORB compositions, whereas non-arc rocks generally plot between E-MORB and OIB end-members. In Cycles III
and V (Figs. 26B, D), non-arc rocks show more variability and have
values that extend toward the N-MORB end-member, implying that
some of these rocks were derived from depleted mantle sources
(e.g., BABB). The role of the depleted mantle in many modern arc
and back-arc magmatic environments is well documented
(McCulloch and Gamble, 1991; Woodhead et al., 1992; Hawkins,
1995; Pearce and Peate, 1995; Gribble et al., 1996; Shinjo et al., 1999).
The presence of depleted mantle wedge sources for YTT arc and
back-arc magmatism suggests that the arc mantle wedge and the
back-arc mantle beneath YTT were similar to modern magmatic arc
and back-arc environments.
In addition to depleted mantle, a component of enriched mantle
is evident in most YTT arc and non-arc rocks (Fig. 26). The occurrence of mafic rocks with more enriched compositions is a feature
that is commonly present in continental arc and back-arc, and in
continental rift geodynamic environments (e.g., Pearce, 1983; Pearce
and Peate, 1995; Shinjo et al., 1999). However, the nature and origin
of this enriched component is a matter of debate. Shinjo et al. (1999)
proposed an asthenospheric source (i.e., new, upwelling asthenospheric mantle) for the enriched component, whereas Pearce (1983),
Hawkesworth et al. (1990) and McDonough (1990) favour a lithospheric origin (i.e., ancient, subcontinental mantle). Pearce (1983)
further argued that the higher HFSE contents of continental arc
magmas relative to intra-oceanic arc magmas are a result of a subcontinental lithospheric contribution to continental arc mafic magmas, in addition to depleted mantle wedge and slab components
(cf. Pearce and Parkinson, 1993). Rogers and Hawkesworth (1989)
illustrated a correlation between increasing incompatible element
enrichment (e.g., Nb/Lamn, Nb/Th mn >1) and decreasing εNd and increasing εSr in Andean basalts, indicating the influence of old subcontinental lithosphere in their genesis. Data for enriched rocks in
YTT favours a lithospheric origin. Most Nb-enriched rocks in YTT
(e.g., OIB-rift, NEB-suite rocks), with positive Nb anomalies relative
to Th and La (Nb/Th mn, Nb/Lamn > 1), have low εNdt values ranging
from +1.7 to -1.6. Furthermore, a broad relationship between increasing Nb/Th mn and Nb/Lamn and decreasing εNdt in some YTT mafic
rocks are features that cannot be explained by crustal contamination
and thus require an enriched, older, subcontinental lithospheric
component (Piercey, 2001; Piercey et al., 2002a, 2004).
Any model that explains the enriched component in the YTT
subarc mantle wedge requires that it operated for most of the mid- to
late Paleozoic evolution of the terrane (Fig. 26). We favour a simple
two-component mixing model between magmas derived from a depleted mantle wedge, or depleted back-arc asthenosphere, and an
enriched lithospheric component, to explain the heterogeneity of
YTT mafic rocks. The linear array between depleted and enriched
components on the Zr/Yb-Nb/Yb diagram supports this hypothesis.
Nevertheless, there are clearly end-member magmas that do not require a mixed component. For example, the boninitic, island arc
tholeiitic and N-MORB-type magmas likely reflect derivation from
solely depleted to ultradepleted mantle sources (Piercey, 2001;
Piercey et al., 2001a, 2004), which would probably be involved in
normal arc and back-arc magmatic activity. In contrast, OIB-rift type
rocks (within-plate/extensional) probably represent derivation from
solely enriched lithospheric sources in response to arc rifting and
the initiation of back-arc magmatic activity in the bulk of the YTT,
as well as continental margin extension in the Yukon-Tanana Upland
and Alaska Range (Piercey et al., 2002b; Colpron, 2001; Simard et al.,
311
Piercey et al .
