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Lithosphere
U-Pb-Hf characterization of the central Coast Mountains batholith:
Implications for petrogenesis and crustal architecture
M. Robinson Cecil, George Gehrels, Mihai N. Ducea and P. Jonathan Patchett
Lithosphere 2011;3;247-260
doi: 10.1130/L134.1
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RESEARCH
U-Pb-Hf characterization of the central Coast Mountains
batholith: Implications for petrogenesis and crustal
architecture
M. Robinson Cecil*, George Gehrels, Mihai N. Ducea, and P. Jonathan Patchett
DEPARTMENT OF GEOSCIENCES, UNIVERSITY OF ARIZONA, TUCSON, ARIZONA 85721, USA
ABSTRACT
We present U-Pb geochronologic and Hf isotopic data from 29 plutonic samples within the Coast Mountain batholith, north-coastal British
Columbia and southeast Alaska. Hf isotopic values do not correlate with age or variation in magmatic flux, but rather they increase systematically from west (εHf[t] = +2 to +5) to east (εHf[t] = +10 to +13) in response to changing country rock assemblages. By comparing our pluton
Hf data with previously reported Nd-Sr and detrital zircon characteristics of associated country rocks, we identify three crustal domains in
an area where crustal affinity is largely obscured by metamorphism and voluminous pluton intrusion: (1) a western domain, emplaced into
continental-margin strata of the Banks Island assemblage; (2) a central domain, emplaced into the Alexander terrane; and (3) an eastern
domain, underlain by the Stikine terrane and its inferred metamorphic equivalents. Between the interpreted Alexander and Stikine terranes,
there is a zone of variable εHf(t) (+2 to +13) that coincides with the suture zone separating inboard (Stikine and Yukon-Tanana) from outboard
(Alexander and associated) terranes. This variation in εHf(t) values apparently results from the structural imbrication of juvenile (Alexander
and Stikine) and evolved (Yukon-Tanana) terranes along mid-Cretaceous thrust faults and the latest Cretaceous–early Tertiary Coast shear
zone. Shifts in the Hf values of plutons across inferred terranes imply that they are separated at lower- to midcrustal levels by steep boundaries. Correlation between these Hf values and the isotopic character of exposed country rocks further implies the presence of those or similar
rocks at magma-generation depths.
LITHOSPHERE; v. 3; no. 4; p. 247–260.
INTRODUCTION
The Coast Mountains batholith is a 1700-km-long belt of Jurassic
through Tertiary plutonic rocks that extends along the length of coastal
British Columbia, southeast Alaska, and southwestern Yukon (Fig. 1).
These plutonic rocks are emplaced along the suture zone between two
large arc-type fragments: the Alexander and Wrangellia terranes to
the west, and the Stikine and associated terranes to the east. Although
clearly related to plate convergence along the western margin of North
America (e.g., Engebretson et al., 1985), it has long been suspected
that Jurassic–Cretaceous collision of terranes along this suture played a
significant role in the generation and exhumation of igneous rocks that
make up the Coast Mountains batholith (e.g., Monger et al., 1982). This
report uses Hf isotope data from plutonic rocks within and adjacent to
the Coast Mountains batholith to investigate the petrogenesis of granitoids that make up this segment of the Coast Mountains batholith, and
the architecture of the terranes that comprise the central-western Coast
Mountains. The coupled Hf and geochronologic data shed new light
on the nature of terranes at depth and the structural boundaries that
separate them.
Much attention has been paid to the metasedimentary assemblages
that occur as pendants within the widespread Jurassic through Eocene
Coast Mountains batholith (e.g., Samson et al., 1991a; Jackson et al.,
1991; Boghossian and Gehrels, 2000; Gareau and Woodsworth, 2000)
(Fig. 2). Typically, these rocks are highly deformed and metamorphosed
*Current address: Division of Geological and Planetary Sciences, California
Institute of Technology, Pasadena, California 91125, USA; [email protected].
doi: 10.1130/L134.1
to amphibolite or even granulite grade (Hollister and Andronicos, 2000).
Because of the voluminous nature of the plutonic rocks and the high
grade of metamorphism, contacts are commonly obscured, and protolith
determinations are difficult to make. This in turn has caused difficulties in correlating pendant rocks with terranes described for other parts
of the Cordillera, and it has led to a limited understanding of the tectonostratigraphic relationships between the various crustal assemblages
making up the central Coast Mountains batholith. In various parts of the
batholith, however, the Nd and Sr isotopes of the metamorphic country
rocks have been studied (Samson et al., 1990, 1991a, 1991b; Jackson et
al., 1991; Patchett et al., 1998; Boghossian and Gehrels, 2000; Gareau
and Woodsworth, 2000). These isotopic data, together with published
U-Pb detrital zircon data and geologic observations, allow us to define
the isotopic character of the various amalgamated terranes that make
up the Coast Mountains. Hf isotopes of magmatic rocks that have interacted, even to a small degree, with a given terrane, should record signatures consistent with that region. Hf data from widely distributed plutons
within the Coast Mountains batholith can therefore be used to identify
the terranes with which the plutons interacted and/or partly assimilated.
Hafnium isotopes in magmatic zircons also act as probes for the chemical maturity of the lithosphere from which the melts were extracted. As
such, the distribution of Hf isotopic signatures across the batholith can
be used to glean important information about the tectonic construction
of the terranes that make up the central Coast Mountains. Terranes are
commonly conceptualized as discrete, coherent blocks of lithosphere
that are accreted to, or transported laterally along, the margins of continents. In such a conceptual framework, terranes form “side-by-side”
panels separated from adjacent terranes or continents by through-going,
| Volume
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CECIL ET AL.
GEOLOGIC AND TECTONIC FRAMEWORK OF THE COAST
MOUNTAINS BATHOLITH
13
8ºW
Skagway
Coast
Mountains
batholith
58
ºN
Juneau
Undifferentiated
metamorphic rocks
(locally YukonTanana terrane)
Chugach
terrane
58º
Petersburg
N
128
ºW
Gravina and
TyaughtonMethow Basins
Alexander terrane
Ketchikan
Bowser
Basin
Western Portion of the Coast Mountains Batholith
Wrangellia terrane
Prince Rupert
Stikine
terrane
Queen
Kitimat
Charlotte
Taku terrane
Islands
Yukon-Tanana
terrane
52º
N
Pacific
Ocean
Banks Island
Assemblage
Bella Coola
Fig. 2
Thrust fault
?
