Boninitic magmatism in a continental margin setting, YukonTanana terrane, southeastern Yukon, Canada
Stephen J. Piercey*
Mineral Deposit Research Unit, Department of Earth and Ocean Sciences, University of British
Columbia, 6339 Stores Road, Vancouver, British Columbia V6T 1Z4, Canada
Donald C. Murphy 2-Yukon Geology Program, P.O. Box 2703 (F-3), Whitehorse, Yukon Y1A 2C6, Canada
James K. Mortensen Mineral Deposit Research Unit, Department of Earth and Ocean Sciences, University of British
Columbia, 6339 Stores Road, Vancouver, British Columbia V6T 1Z4, Canada
Suzanne Paradis Mineral Resources Division, Geological Survey of Canada, 9860 West Saanich Road, Sidney, British
Columbia V8L 4B2, Canada
ABSTRACT
Mid-Paleozoic mafic rocks in the Finlayson Lake region of the Yukon-Tanana terrane,
southeastern Yukon, Canada, have the diagnostic geochemical signatures of boninites:
high MgO, Cr, Ni, and Co contents, intermediate SiO2 contents, high Mg#’s (MgO/
(MgO1FeO*), Al2O3/TiO2, and Zr(Hf)/middle rare earth element (REE) ratios; low TiO2,
REE, and high-field-strength element contents; and U-shaped primitive mantle–normalized trace element patterns. However, unlike most modern and ancient boninitic rocks
that are typically associated with intraoceanic realms, those from the Finlayson Lake
region are part of a mid-Paleozoic continental margin arc-backarc magmatic system. We
propose a model in which the boninitic rocks from the Finlayson Lake region formed as
a result of spreading ridge propagation into an arc built on composite basement of oceanic
and continental crust. In the oceanic segment, upwelling asthenosphere induced melting
of a subducted-slab metasomatized refractory mantle source to form boninitic magmatism.
In the continental sector, upwelling asthenospheric mantle, and/or the melts derived thereof, induced crustal melting, which explains the large volume of temporally equivalent felsic
volcanic and intrusive rocks.
Keywords: boninite, geochemistry, magmatism, continental crust, Yukon-Tanana terrane, arcs.
INTRODUCTION
Boninitic magmatism represents an enigmatic style of magmatism that is interpreted
to reflect melting of highly refractory mantle
source regions that have been variably metasomatized by subducted slab fluids (e.g.,
Crawford et al., 1989; Pearce et al., 1992).
These primitive arc rocks have distinctive
geochemical characteristics, including (1) high
MgO, Mg#’s, compatible element contents
(Ni, Cr, Co) and Al2O3/TiO2 ratios, (2) low
TiO2, (3) intermediate (andesitic) SiO2 contents (.53wt%), (4) U-shaped rare-earth element (REE) patterns, (5) extreme high-fieldstrength element (HFSE) depletions, and (6)
commonly, but not always, Zr and Hf enrichments relative to the middle REE (Crawford
et al., 1989; Pearce et al., 1992).
These compositionally distinct rocks have
been reported in a variety of settings, including modern forearc regions of intra-oceanic island arcs (Falloon and Crawford, 1991; Pearce
et al., 1992), Phanerozoic and Proterozoic supra–subduction zone (SSZ) ophiolites (Rogers
et al., 1989; Meffre et al., 1996; Wyman,
1999), and Archean greenstone belts (Kerrich
et al., 1998). A common feature of all these
boninite suites is their occurrence in SSZ (intraoceanic) settings far removed from conti*E-mail: [email protected].
nental crust. Although boninite-like rocks
have been documented in continental margin
environments (e.g., bajaiites, sanukitoids;
Rogers and Saunders, 1989), to our knowledge boninites (sensu stricto) have not previously been documented within a continental
or epicontinental setting (e.g., continental arc
and/or backarc). In the Finlayson Lake region
in southeastern Yukon, Canada, boninitic
(sensu stricto) volcanic rocks are spatially,
temporally, and stratigraphically linked to
continental arc and backarc magmatism in a
region partially underlain by continental crust
or continent-derived rocks. In this paper we
document boninitic magmatism within such
an unusual setting.