1000
1000
A)
B)
Cycle I
100
Cycle II
Zr/Yb
Zr/Yb
100
10
Non-Arc
1
.1
1000
1
C)
Nb/Yb
10
OIB
E-MORB
N-MORB
10
Arc
1
100
.1
1000
1
D)
Cycle III
OIB
E-MORB
N-MORB
Non-Arc
Nb/Yb
10
100
Cycle IV
100
Zr/Yb
Zr/Yb
100
10
Arc
1
.1
1000
1
Non-Arc
Nb/Yb
10
OIB
E-MORB
N-MORB
10
Arc
1
100
1000
E)
1
F)
Cycle V
Non-Arc
Nb/Yb
10
100
Cycle VI
100
Zr/Yb
100
Zr/Yb
(e)
10
Arc
1
.1
OIB
E-MORB
N-MORB
.1
1
Non-Arc
Nb/Yb
10
OIB
E-MORB
N-MORB
10
Arc
100
1
.1
1
OIB
E-MORB
N-MORB
Non-Arc
Nb/Yb
10
100
Figure 26. Zr/Yb-Nb/Yb plots for mafic rocks from the various magmatic cycles in YTT evolution. Cycles I through VI represented in (A)
through (F) respectively. Diagram modified after Pearce and Peate (1995). Average values of mantle reservoirs are from Sun and McDonough
(1989): N-MORB = normal mid-ocean ridge basalt (depleted); E-MORB = enriched MORB (moderately enriched); and OIB = ocean-island
basalt (enriched).
312
Paleozoic magmatism and crustal recycling
2003). Mixing between these depleted and enriched end-members
is an expected response to concurrent tectonic activity. The presence
of an enriched component in arc rocks likely reflects mixing of material from a depleted mantle wedge with subcontinental lithospheric
mantle during migration of the melts to their ultimate destination
within and upon the crust. Similarly, the occurrence of E-MORB-type
signatures within YTT back-arc basins would signal mixing between
upwelling depleted N-MORB type asthenosphere and enriched
lithospheric material, en-route to emplacement within the back-arc
regions (e.g., Campbell Range basalts and Sylvester allochthon).
It is notable that there are no temporal variations in the incompatible element behaviour of YTT mafic rocks from the mid- to late
Paleozoic. This suggests that mantle sources for mafic rocks of YTT
did not change appreciably throughout the terrane’s evolution, and
that the mantle heterogeneity exhibited by YTT is a fundamental
feature of the terrane.
Importance of Recycled Continental Crust During
YTT Evolution
The importance of continental crustal recycling in modern and ancient continental margin arc and back-arc, and in rifted continental
margin geodynamic environments is well established (Rogers and
Hawkesworth, 1989; Shinjo et al., 1999; Whalen et al., 1998; Piercey
et al., 2003). In YTT, the influence of evolved continental crust is
shown by the Pb, Nd and Sr isotope systematics of granitic rocks
(Mortensen, 1992a). Uranium-lead zircon data from geochronological studies of granitoid rocks from the terrane commonly indicate
Proterozoic and Archean inheritance (Mortensen, 1990; 1992a;
Dusel-Bacon et al., 2004). Similarly, Nd isotopic studies of YTT
sedimentary and felsic igneous rocks exhibit strong evidence for
recycled ancient continental crust in their genesis (εNdt<0 and
Proterozoic TDM ages; e.g., Stevens et al., 1995; Creaser et al., 1997;
Piercey et al., 2003).
Figure 27 shows plots of La and Sm (both normalized to UCC
values) for felsic rocks from all magmatic cycles in YTT. Rocks derived from crustal sources lie on or near the line with a slope of 1,
equivalent to La/SmUCN (UCN = Upper Crust Normalized) = 1
(Fig. 27). Samples that plot below this line, with La/SmUCN < 1, likely
come from sources that are depleted relative to UCC, for example,
mafic crust; or they may have fractionated LREE-bearing accessory
phases. The case of La/SmUCN >>1 is very rare, as it would require
crustal sources with much higher LREE than the UCC; however,
high La to Sm ratios could be achieved through preferential mobilization of LREE in hydrothermal fluids, or through kinetically controlled melting of LREE-enriched accessory phases at high temperatures (e.g., Bea, 1996a, b).