Main trace of
Coast shear zone
0
50
100
km
150
124
ºW
Strike-slip fault
52º
N
N
Figure 1. Geologic framework of the Coast Mountains batholith (CMB),
modified from Wheeler and McFeely (1991), Wheeler at al. (1991), and
Gehrels et al. (2009). Outline represents the study area shown in Figure 2.
vertical structures. Alternatively, during their accretion to or collision
with an existing margin, terranes can become imbricated by large-scale
thrusts, making the boundaries between them more diffuse and less
steep, as is the case with the mid- to Late Cretaceous fold-and-thrust system that developed along the boundary between the Insular (Wrangellia
and Alexander) and Intermontane (Stikine and Yukon-Tanana) superterranes (Rubin et al., 1990). The thrust belt thickened the crust and effectively stacked rocks of the various existing terrane assemblages, laterally
smearing them along thrusts. We use the arc-perpendicular distribution
of Hf isotopes to evaluate the nature of the terrane boundaries and the
degree to which the terranes have been structurally interleaved.
248
Most of the igneous rocks that make up the Coast Mountains batholith
are tonalitic and range in age from 160 Ma to 50 Ma (Gehrels et al., 2009).
In general, the ages become younger progressively eastward, although
there are also Jurassic ages in the easternmost part of the Coast Mountains
batholith at the latitude of the study area. This is interpreted to result from
sinistral strike-slip duplication of the Jurassic portion of the batholith during Early Cretaceous time (Gehrels et al., 2009).
Country rocks of the batholith are generally mid- to high-grade
metasedimentary assemblages derived from marine strata (Wheeler and
McFeely, 1991). Due to the interpreted geologic setting of these protoliths, and supported by Nd-Sr (e.g., Samson et al., 1989) and detrital
zircon (e.g., Gehrels and Boghossian, 2000) data, the country rocks are
interpreted to have formed in settings ranging from juvenile volcanic arcs
to pericratonic passive margins.
The lithologic and isotopic character of the main plutonic suites and
their country rocks are described next and shown in Figures 1 and 2.
The western portion of the Coast Mountains batholith is underlain by
three distinct belts of plutonic rocks of Late Jurassic, Early Cretaceous,
and mid-Cretaceous age. The ages of these bodies decrease systematically
eastward (van der Heyden, 1989, 1992; Butler et al., 2006). Their composition also changes eastward, from predominantly quartz diorite on the
west to mainly tonalite on the east. The emplacement depth of these bodies ranges from ~10 km on the west to ~25 km on the east (Butler et al.,
2001, 2006), a change that is also reflected in the increasing metamorphic
grade of the metasedimentary host rocks. A magmatic flux curve for the
western portion of the Coast Mountains batholith, based on ages from
these plutons, suggests high-flux periods from 160 to 140 Ma and 120 to
80 Ma, with little magmatism between 140 and 120 Ma (Gehrels et al.,
2009, their fig. 9).
Country rocks to these plutons (Fig. 1), from west to east, include the
Wrangellia terrane, Banks Island assemblage, Alexander terrane, Gravina
belt, and Yukon-Tanana terrane, as described in the following.
The Wrangellia terrane in the study area consists of Upper Paleozoic arc-type metavolcanic and metasedimentary rocks and Triassic riftrelated(?) pillow basalts (Monger et al., 1992). These rocks presumably
formed in a marine volcanic arc setting on the basis of their primitive Nd-Sr
signature of correlative rocks on Vancouver Island (Samson et al., 1990).
The Banks Island assemblage consists mainly of highly folded
quartzite interlayered with marble and subordinate pelitic schist (Gehrels and Boghossian, 2000). These rocks are intruded by a ca. 357 Ma
orthogneiss and are crosscut by nondeformed dikes that are ca. 147 Ma
in age (G. Gehrels, 2010, personal commun.). Detrital zircons in the
quartz-rich strata yield dominant ages of 410–480 Ma, 1700–1850 Ma,
1940–2250 Ma, and 2620–2940 Ma. Metamorphic rocks of the Banks
Island assemblage have evolved continental isotopic signatures, with initial Nd values ranging between +0.5 and −9.9, and relatively radiogenic
Sr values ranging from 0.71178 to 0.71934 (Boghossian and Gehrels,
2000). These rocks are interpreted to have formed in a continental-margin environment because of the occurrence of interlayered metaclastic
quartzite and marble and cratonal detrital zircon and Nd-Sr signatures.
The occurrence of ca. 410–480 Ma detrital zircons, however, suggests
possible connections with the Alexander terrane.
The Alexander terrane is composed of Neoproterozoic–Cambrian and
Ordovician–Silurian meta-igneous and metasedimentary rocks that are
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Hf isotopes of the Coast Mountains batholith
498 Ma
9.8
405 Ma
10.4
445 Ma
12.0
0°
Coast
shear
zone
394 Ma
12.2
Al
W
414 Ma
12.9
13
430 Ma
8.6
| RESEARCH
a
Ca ska
na
da
Coast
Mountains
batholith
68 Ma
10.3
Banks Island
assemblage
101 Ma
5.4
Prince Rupert
Gravina belt
92 Ma
6.0
Metamorphic
rocks of the
CGC (locally
YTT and
Stikine)
Terrace
58 Ma
1.5
119 Ma
7.5
12
8°
W
Alexander
terrane
X′
53 Ma
2.0
125 Ma
12.0
Kitimat
89 Ma
8.1
60 Ma
9.7
Stikine
terrane
142 Ma
2.7
142 Ma
5.4
Assemblage
boundary
Coast
shear
zone
94 Ma
6.8
56 Ma
9.2
55 Ma*
9.6
54
°N
X
153 Ma
4.3
142 Ma
5.2
Klemtu
N
0
50
25
95 Ma
11.6
124 Ma
2.6
km
82 Ma*
11.2
Pb/238U age
(*denotes samples
with zircon
inheritance)
εHf(t)
12
206
6°
W
97 Ma
8.1
Bella
Bella
76 Ma
7.3
82* Ma
11.2
151 Ma
12.3
96* Ma
9.8
Bella Coola
52
°N
Figure 2. Locations, U-Pb ages, and εHf(t) values of plutonic rocks sampled from the west-central Coast Mountains batholith, British Columbia, and from the Alexander terrane, southeast Alaska. Generalized geology is
modified from Wheeler and McFeely (1991), and Wheeler et al. (1991). Hf isotope and U-Pb age data are also
available in Table 2. CGC—central gneiss complex; YTT—Yukon-Tanana terrane.
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CECIL ET AL.
interpreted to have formed in a marine volcanic arc (Gehrels and Saleeby,
1987). There is no sign of continental input in these assemblages in the
geologic units (e.g., quartz-rich clastic strata; Gehrels and Saleeby, 1987),
U-Pb geochronologic data (e.g., Precambrian inherited or detrital zircons; Gehrels et al., 1987, 1996), or Nd-Sr isotopes (Samson et al., 1989).
Beginning in Early Devonian time, conglomeratic strata (referred to as the
Karheen Formation) were shed from a source to the southwest (in present coordinates) that included rocks of 1120–2230 Ma age (Gehrels et
al., 1996). Middle and Upper Paleozoic strata consist of shallow-marine
clastic strata and carbonates with little sign of arc- or craton-derived detritus. The sequence is capped by a Triassic assemblage of rift-related(?)
metavolcanic and metasedimentary rocks.