GEOLOGIC SETTING AND
STRATIGRAPHY
The Yukon-Tanana terrane in the Finlayson
Lake region of southeastern Yukon (Fig. 1)
consists of variably deformed and metamorphosed volcanic and sedimentary rocks of
pre–Late Devonian to Permian age (Murphy,
1998; Murphy and Piercey, 1999, 2000). Although these rocks are locally highly deformed, an original stratigraphic succession
has been determined from regional mapping.
The terrane in the Finlayson Lake region has
been subdivided into three unconformitybounded stratigraphic successions: the Grass
Lakes succession (upper Devonian to lowest
Mississippian), Wolverine Lake succession
(Lower Mississippian), and Campbell Range
succession (Pennsylvanian-Permian) (Fig. 1;
Murphy and Piercey, 2000).
Boninitic rocks form part of the Fire Lake
unit of the Grass Lake succession. The lowermost stratigraphic unit within the Grass
Lakes succession consists of a pre-Devonian
(.365 Ma) assemblage of quartz-rich, noncarbonaceous siliciclastic, with lesser carbonate, metasedimentary rocks (Fig. 1). Overlying this unit is the Fire Lake unit; the contact
between the two units is marked by interlayering of psammite and biotite-chlorite schist
identical to the overlying mafic schist of the
Fire Lake unit, implying a transitional contact.
Alternatively, interlayered mafic schist may be
transposed dikes feeding the Fire Lake unit.
In either case, however, the Fire Lake unit is
tied stratigraphically to unit 1. Above this contact, the Fire Lake unit consists of variably
metamorphosed and deformed ;365–360m.y.-old (Mortensen, 1992a; Grant, 1997) mafic-dominated succession of volcanic rocks
with geochemical signatures that vary from
boninitic through island-arc tholeiitic, calcalkalic and non-arc (Figs. 1 and 2; Grant,
1997; Piercey et al., 1999). The Fire Lake unit
passes conformably upward into the ;360m.y.-old felsic volcanic– and sedimentary
rock–dominated Kudz Ze Kayah unit, which
is in turn unconformably overlain by the Mississippian felsic volcanic– and sedimentary
rock–dominated Wolverine succession and the
mafic-dominated late Paleozoic Campbell
Range succession (Murphy and Piercey,
2000).
The arc-backarc systems of the Yukon-Tanana terrane are thought to have been formed
on crust of continental or transitional composition. This conclusion is based on the ubiquitous presence of inherited zircons in felsic
volcanic and intrusive rocks and their radiogenic isotopic compositions (Mortensen,
1992a, 1992b, 1994; Grant, 1997). The quartzrich and noncarbonaceous nature of the basal
clastic unit of the Grass Lakes succession also
implies proximity to a continental mass.
q 2001 Geological Society of America. For permission to copy, contact Copyright Clearance Center at www.copyright.com or (978) 750-8400.
Geology; August 2001; v. 29; no. 8; p. 731–734; 3 figures; Data Repository item 2001086.
731
middle REE (Zr/Zr* 5 1.2–2.6; Hf/Hf* 5
1.3–2.8; Zr/Sm 5 26.2–58.6; Fig. 3A), typical
of boninites (Fig. 3A; Hickey and Frey, 1982;
Pearce et al., 1992).
The REE-HFSE and transitional element
(TE) systematics of boninitic rocks are distinct
from those of most ocean floor basalts (Fig.
3, B and C). The boninites generally have low
TiO2 contents and overlap fields for modern
boninites, for Cambrian low-Ti tholeiites
(LOTI) from Tasmania, and they are similar
to the north Tonga Ridge HCB but are displaced to lower La/Sm values (Fig. 3). The
low TiO2 values coupled with the elevated Sc
and V contents result in low Ti/Sc (17.5–50.3)
and Ti/V values (3.6–9.6), typical of boninites
(Hickey and Frey, 1982), and overlapping the
fields for modern boninites, the Tongan HCB,
and HCB from the Troodos ophiolite (Fig. 3).
The geochemical features outlined above
and comparisons to modern and ancient boninites clearly illustrate that volcanic rocks
from the Fire Lake unit are boninitic in composition, and, more specifically, similar to
HCB.
Figure 1. Simplified geologic map of Yukon-Tanana terrane in Finlayson Lake
region, Yukon, Canada. Stars north of Fyre Lake deposit are boninite locations,
all other boninitic rocks come from drill core from Fyre Lake volcanic-hosted
massive sulfide deposit. Modified after Murphy and Piercey (2000).