Most felsic rocks from YTT cluster close to the La/SmUCN ≈1
control line, implying that recycling of continental crust has been
an important process throughout the history of the terrane.
Furthermore, although there are variations in the absolute La and
Sm concentrations between arc and non-arc rocks, there is no significant separation in terms of La/SmUCN ratios between the two
groups, suggesting that both arc and non-arc felsic rocks were derived
from predominantly recycled upper continental crustal material.
This geochemical data for felsic rocks, unfortunately, can only
point to an UCC-like source and cannot specifically delineate what
the source of that crust was. In particular, it cannot delineate whether
this crust was once part of the North American craton, or an exotic
crustal fragment with UCC geochemical attributes. The similarity
in stratigraphic and geochemical attributes between rocks of the
Yukon-Tanana Upland and parts of the Alaska Range and those of
Selwyn basin in Yukon does suggest that the basement to YTT may
have once been part of the North American miogeocline. Other aspects of YTT also share similarities with the North American miogeocline including: (1) Nd isotopic attributes for YTT felsic and
sedimentary rocks (Mortensen, 1992a; Stevens et al., 1995;
Boghossian et al., 1996; Grant, 1997; Creaser et al., 1997; Garzione
et al., 1997; Patchett et al., 1999; Piercey et al., 2003); (2) detrital
zircon geochronology (Gehrels et al., 1995); (3) Pb isotopic composition of syngenetic sulphide deposits (Nelson et al., 2002; Mortensen
et al., this volume); and (4) broadly similar Devonian-Mississippian
geodynamic history (Paradis et al., 1998; Nelson et al., 2002, this
volume). Collectively these features point to a likely link between
the North American miogeocline and YTT crust, at least prior to
Devonian-Mississippian back-arc initiation along the ancient Pacific
continental margin (see also Nelson et al., this volume).
Importance of Recycled Oceanic Crust
It is well established that basaltic rocks can provide probes to their
past mantle history, their mantle sources, and the relative importance
of oceanic and continental crustal recycling in their genesis (Zindler
and Hart, 1986). The abundance of magmatic rocks with incompatible element-enriched OIB signatures throughout the evolution of
YTT points to the potential for a recycled oceanic crustal component
in YTT magmas.
It has been shown that mafic rocks erupted in ocean islands and
other large igneous province (LIP) environments commonly exhibit
geochemical and radiogenic isotopic evidence for past oceanic and
continental crustal recycling (White and Hofmann, 1982; Zindler
and Hart, 1986; Hart et al., 1992; Hofmann, 1997, and references
therein). Of particular interest to this paper is the presence of excess
Nb and Ta relative to Th, La and other LREE/LFSE compared to
primitive mantle values (i.e., positive Nb and Ta anomalies), features
interpreted to indicate a recycled oceanic crustal component in these
magmas (e.g., McDonough, 1991; Niu and Batiza, 1997; Niu et al.,
1999).
The origin of positive Nb and Ta enrichments is attributed to
the recycling of oceanic crust in subduction zones and its re-incorporation into the mantle wedge via convective stirring into the upper
mantle (e.g., Allègre and Turcotte, 1986; Meibom and Anderson,
2003), and/or by incorporation into mantle plumes (e.g., Weaver,
1991; McDonough, 1991; Hart et al., 1992; Stein and Hofmann, 1994;
Hofmann, 1997; Niu and Batiza, 1997; Niu et al., 1999; Condie, 1998,
2000). As the subducted slab descends into the subduction zone, it
undergoes dehydration and transport of Th, U, LFSE and in some
cases LREE, to the overlying mantle wedge, leading to the characteristic “arc” signature in subduction related basalts (e.g., Pearce,
1983; You et al., 1996; Jenner, 1996; Swinden et al., 1997; Johnson
313
Piercey et al .