The Gravina belt consists of Upper Jurassic through Lower Cretaceous volcaniclastic turbidites and subordinate mafic and felsic
metavolcanic rocks (Berg et al., 1972). These strata are interpreted to
depositionally overlie the Alexander terrane to the west and the YukonTanana terrane (described next) to the east (Gehrels, 2001). Detrital
zircons in the metaclastic rocks record derivation from both the Alexander and Yukon-Tanana terranes, which suggests proximity of the
two terranes by Late Jurassic time (Gehrels, 2001), as indicated on the
basis of geologic relations examined by McClelland et al. (1992) and
Saleeby (2000).
The Yukon-Tanana terrane consists of a Proterozoic–Lower Paleozoic assemblage of mainly quartz-rich metaclastic rocks (psammitic
schist) and marble, a mid-Paleozoic assemblage of metavolcanic rocks,
and Upper Paleozoic metasedimentary and metavolcanic rocks (Gehrels
et al., 1992; Gehrels, 2001). In the study area, these rocks are locally
referred to as the Scotia-Quaal assemblage, and they consist mainly of
Devonian–Mississippian metavolcanic rocks and orthogneisses (Gareau,
1989). Although somewhat ambiguous, Nd-Sr data from the Scotia-Quaal
are more evolved than those from the Wrangellia, Alexander, and Stikine
terranes, and they indicate the presence of old continental crust (Gareau
and Woodsworth, 2000).
Primary relations between these assemblages are difficult to document
because of younger deformation, pluton intrusion, and/or lack of exposure. For example, the contact between Wrangellia and the Banks Island
assemblage is everywhere under water or intruded by Mesozoic plutons,
the contact between the Banks Island assemblage and Alexander terrane is
a major sinistral strike-slip fault (Kitkatla shear zone) of Early Cretaceous
age, and the Yukon-Tanana terrane and eastern Gravina belt are highly
imbricated along west-vergent thrust faults of mid-Cretaceous age (Chardon et al., 1999; Gehrels et al., 2009).
the Shames mylonite zone (Heah, 1990, 1991; Andronicos et al., 2003) or
eastern boundary detachment (Rusmore et al., 2005).
Metasedimentary assemblages within axial portions of the Coast
Mountains batholith, which are commonly referred to as the Central
Gneiss complex, consist mainly of pelitic and psammitic schist with subordinate quartzite, marble, and calc-silicate gneiss (undivided metamorphic rocks in Figs. 1 and 2). These rocks are generally sillimanite grade,
locally with sillimanite replacing kyanite and/or staurolite (Stowell and
Crawford, 2000; Hollister and Andronicos, 2000; Rusmore et al., 2005).
The tectonic affinity of these metasedimentary rocks in the study area is
uncertain. To the north, in southeast Alaska, detrital zircon and Nd-Sr analyses record derivation primarily from continental source regions (Samson
et al., 1990; Gehrels et al., 1992). Regional correlations and northward
continuity suggest that these rocks belong to the Yukon-Tanana terrane.
In the study area, however, quartzites and marbles that are characteristic
of the Yukon-Tanana terrane are rare, and some workers have suggested
correlations with strata of the Stikine terrane (Hill, 1985).
Eastern Portion of the Coast Mountains Batholith
The eastern portion of the Coast Mountains batholith consists of
Jurassic through Eocene plutons that intrude low-grade Upper Paleozoic through Tertiary sedimentary and volcanic rocks of the Stikine terrane and overlying strata of the Bowser Basin (Wheeler and McFeely,
1991; Haggart et al., 2006a, 2006b, 2007; Mahoney et al., 2007a, 2007b,
2007c, 2007d, 2007e, 2009). The Jurassic–Cretaceous history of this
portion of the batholith is somewhat different from that of the western
portion because magmatism did not migrate eastward, and it was apparently continuous through Early Cretaceous time (Gehrels et al., 2009;
Mahoney et al., 2009).
The Stikine terrane consists largely of widespread Triassic and Jurassic arc-type assemblages blanketed by Jurassic–Cretaceous marine strata
of the Bowser Basin (Monger et al., 1992). Locally, arc-type volcanic
and sedimentary assemblages as old as Devonian are exposed. Available
Nd-Sr data from these rocks suggest formation in a juvenile arc setting
with little continental influence (Samson et al., 1989). The only exception
to this is the occurrence of inherited zircons of Precambrian age in Early
Jurassic plutons, which may reflect the presence of Precambrian basement
in some portions of the terrane (Thorkelson et al., 1995). Alternatively, it
has been suggested that the Stikine terrane rests depositionally on rocks
of the Yukon-Tanana terrane (e.g., McClelland et al., 1992; Jackson et al.,
1991), which carries predominantly Precambrian detrital zircons.
Axial Portion of the Coast Mountains Batholith
ANALYTICAL STRATEGY AND METHODS
Axial portions of the Coast Mountains are underlain primarily by
Paleocene tonalitic sills, large bodies of Eocene granodiorite, and highgrade metasedimentary rocks. Emplacement and ductile deformation of
the Paleocene tonalitic sills occurred during east-side-up motion along the
Coast shear zone, which can be traced for most of the length of the Coast
Mountains (Gehrels et al., 2009). Elsewhere, the Coast shear zone reveals
evidence for older dextral motion (Gehrels, 2000) and younger east-sidedown motion (Rusmore et al., 2001; Hollister and Andronicos, 2006).
Barometric studies of tonalitic sills in southeast Alaska indicate emplacement depths of ~15–20 km (Hollister et al., 1987; Stowell and Crawford,
2000; Rusmore et al., 2005).
East of the sills and Coast shear zone, there are large plutons of homogeneous granodiorite that are primarily of Eocene age. These plutons are
generally nondeformed, but locally they were emplaced along an eastside-down normal fault and associated ductile shear zone referred to as
Zircons from 29 individual plutonic samples were separated, picked,
and mounted, along with appropriate U-Pb and Hf standards, in 2.5 cm
epoxy mounts. Epoxy mounts were imaged in plain light with a binocular microscope, and grain maps were produced. U-Pb analysis was first
performed on individual zircon grains from each sample via ablation of
40-μm-diameter pits using methodology described next. After all U-Pb
analyses for a sample were completed, Hf isotope measurements were
made via ablation on top of the preexisting U-Pb pits; the following sections describe the details of these analyses. This analytical technique
allows for the measurement of both U-Pb and Hf in the same part of the
zircon crystal, which is important for two reasons: (1) Crystallization age
of the analyzed zircons is required for calculating initial 176Hf/177Hf; and (2)
zircons with complex growth histories may record multiple ages, such that
it is necessary to collect Hf and U-Pb data from the same zircon domain.
Because the data are acquired sequentially, and not simultaneously, it is
250
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Hf isotopes of the Coast Mountains batholith
possible that the successive pits drilled for U-Pb and Hf measurement
are sampling different crystal depth domains (Kemp et al., 2009). This is
likely not the case with zircons from the Coast Mountains batholith samples, however, given the characteristic homogeneity of analyzed crystals,
as determined by cathodoluminescence imaging and age mapping of large
grains, and the fact that we observed no marked changes in Hf isotopic
ratios with time during data acquisition.