GEOCHEMICAL CHARACTERISTICS
OF THE BONINITIC ROCKS
The boninitic rocks of the Fire Lake unit
(Fig. 3; Table 11) have variable SiO2 (48.9–
59.2 wt%), MgO (8.8–16.8 wt%), and Mg’s
(52.5–65.71), reflecting element mobility during seawater alteration and/or metamorphism.
Nevertheless, the average SiO2 (52.9 wt%),
MgO (13.3 wt%), and Mg# (60.4) of these
rocks are consistent with Crawford et al.’s
(1989) boninite definition (SiO2 $ 53 wt%;
Mg# $ 60). The high MgO contents are coupled with high Ni (80–507 ppm), Co (26–80
ppm), and Cr (282–1590 ppm) contents. In
contrast, they have low TiO2 content (0.1–0.3
wt%), moderate Al2O3 content (7.7–14.6
wt%), and high Al2O3/TiO2 ratios (45.1–83.3).
The CaO/Al2O3 ratio of the samples from the
Fire Lake unit range from 0.3 to 1.2, reflecting
1GSA Data Repository item 2001086, Table 1,
Geochemical data for boninitic rocks from the Finlayson Lake region, is available on request from
Documents Secretary, GSA, P.O. Box 9140, Boulder, CO 80301, [email protected] or at
www.geosociety.org/pubs/ft2001.htm.
732
likely CaO mobility due to seawater alteration
and/or regional metamorphism; however, the
average CaO/Al2O3 ratio of the samples (0.7)
is similar to Crawford et al.’s (1989) high-Ca
boninite (HCB) designation (CaO/Al2O3 .
0.8). Furthermore, the presence of preserved
plagioclase phenocryst pseudomorphs in
many samples and moderate Sc contents (30–
55 ppm) is typical of HCB (Crawford et al.,
1989; Meffre et al., 1996). The Fire Lake unit
samples have Zr/Y 5 0.9–3.0, and Zr/Hf 5
26.2–38.1. On a primitive mantle–normalized
plot (Fig. 3A) the Fire Lake boninites have
distinctive U-shaped trace element patterns
with strong medium REE depletion relative to
the heavy REE ([Gd/Yb]n 5 0.3–0.8) and minor light REE depletion to slight enrichment
([La/Sm]n 5 0.7–2.6) typical of boninites
(e.g., Hickey and Frey, 1982; Pearce et al.,
1992). The extreme HFSE depletion of the
Fire Lake unit boninites (Fig. 3) is illustrated
by their very low Nb (0.4–0.9 ppm), Zr (5.4–
20.0 ppm), and Hf (0.2–0.5 ppm) contents.
However, in spite of their overall depletion, Zr
and Hf are slightly enriched relative to the
EVIDENCE FOR BONINITIC
MAGMATISM LINKED TO
CONTINENTAL CRUST
Numerous lines of field, geochemical, and
isotopic evidence suggest that the basement
beneath the Fire Lake unit was, at least in part,
continental. Isotopic evidence from igneous
rocks throughout the Finlayson Lake area
clearly points to a basement of continental
crust or derivation from continental crust (sedimentary). Most Late Devonian to midMississippian felsic igneous rocks in the Finlayson Lake area are isotopically evolved and
have abundant inherited Proterozoic zircon
(Mortensen, 1992a, 1992b, 1994; Grant, 1997;
summarized in Fig. 2). These include felsic
volcanic rocks in the Money Creek thrust
sheet that are coeval with the Fire Lake unit;
younger felsic volcanic rocks of the Kudz Ze
Kayah unit of Grass Lakes succession and felsic volcanic and subvolcanic rocks of Wolverine succession; and younger intrusions of the
Grass Lakes and Simpson Range plutonic
suites, both of which intrude the Fire Lake
unit (Fig. 2). The isotopic character and peraluminous nature of the Grass Lakes suite, in
particular, suggest that it is the product of
partial melting of continental crustal basement (Mortensen, 1992a). Second, mafic
rocks of the Fire Lake unit have eNdt values
that range from 13.8 to 24.8, compatible
with variable influence from juvenile mantle
and evolved (TDM 5 0.78–2.00 Ga) crustal
material (Fig. 2; Grant, 1997). Third, Pbisotopic data on sulfides from felsic volcanicassociated and syngenetic volcanic-hosted
massive sulfide (VHMS) mineralization
(Mortensen, 1994) have evolved crustal PbGEOLOGY, August 2001
Figure 2. Schematic cross section through post–boninite formation Money Creek thrust illustrating stratigraphic relationships of boninitic rocks with various successions and relationships to rocks of interpreted continental origins. U-Pb zircon ages and inheritance information are from Mortensen (1992a, 1992b, 1994) and Grant (1997). Nd and Sr isotopic
data are from Grant (1997), with exception of Grass Lakes granitoid suite, which is from
Mortensen (1992a, 1994). Pb isotopic data are from Mortensen (1992a, 1994). Legend as in
Figure 1. Modified from Murphy and Piercey (2000).