5
(a)
!
5
Cycle I
"
Cycle II
4
La/SmUCN > 1
Enriched
3
LaUCN
LaUCN
4
(b)
2
La/SmUCN > 1
Enriched
3
2
La/SmUCN < 1
Depleted
1
La/SmUCN < 1
Depleted
1
Non-Arc
Non-Arc
0
0
5
1
2
(c)
#
SmUCN
3
4
0
5
LaUCN
LaUCN
La/SmUCN > 1
Enriched
2
2
(d)
$
SmUCN
3
4
5
Cycle IV
La/SmUCN > 1
Enriched
3
2
La/SmUCN < 1
Depleted
1
La/SmUCN < 1
Depleted
1
Non-Arc
Arc
0
5
1
2
SmUCN
3
4
0
5
Arc
0
5
(e)
%
1
2
(f)&
Cycle V
SmUCN
3
4
5
Cycle VI
4
4
La/SmUCN > 1
Enriched
3
LaUCN
LaUCN
1
4
3
La/SmUCN > 1
Enriched
3
2
2
La/SmUCN < 1
Depleted
1
0
0
5
Cycle III
4
0
Arc
Arc
0
1
2
SmUCN
3
La/SmUCN < 1
Depleted
1
4
5
0
Arc
0
1
2
SmUCN
3
4
Figure 27. LaUCN-SmUCN plots for YTT felsic rocks. Cycles I through VI represented in (A) through (F). Details of the diagram provided in
the text. Solid (filled) symbols in each plot represent non-arc rocks and open (unfilled) symbols represent arc rocks. Normalization values
from McLennan (2001).
314
5
Paleozoic magmatism and crustal recycling
3
A)
Cycle I
3
Enriched Mantle
(Recycled Oceanic Crust)
Slab Input
or
Crustal
Contamination
2
Cycle II
Enriched Mantle
(Recycled Oceanic Crust)
Slab Input
or
Crustal
Contamination
Nb/Thmn
Nb/Thmn
2
B)
1
1
OIB
0
Non-Arc
0
3
1
E-MORB
N-MORB
Non-Arc
N-MORB
2
3
Enriched Mantle
(Recycled Oceanic Crust)
0
1
D)
Cycle IV
Nb/Lamn
2
Enriched Mantle
(Recycled Oceanic Crust)
Nb/Thmn
1
OIB
Arc
0
1
E)
Cycle V
Nb/Lamn
N-MORB
2
3
0
Cycle VI
Nb/Lamn
N-MORB
2
3
Enriched Mantle
(Recycled Oceanic Crust)
Slab Input
or
Crustal
Contamination
2
1
1
F)
Nb/Lamn
Nb/Thmn
0
E-MORB
Non-Arc
3
Enriched Mantle
(Recycled Oceanic Crust)
Slab Input
or
Crustal
Contamination
2
OIB
Arc
E-MORB
Non-Arc
3
1
OIB
0
3
Slab Input
or
Crustal
Contamination
2
1
0
0
3
Slab Input
or
Crustal
Contamination
Nb/Thmn
Arc
Cycle III
C)
2
Nb/Lamn
OIB
E-MORB
0
1
OIB
Arc
E-MORB
Arc
E-MORB
Non-Arc
N-MORB
Non-Arc
N-MORB
Nb/Lamn
2
3
0
0
1
Nb/Thmn
2
3
Figure 28. Nb/Thmn-Nb/Lamn diagram for YTT mafic rocks. Symbols as in Figure 22. Cycles I through VI represented in (A) through (F).
Details of the diagram provided in the text. Diagram constructed from the concept of Niu et al. (1999) (e.g., Ta/Upm-Nb/Thpm diagram).