Instrumentation
Reported U-Pb and Hf isotope data were collected using a New Wave
193 nm ArF laser ablation system coupled to a Nu Plasma HR inductively coupled plasma–mass spectrometer (ICP-MS) at the University of
Arizona (for additional information, see http://sites.google.com/a/laserchron.org/laserchron/). Ablation was performed in a New Wave SuperCell™, and sample aerosol was transported with He carrier gas through
Teflon-lined tubing, where it was mixed with Ar gas before introduction
to the plasma torch. The multicollector (MC) ICP-MS utilizes 12 Faraday
detectors equipped with 3 × 1011 Ω resistors and four discrete dynode ion
counters, which remain fixed as beams are directed into them via an electrostatic zoom lens. For U-Pb analyses, U, Th, and Pb isotopes were measured simultaneously in Faraday collectors, with the exception of 204Pb,
which was measured using an ion counter. For Hf analyses, masses 171
through 180 were all measured simultaneously in Faraday collectors. Pure
Hf solutions and Hf solutions doped with various amounts of Yb and Lu
were introduced in Ar carrier gas via a Nu DSN-100 desolvating nebulizer.
U-Pb Geochronology
Geochronologic analyses presented here were performed by laser
ablation (LA) ICP-MS at the Arizona Laserchron Center using methods described by Gehrels et al. (2008). Laser ablation was done using a
40-μm-diameter spot and a pulse rate of 7 Hz. The laser was run in constant energy mode with output energy of 8 mJ/pulse, which corresponds
to an energy density of ~2 J/cm2 and an estimated excavation rate of 0.7
μm/s. The analytical routine consisted of a 15 s on-peak background measurement with the laser off, followed by 15 s of peak measurement, performed at 1 s integration times, with the laser firing. This results in an
analysis pit of ~15 μm depth.
The samples we used are a subset of those analyzed by Gehrels et
al. (2009), and they generally can be characterized by simple, prismatic
zircons with internal oscillatory zoning and rare inherited components or
younger growth rims. For each sample, single pits were ablated on 20–30
individual zircons. In the case of samples 04GJP-09, 04GJP-13, and
05MT-135, which had zircons with rims and cores distinguishable in cathodoluminescence images and of variable age, multiple analyses (2–5 spots
at 40 μm per spot) were performed on single crystals. Weighted mean
ages were then determined for each component, and a magmatic age was
assigned based on the interpreted igneous domain. Fractionation between
U and Pb was accounted for by bracketing every five measurements with
analysis of a Sri Lankan zircon standard of known age (see Gehrels et al.,
2008). Corrections for the interference of mercury were made by monitoring 202Hg and using the natural ratio of 202Hg/204Hg to subtract the Hg
contribution from mass 204. Corrections for common Pb were made by
measuring 204Pb and assuming an initial Pb composition based on the Pb
evolution model of Stacey and Kramers (1975). Uncertainties for reported
238
U-206Pb ages are ~1%–2% (2σ) and include both a systematic error (typically ~1%–2%), and an error associated with the scatter and precision of
a set of measurements for a given sample (~1%, 2σ) (for details of error
analysis, see Gehrels et al., 2009).
LITHOSPHERE | Volume 3 | Number 4 | www.gsapubs.org
| RESEARCH
Hf Isotope Measurement
Interference Correction
Accurate in situ measurement of Hf isotopes in zircon is made difficult
by the isobaric interferences of 176Yb and 176Lu on 176Hf, the correction of
which has been discussed in detail (e.g., Griffin et al., 2002; Woodhead et
al., 2004; Iizuka and Hirata, 2005; Hawkesworth and Kemp, 2006; Gerdes
and Zeh, 2009; Wu et al., 2006; Kemp et al., 2009). Properly correcting
for 176Yb (and to a lesser degree 176Lu) is critical given that 176Yb/176Hf of
typical zircons is commonly between 10% and 30% and can be as much
as 70%. The ratio of stable isotopes 179Hf/177Hf is used for mass bias corrections, and an exponential mass bias function is used in all calculations.
Interference-free 173Yb and 171Yb were monitored during the Hf analysis
to calculate Yb mass bias (βYb) and the contribution of Yb to the measurement of 176(Hf + Lu + Yb). Because the magnitude of the Yb correction
is so great, small inaccuracies in the Yb mass bias can lead to large analytical errors (Woodhead et al., 2004). Unlike 179Hf/177Hf, the precision
of the 173Yb/171Yb measurement, and consequently the accuracy of βYb,
is dependent upon the Yb signal intensity (Fig. 3). At 171Yb intensities of
less than 0.015 V, it becomes very difficult to reliably estimate βYb, and for
those analyses, Hf mass bias (βHf) was used to correct 176Yb/171Yb. Unfortunately, Chu et al. (2002) and Woodhead et al. (2004) have shown that
Hf and Yb exhibit slightly different fractionation behavior, which we also
observed to be true (Fig. 3C). So, although it is not ideal to use Hf fractionation factors to correct for Yb mass bias, low-Yb zircons require relatively
minor correction, and as such it is possible to use βHf without introducing
large errors to the corrected 176Hf/177Hf ratio. The scatter introduced by
the interference correction is not included in the final error attached to
176
Hf/177Hf values, but it is believed to be a relatively minor contribution to
the quoted uncertainty. If there were a source of significant, unaccounted
error, we would expect a given set of measurements to be overly dispersed.
This is not the case, as evidenced by a mean square weighted deviation
(MSWD) of less than one for all reported sample data (see GSA Supplementary Data1).
The Lu correction was done by monitoring 175Lu and using 176Lu/175Lu
= 0.02653 (Patchett, 1983) and βYb, assuming that Lu behaves similarly to
Yb. All corrections are performed on a line-by-line basis, and in all cases,
Hf and Yb isotope data were normalized to 179Hf/177Hf = 0.72350 (Patchett
and Tatsumoto, 1980) and 173Yb/171Yb = 1.132338 (Vervoort et al., 2004),
respectively. A 176Lu decay constant of 1.876 × 10 −11 (Scherer et al., 2001;
Söderlund et al., 2004) was used in all calculations. Chondritic values of
Bouvier et al. (2008) were adopted for the calculation of εHf values.
Hf Solution Analysis
Analyses of pure Hf solutions, as well as Hf solutions doped with variable amounts of Yb and Lu, were performed to test our ability to reliably
correct for Yb and Lu interferences. Solution analyses were run in three
blocks of 20 measurements, with additional background measurements
being automatically performed between blocks. Backgrounds were measured using electrostatic analyzer deflection for 60 s at the start of the run,
and measurements were integrated over 5 s. For 10 ppb solutions, total Hf
beams of ~5 V were achieved (this is the maximum possible with our 3
× 1011 Ω resistors). Solution data were collected during many analytical
sessions over the course of this study, and Hf standard solution measurements were always made after instrument tuning and before acquisition
1
GSA Data Repository Item 2011234, Weighted mean and concordia plots for all
U-Pb and Hf data presented, is available at www.geosociety.org/pubs/ft2011.htm,
or on request from [email protected], Documents Secretary, GSA, P.O. Box
9140, Boulder, CO 80301-9140, USA.