isotope signatures that lie on the ‘‘shale
curve,’’ a strongly radiogenic Pb-isotopic
growth curve (m 5 12.16) defined for miogeoclinal rocks of the Cordillera (Godwin
and Sinclair, 1982; Fig. 2).
The Fire Lake unit is locally stratigraphically sandwiched by quartz-rich clastic rocks
of pericratonic nature and provenance. It everywhere overlies with transitional contact a
unit of quartz-rich clastic rocks, pelite, and
marble with negative eNdt (t 5 350 Ma; 21.7
to 211.4) values and Proterozoic depletedmantle model ages (TDM 5 1.38–1.98 Ga)
(Fig. 2). At the Fyre Lake massive sulfide deposit, boninitic volcanic rocks are overlain by
massive sulfide and up to 700 m of quartzrich turbiditic sedimentary rocks (Foreman,
1998). Within this sedimentary package, Foreman (1998) has documented mature sandstone
layers up to 10 m thick, with lesser volcanicderived sandstone, chert, and limestone. It is
uncertain whether these are first-cycle quartzrich material from a cratonal basement or a
recycled component from sedimentary and igneous rocks derived from a continental block.
Nonetheless, the abundance of mature quartzose clastic material in rocks proximal to the
boninites implies a nearby continental source.
Although the bulk of the evidence presented
above points to a dominantly continental character of the Yukon-Tanana terrane lithosphere,
we cannot rule out that the lithosphere immediately underlying the Fire Lake boninites
was of oceanic character. The current location
of the Yukon-Tanana terrane, sandwiched between rocks of the ancestral North American
GEOLOGY, August 2001
continental margin and dominantly allochthonous terranes of intra-oceanic origin,
leaves open the possibility that Yukon-Tanana
terrane is underlain by lithosphere transitional
between continental and oceanic—i.e.,
thinned continental lithosphere with rifted domains within which oceanic crust may have
formed.
DISCUSSION AND CONCLUSIONS
The generation of boninitic magmatism requires unique thermal and petrological conditions (shallow melting, elevated geothermal
gradient, subducted slab flux; Crawford et al.,
1989; Pearce et al., 1992). These conditions
have led to a variety of models for their setting and genesis, including (1) arc formation
and catastrophic melting (Stern and Bloomer,
1992; Pearce et al., 1992), (2) ridge subduction (Crawford et al., 1989), (3) backarc basin
formation (Coish et al., 1982), (4) plume–
island arc interaction (Kerrich et al., 1998),
and (5) ridge propagation into a forearc region
(Falloon and Crawford, 1991; Monzier et al.,
1993; Meffre et al., 1996). A feature common
to all of these models and settings, however,
is that they all form in SSZ or SSZ-like environments (e.g., greenstone belts), with the
imposition of an external heat source on a variably metasomatized, depleted mantle wedge
to generate the boninites.
The geochemical attributes of the Fire Lake
boninites also require satisfaction of these
thermal and petrological source characteristics. Furthermore, their similarity to modern
and ancient HCBs provides additional con-
Figure 3. A: Primitive mantle–normalized
trace element plot for Fire Lake unit boninitic rocks, primitive mantle values from Sun
and McDonough (1989). HCB—high-Ca boninite. B: Modified after Wyman (1999), Brown
and Jenner (1989), and Meffre et al. (1996);
N-MORB—normal mid-oceanic ridge basalt;
LOTI—low-Ti tholeiite. C: From Hickey and
Frey (1982), with additional data from Troodos, Tonga, and Limassol Forest; SHMB—
siliceous high-magnesian basalt. Data from
Troodos, Limassol Forest complex, and Tonga from Cameron (1985), Rogers et al.