315
Piercey et al .
and Plank, 1999). In contrast, the stability of phases such as rutile
during eclogitization results in the retention of HFSE in the subducted slab (in particular Nb, Ta and Ti; Saunders et al., 1988;
McDonough, 1991; Rudnick et al., 2000; Foley et al., 2000). The
preferential retention of Nb, Ti and Ta in the slab results in arc basalts
with characteristic Nb depletions relative to Th and La (and other
LFSE and LREE), and primitive mantle normalized Nb/La and
Nb/Th ratios of less than one (Nb/Th mn, Nb/Lamn < 1; Pearce and
Peate, 1995). The subducted slab, in contrast, has antithetical Nb/La
and Nb/Th ratios relative to arc basalts, with primitive mantle normalized values greater than 1 (Nb/Th mn, Nb/Lamn >1). It has been
argued that the Nb/Thmn and Nb/Lamn >1 in OIB and E-MORB imply
a recycled oceanic crustal component in the source of these basaltic
rocks (e.g., Saunders et al., 1988; McDonough, 1991; Niu and Batiza,
1997; Niu et al., 1999; Rudnick et al., 2000).
On Figure 28, the data for YTT mafic rocks are shown on a
Nb/Th mn vs. Nb/Lamn diagram. This diagram divides mafic rocks
derived from magmas of arc parentage, or that have been contaminated by continental crust (Nb/Th mn and Nb/Lamn <1), from rocks
that come from enriched sources with an inherited oceanic crustal
component (Nb/Th mn and Nb/Lamn >1; Fig. 28). Arc and crustally
contaminated rocks invariably lie within the arc/contamination field
(Nb/Th mn and Nb/Lamn<1; Fig. 28). Non-arc rocks are more variable.
Those with a weak subduction signature (e.g., BABB) or that are
crustally contaminated typically plot in the arc/contamination field
(Nb/Th mn and Nb/Lamn<1; Fig. 28). Mafic rocks with N-MORB ±
E-MORB geochemical signatures lie near the junction between
Nb/Th mn < 1 and Nb/Lamn > 1, and those with OIB ± E-MORB signatures fall within the enriched field (Fig. 28).
These types of enriched rocks with oceanic crustal components
are commonly associated with large igneous provinces (LIP). The
occurrence of enriched rocks in all cycles of YTT magmatism suggests the influence of a recycled oceanic crustal component, but may
also highlight the possibility that these rocks represent formation
within a LIP. If this is the case, then this LIP must have existed during the entire mid- to late Paleozoic magmatic evolution of YTT.
This is highly unlikely, as it would require that the LIP persisted for
more than 150 m.y. and influenced an area in excess of 250,000 km 2
in the northern Cordillera. Furthermore, the geological events that
dominated YTT history such as arcs, arc rifts, intra-arc compressional events, and extensional back-arc basin activity (e.g., Nelson
et al., this volume), coupled with the lack of evidence for voluminous,
high-temperature basaltic magmatism, are inconsistent with a LIP
geodynamic environment such as an ocean island or oceanic plateau
(e.g., Hawaii, Ontong-Java).
Although formation within a LIP can be ruled out for the mid- to
late Paleozoic evolution of YTT, the presence of mafic rocks with
signatures similar to rocks from such environments highlight the
potential role that a LIP may have had in its ancestry. In particular,
the recycled oceanic lithospheric signatures in enriched YTT basalts
could be explained by inherited LIP melts residing as veins within
a subcontinental lithospheric mantle domain in the terrane. For example, Stein and Hofmann (1992, 1994) argued that in the ArabianNubian shield, past LIP-related material (frozen plume heads) froze
316
against and/or was incorporated into the lithospheric mantle. With
subsequent rifting and melting of the lithospheric mantle, these ancient LIP components were recycled into younger basalts with signatures akin to the preexisting LIP (Stein and Hofmann, 1992, 1994).
Pre-Paleozoic fossilized LIP material in the YTT subcontinental
lithospheric mantle could also explain the recycled oceanic crustal
component in YTT enriched mafic magmas. An interesting consequence of this model is the question of the provenance of the LIP
magmatism (plume?) that fertilized the YTT lithospheric mantle.