251
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CECIL ET AL.
0
20
A
-1
B
-1.1
-0.4
-0.8
-1.2
-1.2
βYb(173/171)
βYb(173/171)
βHf(179/177)
10
0
-1.3
-1.4
1
1:
-1.5
-10
-1.6
r)
0.4
0.8
177
1.2
1.6
2.0
-20
ea
lin
-1.6
-2.0
0
C
Temora-2
FC-52
st
(
fit
Be
0
0.02
0.04
171
Hf (V)
0.06
0.08
0.10
Yb (V)
-1.7
-1.7
-1.6
-1.5
-1.4
-1.3
βHf(179/177)
-1.2
-1.1
-1
Figure 3. Plots showing the relationship between Hf and Yb fractionation factors and signal intensity. βHf and βYb were measured simultaneously during
laser-ablation analysis of zircon standards. All laser standard data (line by line data, not analytical averages) are included in plots A and B, whereas
only a subset of averages from Temora-2 and FC-52 standard runs is included in plot C, because those standard zircons have higher concentrations of
Yb. The Hf fractionation factor does not vary with Hf signal intensity, making for a reliable Hf mass bias correction (A). On the other hand, 173Yb/171Yb
can only be used to correct for Yb mass bias when 171Yb signal intensity is at least 0.015 V (B). Although broadly correlated, βHf and βYb do not have a
simple 1:1 relationship, making it important to use βYb for Yb mass bias corrections when possible (C).
of laser data. Repeated analysis of JMC 475 (n = 71) over the course of
this study yielded a weighted mean of 176Hf/177Hf = 0.282159 ± 15, which
was nearly identical to the accepted JMC 475 value of 0.282160 (Vervoort
et al., 2004). No normalization of the data to JMC 475 was performed.
Hafnium Spex solution, although not an ultrapure interlaboratory standard, was also analyzed (n = 50) and found to be isotopically the same
as JMC 475 (Hf Spex 176Hf/177Hf = 0. 282159 ± 12). Hafnium Spex solutions doped with natural Yb and Lu produced corrected 176Hf/177Hf values
similar to that of the pure solution, although scatter in the high (Yb + Lu)/
Hf data was greater (Fig. 4). No statistical correlation was found between
0.28228
Weighted mean 176Hf/177Hf = 0.282161 ± 0.000013
MSWD = 3.5; n = 293
0.28224
176
Hf/177Hf
0.28220
0.28216
0.28212
0.28208
0.28204
0.28200
0
20
40
60
80
100
(176Yb + 176Lu)/176Hf (%)
Figure 4. 176Hf/177Hf plotted against relative proportion of 176(Yb +
Lu)/176Hf (shown as a percentage), for all solution analyses performed over the course of this study. Data were normalized to
179
Hf/177Hf = 0.72350 (Patchett and Tatsumoto, 1980; Patchett, 1983),
and 173Yb/171Yb = 1.132338 (Vervoort et al., 2004). Error bars represent the internal precision of individual measurements (1 standard
error). The weighted mean of all solution analyses (0.282161
± 0.000013; solid line) is identical to the weighted mean of pure Hf
solution analyses. MSWD—mean square of the weighted deviates,
a statistical measure of scatter compared to analytical precision.
252
176
Hf/177Hf and 176(Lu + Yb)/176Hf, indicating that Yb and Lu interferences
were adequately removed.
Hf Laser-Ablation Analysis
In situ Hf isotope data were acquired using a 40 μm beam centered
directly on top of the pit previously excavated for U-Pb analysis. Laser
run conditions were the same as those described for U-Pb geochronology.
Under those conditions, total Hf beams ranged from 2 to 7 V for standard
zircons. The in situ analytical routine began with a 40 s on-peak background measurement, followed by 60 s of laser ablation with a 1 s data
integration time. This resulted in a laser pit that was ~50 μm in depth, with
~15 μm for the U-Pb analysis and 35 μm for the Hf analysis. All corrections were automatically calculated during the run on a line-by-line basis,
and a 2σ filter was applied to each 60 measurement data block offline to
remove outliers.
The zircon standards Mud Tank, Temora-2, FC-52 (compositionally
similar to FC-1, from an anorthosite of the Duluth complex), 91500 (all
described in Woodhead and Hergt, 2005), and Plesovice (Sláma et al.,
2008) were analyzed. The results of repeated in situ analysis of these zircons during many analytical sessions over the course of roughly 6 mo are
given in Figure 5 and Table 1. All zircon standards were added to each
sample mount (3–4 samples per mount) and were analyzed between each
set of unknowns in order to monitor laser stability and Hf ratio accuracy.
In most cases, the long-term measured laser-ablation averages overlap
(within error) the long-term solution values for those zircons, indicating
that the previously described Lu and Yb interference correction method is
also successful for laser analyses. A small discrepancy exists between the
long-term 176Hf/177Hf laser average of FC-52 (0.282169 ± 10; 95% confidence; n = 74) and the long-term solution average of FC-1 (0.282184
± 16; 2 standard error [S.E.]; n = 42) (Woodhead and Hergt, 2005). The
cause of this difference is not clear, although it is likely not a function of
the interference correction, as discussed later.
The five standards chosen have rare earth element (REE) concentrations ranging from REE-poor (Mud Tank) to REE-rich concentrations
(Temora-2 and FC-52). Because of their high and variable REE content,
Temora-2 and FC-52 are the most useful for testing the reliability of in situ
Hf isotope data. Both have 176(Yb + Lu)/176Hf values that range from only
a few percent to almost 50% (similar to the range that we observe in rocks
from the Coast Mountains), and they are not correlated with 176Hf/177Hf.
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Hf isotopes of the Coast Mountains batholith
0.28270
FC-52
Plesovice
Average = 0.282169 ± 10
(MSWD = 0.68)
0.28235
0.28260
0.28225
0.28250
0.28215
0.28240
0.28205
0.28230
Average = 0.282476 ± 11
(MSWD = 0.33)
176
Hf/177Hf
0.28245
| RESEARCH
0.28220
0.28195
10
176
Hf/177Hf
0.28295
20
30
40
50
60
70
10
80
30
40
50
60
70
80
0.28275
Temora-2
Mud Tank
Average = 0.282684 ± 11
(MSWD = 0.51)
0.28285
0.28265
0.28275
0.28255
0.28265
0.28245
0.28255
0.28235
0.28245
20
Average = 0.282508 ± 6
(MSWD = 0.60)
0.28225
10
20
30
40
50
60
70
80
20
40
60
80
100
120
140
160
Analysis number
0.28260
91500
Figure 5. Laser-ablation Hf data from five zircon standards measured over the course of this study. Error bars
on individual analyses represent within-run precision (1
standard error), and weighted mean averages are quoted
at 95% confidence. Horizontal solid lines represent the
weighted mean of measured laser ablation–inductively
coupled plasma–mass spectrometry (LA-ICP-MS) 176Hf/177Hf
values; the dashed horizontal line represents average
176
Hf/177Hf values measured from solutions after chemical
purification from Woodhead and Hergt (2005; Temora-2,
Mud Tank, FC = 52, and 91500), and Sláma et al. (2008;
Plesovice). Where only one line is visible, the mean in situ
value reported here matches solution averages previously
measured. Shaded boxes represent the envelope of analytical error from the laser 176Hf/177Hf averages. MSWD—mean
square of the weighted deviates.