(1989), and Falloon and Crawford (1991), respectively. Triangles are boninitic samples
from Finlayson Lake region.
straints on their petrotectonic origin. The HCB
subclass of boninites is found in many ancient
(e.g., Troodos, Koh) and modern (e.g., Tonga,
North Fiji Basin) SSZ settings. In these regions, HCB generation is commonly interpreted to have formed near areas of backarc
spreading (e.g., Falloon and Crawford, 1991;
Monzier et al., 1993; Meffre et al., 1996;
Hawkins and Castillo, 1999), and in the case
of Tonga and the North Fiji Basin, HCB are
associated with the propagation of spreading
ridges into forearc regions (Falloon and Crawford, 1991; Monzier et al., 1993). We propose
that the formation of boninitic rocks within
the Finlayson Lake region and their associa733
tion with continental crust may be due to the
propagation of a spreading ridge into an arc
built on composite basement of continental
and oceanic affinity. In this model, the normal
island arc and calc-alkalic arc volcanism
(Grant, 1997; Piercey et al., 1999) would be
disrupted due to the propagating ridge. Boninite genesis would be due to the influx of
H2O from the subducted slab and to heat associated with ridge-related asthenospheric uprise inducing melting of the refractory mantle
wedge (e.g., Falloon and Crawford, 1991;
Monzier et al., 1993; Meffre et al., 1996); the
subsequent decompression of the asthenosphere would form non-arc (MORB and oceanic-island basalt) volcanic rocks (Piercey et
al., 1999). In the region overlain by continental crust, the upwelling asthenospheric mantle
and/or melts derived from it were likely important in inducing crustal melting, leading to
the formation of the temporally equivalent felsic magmas evident in the Grass Lakes succession (Piercey et al., 2000).
These petrological models can explain the
thermal and petrological requirements of the
Fire Lake boninites; however, those boninites
require satisfaction of an additional constraint—escaping crustal contamination. We
suggest that rapid arc rifting associated with
ridge propagation resulted in extreme extension and attenuation of the crust (e.g., Stern
and Bloomer, 1992). Rapid arc extension resulted in lithosphere-penetrating synvolcanic
faults that would have quickly removed any
crustal material that could have contaminated
the boninites and would have provided conduits that would permit rapid ascent of the
boninites to the surface. Field evidence for
rapid extension, arc rifting, and synvolcanic
faulting is supported by an increase in the intensity of mafic-ultramafic intrusions proximal
to the boninitic rocks and by the thick clastic
infill sequence overlying the Fyre Lake
VHMS deposit (Figs. 1 and 2; Foreman, 1998;
Murphy and Piercey, 2000).
We do not necessarily advocate that spreading-ridge propagation is the sole model for
boninite genesis within the Finlayson Lake region; other models may be plausible. Additional chemostratigraphy and temporal constraints are required to evaluate this and other
possible models (e.g., arc initiation, backarc
initiation, slab window). What is significant,
however, is that numerous lines of field, geochemical, and isotopic evidence can link boninitic magmatism (sensu stricto) to continental
margin arc and backarc magmatism, an ex-
734
tremely unusual
magmatism.
setting
for
boninitic
ACKNOWLEDGMENTS
We thank our colleagues for discussions; W. Roberts and
Columbia-Yukon Gold Mines Ltd. for providing access to
the Fyre Lake deposit; and Alan Galley, James Hawkins,
Richard Arculus, and Rosemary Hickey-Vargas for reviews
of the manuscript. Funding was provided by the Yukon Geology Program, Geological Survey of Canada and Ancient
Pacific Margin NATMAP Project, Atna Resources and Expatriate Resources, a Natural Sciences and Engineering Research Council (NSERC) of Canada operating grant, a postgraduate NSERC scholarship, the Hickok-Radford Fund of
the Society of Economic Geologists, and a Geological Society of America student research grant. This is Mineral
Deposit Research Unit contribution P-141.
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Manuscript received October 30, 2000
Revised manuscript received April 4, 2001
Manuscript accepted May 3, 2001
Printed in USA
GEOLOGY, August 2001