A) Neoproterozoic Breakup of Rodinia
(~780-720 Ma)
Continental Crust
SCLM
Low Degree Melts
Fertilization of
Lithosphere
Franklin-Gunbarrel
LIP Event
B) Mid- to Late-Paleozoic
Convergent Margin Magmatism.
CC
SCLM
Subduction and Arc-Rifting /
Back-Arc Basin Magmatism
Recycling the
Ancient LIP Component
Figure 29. The occurrence of OIB-like signatures in YTT rocks over
150 m.y. points to a potential plume component in their genesis.
Enriched rocks associated with the Gunbarrel and Franklin large
igneous provinces (LIP) resulted in widespread magmatism and
igneous activity along the northwestern edge of Laurentia in
Neoproterozoic time (~780-720 Ma; Heaman et al., 1992; Ernst and
Buchan, 2001; Harlan et al., 2003). During this event, partial melts
from the Franklin plume fertilized the subcontinental lithospheric
mantle of northwestern Laurentia resulting in a veined lithospheric
mantle, which retained the geochemical signature of plume-derived
rocks (i.e., OIB to E-MORB). These veins were later liberated during
partial melting related to YTT arc and back-arc magmatism, which
developed on top of western Laurentian continental margin fragments during the middle to late Paleozoic. SCLM = Subcontinental
lithospheric mantle.
Paleozoic magmatism and crustal recycling
The ancient Pacific margin of North America was the locus of
a LIP in the Late Proterozoic. In particular, two Neoproterozoic
events, the Gunbarrel (~780 Ma; Harlan et al., 2003) and Franklin
igneous events (~720 Ma; Heaman et al., 1992; Park et al., 1995;
Dupuy et al., 1995; Dudas and Lustwerk, 1997), resulted in more
than 1,250,000 km 2 of mafic magmatism extending from Wyoming,
along the cratonic margin of western North America, through Arctic
Canada and into Greenland (Ernst and Buchan, 2001). These events
resulted in widespread mafic dike swarms, mafic sills and flood basalt
extrusions over this extended region and collectively have been interpreted to be a product of the Neoproterozoic breakup of Rodinia
along the western margin of Laurentia (Fig. 29; Heaman et al., 1992;
Park et al., 1995; Dudas and Lustwerk, 1997; Harlan et al., 2003). It
is possible that the widespread magmatic activity associated with
these LIP events was responsible for the fertilization of the subcontinental mantle of the northern Cordillera, resulting in a lithospheric
mantle with veins of Nb-enriched material within it, which upon
later reactivation during continental rifting or arc rifting resulted in
the OIB-like signatures observed in both the miogeocline and YTT
(e.g., Goodfellow et al., 1995; Dusel-Bacon and Cooper, 1999; Piercey
et al., 2002a; Nelson and Friedman, 2004). Although equivocal, the
presence of Neoproterozoic depleted mantle model ages in uncontaminated alkalic basalts supports the hypothesis that these rocks
had their initial origins as part of the LIP associated with the
Neoproterozoic breakup of Rodinia (e.g., Piercey et al., 2002a, 2004;
S.J. Piercey and R.A. Creaser, unpublished data).
SUMMARY AND CONCLUSIONS
Yukon-Tanana terrane has had a varied petrological history with
complex interactions between crust, mantle and subducted slab; ultimately these petrological variations can be tied to the geodynamic
evolution of the terrane. The main conclusions of this paper can be
outlined as follows:
(1) YTT comprises six cycles of magmatic activity prior to its
Mesozoic accretion to the North American craton. The first five
cycles occurred above an east-dipping subduction zone that
developed along the distal edge of the North American craton,
whereas the sixth (Klondike) magmatic cycle took place above
a west-dipping subduction zone.