Average = 0.282315 ± 13
(MSWD = 0.77)
176
Hf/177Hf
0.28250
0.28240
0.28230
0.28220
0.28210
10
20
30
40
50
60
70
80
Analysis number
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253
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CECIL ET AL.
TABLE 1. AVERAGE Lu-Yb-Hf RATIOS OF STANDARD ZIRCONS
Zircon
n
Mud Tank 152
91500
46
Plesovice
77
Temora-2
73
FC-52
73
176
(Yb + Lu)/176Hf
(%)
0.3
6.1
3.0
18.8
17.6
176
Hf/177Hf*
0.282508 (6)
0.282315 (13)
0.282476 (11)
0.282684 (11)
0.282169 (10)
176
Lu/177Hf
Long-term
solution
176
Hf/177Hf†
0.00002
0.003
0.001
0.011
0.001
0.282507
0.282306
0.282484
0.282686
0.282184
*Numbers in parentheses are analytical uncertainties, based on precision and
scatter of all individual measurements, quoted at 2 standard error.
†
Solution values from Mud Tank, 91500, Temora-2, and FC-52 (FC-1) are from
Woodhead and Hergt (2005); Plesovice is from Sláma et al. (2008).
This provides further evidence that Yb and Lu are being properly subtracted from the laser 176Hf/177Hf calculations, as well as suggesting that
any discrepancy in FC-52 Hf isotopic values is not related to REE interference correction. It is likely that the observed difference between the FC-52
long-term solution and laser averages is the result of isotopic heterogeneity in the Duluth anorthosite, given that the solution Hf data were generated from a different batch of zircon crystals than we analyzed.
Measurements from a given sample were highly reproducible, and
uncertainties associated with the precision and scatter of a set of analyses are low (≤1 unit of εHf at the 2σ level). Measured 176Hf/177Hf values
were corrected for the radiogenic in-growth of 176Hf, although that correction is small, given that zircon incorporates relatively little Lu (measured
176
Lu/177Hf values range from 0.0005 to 0.0015). Corrections for the isobaric interference of 176Lu and 176Yb ranged from minor (~5% change in
the 176Hf/177Hf ratio) to large (~35% change in the 176Hf/177Hf ratio). Good
reproducibility of corrected 176Hf/177Hf values, however, inspires confidence that even major changes in isotope ratios were accurately accounted
for. For example, sample 80JA11 yielded zircons that have up to 70% of
their total mass 176 contributed from Yb and Lu, and yet the reproducibility of corrected 176Hf/177Hf values from those sample grains was excellent
(Fig. 6). Plots showing reproducibility of the corrected 176Hf/177Hf as a
function of Yb and Lu interference can be found in the GSA Data Repository (see footnote 1).
Initial εHf values from plutonic rocks of the Coast Mountains batholith range widely from +1 to +13, and values cluster between +9 and +13
for Paleozoic plutonic rocks of the Alaskan Alexander terrane. A general
increase in εHf(t) is observed from west to east across the central Coast
Mountains batholith, although nearly the entire range of Hf values is
found in rocks located within and along the periphery of the Coast shear
zone (see Fig. 2; Table 2).
U-Pb GEOCHRONOLOGY RESULTS
We present 29 new U-Pb zircon ages from two suites of plutonic rocks:
(1) Jurassic–Eocene plutons of the central Coast Mountains batholith (n =
23), and (2) Ordovician–Early Devonian plutons clearly identified as part
of the Alexander terrane in southeast Alaska (n = 6). Detailed geochronology data, including concordia and weighted mean plots, can be found in
the GSA Data Repository (see footnote 1).
Coast Mountains batholith pluton ages range from ca. 151 to 53 Ma and
represent nearly the entire time span of magmatism in the Coast Mountains. West of the Coast shear zone, ages decrease systematically from
west to east, indicating eastward migration of magmatism at ~1 km/m.y.
across this area between 150 and 80 Ma. Jurassic to Early Cretaceous
plutons are also found east of the Coast shear zone, which is ascribed to
duplication of the Jurassic arc by sinistral displacement along strike-slip
faults in the Early Cretaceous (Gehrels et al., 2009). Most samples from
plutons east of the Coast shear zone record younger (Late Cretaceous to
Eocene) ages and show no apparent migratory trends.
In addition to intrusive rocks of the central Coast Mountains batholith,
Alexander terrane plutons from southeast Alaska were analyzed for the
sake of comparing εHf values of the Alexander terrane with those of the
plutons intruding Coast Mountains batholith crust of unknown affinity, as
discussed in later sections. U-Pb ages of the Alexander plutons range from
ca. 480 to 390 Ma and are consistent with the range of ages reported by
Gehrels and Saleeby (1987).
Hf ISOTOPIC RESULTS
Hafnium isotopic compositions were measured in situ via LA-MCICP-MS directly on top of the spot previously excavated for U-Pb analysis, such that each Hf isotopic measurement is directly tied to a corresponding U-Pb age. Hf data, and related ages, are reported in Table 2
for the 29 plutonic samples discussed in the previous section. For each
sample, between 15 and 55 individual spot measurements were made,
and mean values are reported. Individual measurements and weighted
mean plots of all Hf sample data can be found in the GSA Data Repository (see footnote 1).
254
INTERPRETATION OF U-Pb-Hf DATA AND IMPLICATIONS FOR
THE CRUSTAL ARCHITECTURE OF THE COAST MOUNTAINS
BATHOLITH
Hafnium isotopic signatures from intrusive rocks across the westcentral Coast Mountains batholith are relatively juvenile, suggesting
derivation of Coast Mountains batholith plutons from similarly juvenile
mantle or crustal sources. In this respect, the Hf data presented here are
consistent with the notion that the Coast Mountains batholith represents
the growth of new crust in a continental arc system (e.g., Samson et al.,
1989; Friedman et al., 1995). However, the range of measured εHf(t) values is great (+1 to +13), and in all cases, those values are lower than the
depleted mantle array (εHf[500–0 Ma] values of +14 to +18; Vervoort and
Blichert-Toft, 1999), indicating heterogeneity in magma source regions
and/or interaction with more evolved crustal materials. Although absent
in most samples, traces of inherited zircon were present in three of the
plutons analyzed in this study, also indicating the incorporation of older,
recycled crust into melts.