(2) Felsic magmatic rocks associated with the extending continental
margin in the Alaska Range and Yukon-Tanana Upland (Cycle I)
are dominantly crustally-derived peralkalic rocks with lesser
crustally-derived subalkalic rocks; they are accompanied by
mafic rocks of intraplate affinity.
(3) During cycles II through V, magmatism was largely bimodal
in nature, with mafic and felsic end-members. Mafic rocks with
arc signatures are predominantly calc-alkalic and island-arc
tholeiitic, with lesser LREE-enriched island arc tholeiites and
boninites. In corresponding back-arc environments, mafic rocks
are dominated by normal mid-ocean ridge basalts, enriched
mid-ocean ridge basalts and ocean island basalt signatures.
Ocean island basalt-like rocks are also present in many arcdominated successions, and are interpreted to record intra-arc
rift events within YTT. Felsic rocks in arc environments are
dominated by calc-alkalic affinities with lesser tholeiitic rocks;
back-arc rocks are characterized by HFSE- and REE-enriched
(A-type) affinities.
(4) Mafic rocks from both arc and non-arc environments in YTT
originate from variably enriched mantle domains, ranging from
ultra-depleted (boninites) to enriched (OIB). There is a continuum of compositions between these end-members, but with
arc rocks tending toward more depleted compositions, and
non-arc rocks tending toward more enriched compositions.
Rocks with intermediate signatures can be explained by mixing
between the enriched and depleted end-members in the mantle
source region. There are no inter-cycle variations in the composition of the YTT mafic rocks, implying that both the underlying mantle and the processes of magma generation remained
similar throughout its mid- to late Paleozoic evolution.
(5) Felsic rocks from the YTT, although varying in absolute HFSE
and REE contents, are predominantly derived from recycled
upper continental crust (UCC). Most felsic rocks have LREEenrichment and relatively flat REE patterns relative to UCC
(La/SmUCN ≈1). This, coupled with published Nd, Sr and Pb
isotopic data, and the common occurrence of inherited zircon,
all imply derivation from crustal protoliths. This UCC protolith
was similar to the North American craton; however, more data
is required to establish definitively whether the basement of
YTT was the North American craton.
(6) Many mafic rocks from YTT intra-arc rifts and back-arc basins
have Nb/Th mn and Nb/Lamn > 1, implying excess Nb relative to
Th and La relative to the primitive mantle. Excess Nb relative
to Th and La in the primitive mantle in these rocks implies a
recycled oceanic crustal component in their genesis. Recycled
oceanic crust is a feature common in rocks associated with
many large igneous provinces (LIP); however, since this component exists in all cycles of YTT magmatism, it is highly unlikely that these rocks represent a LIP, because YTT magmatism
spans more than 150 m.y. and affects an area in excess of
250,000 km2 over this time frame. This signature in YTT alkalic
basalts likely reflects the reactivation of recycled LIP components resident in the lithospheric mantle that were originally
derived via lithospheric fertilization by LIP magmatism during
the Neoproterozoic breakup of Rodinia.
ACKNOWLEDGEMENTS
Steve Piercey has been supported by Natural Sciences and
Engineering Research Council (NSERC) post-graduate scholarships,
the Yukon Geological Survey (YGS), the Geological Survey of
Canada (GSC), a Geological Society of America student research
grant, the Society of Economic Geologists Hickok-Radford Fund,
Atna, Expatriate Resources and an NSERC Discovery Grant. Mitch
Mihalynuk is thanked for providing unpublished data from the
Big Salmon Complex. Funding for the analytical work presented
herein was provided by the GSC (operating funds and NATMAP
projects), U.S. Geological Survey, YGS and NSERC. We thank our
colleagues for invigorating and stimulating discussions over the
years and continued collaboration, in particular: Don Murphy,
317
Piercey et al .
Jim Mortensen, Jim Ryan, Steve Gordey and Rob Creaser. Reviews
by Dwight Bradley, Edward DuBray and Brendan Murphy are gratefully acknowledged.
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