Because a considerable amount of age control exists for this portion
of the Coast Mountains batholith, our data can be used to evaluate relations between petrogenesis and magmatic flux. Magmatism in the Coast
Mountains batholith is interpreted to be strongly episodic, with distinct
flare-up events at 160–140 Ma, 120–78 Ma, and 55–48 Ma, and a longlived period of relative magmatic inactivity between 140 and 120 Ma
(Gehrels et al., 2009). Our geochronologic data do not reflect that periodicity because we intentionally chose samples that were either known
or inferred to have ages corresponding to both high- and low-flux events.
Isotope pull-downs, or negative excursions in whole-rock initial εNd values, have been temporally correlated with magmatic flare-ups (Ducea and
Barton, 2007; DeCelles et al., 2009), suggesting a link between isotopic
signatures and periods of lithospheric thickening and crustal melt production. Our data show no clear negative excursions or any correlation
between εHf(t) and U-Pb age and/or the timing of magmatic flux events
(Fig. 7). This is probably attributable to the relatively small (n = 23) size
of the Coast Mountains batholith U-Pb-Hf data set presented here. The
range of isotopic values presented here likely records normal variation in
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Hf isotopes of the Coast Mountains batholith
| RESEARCH
TABLE 2. Hf ISOTOPIC DATA
Sample
name
n
176
176
(Yb + Lu)/ Hf
(%)
176
177
176
Lu/177Hf*
Hf/ Hf*
176
εHf(t)*
Hf/177Hf(t)
206
Pb/238U age†
(Ma)
Distance E of the CSZ§
(km)
Data from Jurassic through Eocene plutons of the Coast Mountains batholith
Otter-W
04GJP-55
04GJP-58
05MT-111
04GJP-60
Stephens
McCauley-W
04GJP-43
04GJP-68
04GJP-69
04GJP-29
05MT-106
Ecstall East
04GJP-13
04GJP-84
83GJ53
04GJP-09
04GJP-77
05MT-135
04GJP-89
04GJP-24
05MT-155
05MT-145
24
27
27
24
33
38
35
55
23
35
34
25
22
17
32
22
16
34
29
54
23
25
36
9.5
33.8
36.6
18.9
8.1
14.9
20.4
18.3
10.7
9.4
10.1
11.5
9.6
26.4
10
6.7
13.1
6.1
14.8
15.1
11.2
18.4
17.2
0.282846 (16)
0.282766 (26)
0.282843 (36)
0.282797 (36)
0.282816 (17)
0.282891 (13)
0.282915 (15)
0.282954 (14)
0.282915 (17)
0.282959 (14)
0.282944 (15)
0.283054 (25)
0.282899 (25)
0.282988 (24)
0.283029 (14)
0.282792 (18)
0.283043 (13)
0.283025 (15)
0.283017 (16)
0.283047 (21)
0.283036 (16)
0.283035 (16)
0.282811 (26)
0.0006 (2)
0.0019 (15)
0.0018 (15)
0.0012 (6)
0.0006 (3)
0.0012 (4)
0.0013 (6)
0.0014 (5)
0.0009 (4)
0.0008 (3)
0.0007 (3)
0.0009 (9)
0.0008 (4)
0.002 (19)
0.0008 (7)
0.0005 (3)
0.0011 (16)
0.0006 (2)
0.0012 (7)
0.0007 (2)
0.0008 (5)
0.0015 (6)
0.0009 (4)
0.282844
0.282761
0.282838
0.282794
0.282814
0.282889
0.282912
0.282952
0.282913
0.282958
0.282943
0.283052
0.282898
0.282984
0.283028
0.282791
0.283041
0.283024
0.283014
0.283045
0.283034
0.283033
0.282810
5.4 (0.6)
2.7 (0.9)
5.2 (1.3)
2.6 (1.2)
4.3 (0.6)
5.4 (0.4)
7.5 (0.6)
8.1 (0.5)
6.8 (0.7)
8.1 (0.5)
7.3 (0.5)
11.6 (0.9)
6.0 (0.9)
9.8 (0.9)
9.7 (0.5)
1.5 (0.7)
11.2 (0.5)
9.6 (0.5)
9.2 (1.6)
12.0 (1.0)
12.3 (0.6)
10.3 (0.6)
2.0 (1.0)
141.6 (2.2)
141.5 (2.9)
142.1 (2.7)
124.1 (1.4)
153.0 (2.4)
101.0 (1.1)
118.8 (2.1)
97.4 (2)
94.0 (2.3)
88.5 (1.2)
75.8 (1)
94.5 (2.3)
91.8 (1.3)
96.1 (1.7)
59.6 (1.2)
58.2 (1.1)
82.2 (1.3)
55.2 (1.3)
55.9 (3.0)
125.1 (1.8)
150.8 (1.8)
67.6 (1.6)
52.5 (0.8)
0.0006 (4)
0.0032 (9)
0.0007 (6)
0.0017 (9)
0.0020 (7)
0.0003 (5)
0.282889
0.282824
0.282882
0.282758
0.282841
0.282762
12.9 (0.8)
10.4 (0.7)
12.2 (1.0)
9.8 (0.9)
12 (1.0)
8.6 (0.9)
414.4 (7)
404.6 (9)
393.9 (8)
497.7 (8)
445.3 (9)
429.9 (9)
–88.9
–79
–76.5
–74.1
–64.6
–49.9
–66.1
–47.1
–36.8
–27.2
–10.9
–7.7
–6.6
–0.5
3.2
4.6
12.4
14.4
17.7
20.7
36.6
74.8
90.3
Data from Paleozoic plutons of the Alexander terrane, SE Alaska
79JD975
80JA11
80JA3
82GP702
83GP335
82GP626
26
34
26
24
26
28
7.2
47.7
8.6
23.9
28.8
3.8
0.282893 (23)
0.282848 (21)
0.282887 (27)
0.282774 (24)
0.282858 (29)
0.282764 (25)
–44
–46
–45
–96
–80
–104
*Numbers in parentheses are analytical uncertainties, quoted at 2 standard error (S.E.).
Numbers in parentheses are analytical and systematic uncertainties, quoted at 2 S.E.
CSZ—Coast shear zone (see text for discussion).
†
§
16
176
60
12
50
10
εHf(t)
Hf/177Hf
0.2832
14
80JA11
176
Hf/177Hf = 0.282884 ± 0.00002
MSWD = 0.7; n = 34
0.2830
40
88
30
66
0.2828
20
4
10
2
0.2826
0
40
60
60
80
80
100
100
120
140
160
140
160
U Ma**
age (Ma)
180
180
Avg. magmatic flux (km3/m.y./km)
0.2834
70
0
200
206U-Pb238
age
Pb/
0.2824
20
30
40
176
50
176
60
70
80
176
( Yb+ Lu)/ Hf (%)
Figure 6. Individual 176Hf/177Hf ratio measurements of sample
80JA11 as a function of 176(Yb + Lu) contribution to the total mass
176 signal. (176Yb + 176Lu)/176Hf is both high and variable in this
sample, ranging from 35% to 70%. MSWD—mean square of the
weighted deviates.
LITHOSPHERE | Volume 3 | Number 4 | www.gsapubs.org
Figure 7. Plot showing the εHf(t) values of Coast Mountains batholith
plutons as a function of age and magmatic flux (magmatic flux curve
from Gehrels et al., 2009). Data from Jurassic and Late Cretaceous plutons in the eastern part of the study area are excluded, because those
intrusive bodies were not used in the magmatic flux calculations. Correlation between Hf values and age or the timing of magmatic flux events
is weak, suggesting that either our sample set is not large enough to
capture Hf excursions, or that Hf is relatively insensitive to tectonic processes (e.g., orogenic thickening, backarc extension, delamination, etc.)
that may be controlling episodes of magmatic lulls and flare-ups.
255
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CECIL ET AL.
the background arc–mantle wedge flux (Ducea and Barton, 2007), but it is
nonetheless significant because these variations can be used as tracers of
input from distinctive sources.
Linking Hf Plutonic Signatures and Country Rock Assemblages
A fundamental observation of our data is that εHf(t) increases from west
to east; the lowest values were obtained for plutons along the western coast,
and the highest values were obtained from plutons in the easternmost, inland
areas (Fig. 2). Based on the distribution of Hf signatures, we discriminate
between three distinct crustal domains into which plutons of the Coast
Mountains batholith were emplaced: a western domain, characterized by
relatively evolved Hf isotopic compositions, a central domain, with intermediate Hf compositions, and an eastern domain, characterized by juvenile
Hf signatures. Because these variations coincide with variations in country
rock assemblages, as well as previously reported Nd-Sr and detrital zircon
data, we interpret those domains to be part of the Banks Island assemblage,
the Alexander terrane, and the Stikine terrane, respectively (Fig. 8).
Samples from the Banks Island domain have the most evolved εHf(t)
values (+2.6 to +5.2), which are consistent with evolved continental
Nd-Sr values from metamorphic country rocks of the west-central Coast
Mountains batholith (Boghossian and Gehrels, 2000). Epsilon Nd values
of those rocks are more negative (~+0.5 to −9.9) than our reported zircon
Hf values for younger intrusive rocks of the same area. This appears to
be typical of Coast Mountains batholith plutons, which likely originated
from juvenile mantle, but which are sensitive to small amounts of more
evolved crustal input, such that recorded εHf(t) values represent mixing
between arc-type melts and older, preexisting crust. Old continental
crust has high Hf concentrations and highly negative εHf. For example,
Middle Proterozoic crust of the western United States has modern εHf
16
14
εHf(t)
12
Decreasing SW-ward
trend
Stikine
10
8
?
6
4
K - Eocene plutons
Jurassic plutons
Alexander
Paleozoic intrusives
(SE Alaska)
Banks Island
2
0
-120
YTT (?)
-80
-40
0
40
80
Dist. east
E of the
Distance
of CSZ
the CSZ (km)
Figure 8. The εHf(t) values of all Coast Mountains batholith and Paleozoic
southeast Alaska Alexander terrane plutons as a function of distance
from the Coast shear zone (CSZ). Hf values increase from west to east
across the main central Coast Mountains batholiths, and shifts in isotopic values are used to infer boundaries between crustal terranes. A broad
zone of heterogeneous Hf signatures, including the lowest recorded εHf(t)
value, is observed near the Coast shear zone (gray rectangle in center of
figure). This is interpreted to be a region of structural imbrication of Alexander, Yukon-Tanana (YTT), and Stikine terranes along mid-Cretaceous (K)
thrust faults and the early Tertiary Coast shear zone. Question marks are
meant to indicate those samples for which we are less confident about
their tectonic affinity, due to uncertainty about the geologic context or
isotopic character of the rocks.
256
values ranging from −10 to −30; Early Proterozoic and Archean crust
is even more negative (Vervoort and Patchett, 1996). Only small additions (a few percent) of isotopically depleted continental material would
therefore be necessary to drive down the εHf of magmas, as has been
pointed out for Nd-Sr systematics (e.g., Patchett and Bridgwater, 1984;
Samson et al., 1990, 1991a).
The central domain, which is the southern continuation of the Alexander terrane, consists of a north-south–trending belt of rocks located
between the Banks Island domain to the west and the Gravina belt and
mid-Cretaceous thrust system to the east. Plutons making up this belt have
εHf(t) values that range from +5.9 and +8.1, and they are more juvenile than
those of the Banks Island terrane (Fig. 8). They are interpreted to belong to
the Alexander terrane because of: (1) the occurrence of distinctive Ordovician–Silurian magmatic arc assemblage overlain by Devonian conglomeratic strata; (2) the juvenile nature of the Alexander terrane understood
from Nd-Sr isotopes (Samson et al., 1989), and the lack of continental
input in geologic units and U-Pb zircon populations (Gehrels and Saleeby,
1987; Gehrels et al., 1987, 1996); and (3) geographic position; the crust
into which this part of the batholith was built forms the southern continuation of the main Alexander terrane to the north (see Fig. 1).
To test the assignment of these rocks to the Alexander terrane, we analyzed Hf isotopes from igneous rocks that are known to belong to the
terrane in southern SE Alaska. These Paleozoic (ca. 480–390 Ma) intrusive rocks have juvenile εHf(t) values ranging from +8.6 to +12.9. The εHf
values of the same plutons at 100 Ma, an age which approximates those of
Alexander plutons in the central Coast Mountains batholith, yield highly
consistent values of +2.5 to +7. Hafnium signatures of Cretaceous plutons
intruding the inferred Alexander terrane (+5.9 to +8.1) are therefore likely
recording either (1) direct melting of Paleozoic Alexander basement; or
(2) melting of the mantle wedge followed by partial melting and assimilation of Alexander and/or Banks Island assemblages. It is also possible
that the interpreted Alexander terrane of British Columbia represents a
transitional zone between primitive Alexander of southeast Alaska and the
more evolved Banks Island assemblage to the south. Connections between
the northern Alexander terrane and Banks Island are consistent with the
presence of ca. 480–410 Ma detrital zircons in Banks Island strata (Gehrels and Boghossian, 2000). Furthermore, Hf isotopic signatures within
plutons of the southeast Alaskan Alexander terrane become more evolved
to the southwest, suggesting a gradational relationship between the Alexander and Banks Island terranes (Fig. 8).
The eastern domain is characterized by juvenile εHf(t) values ranging from +10.2 to +15.1, and it is interpreted as Stikine terrane based
on proximity with Stikine to the east, lack of a major tectonic boundary
separating the two, the recognition by Hill (1985) that the central gneiss
complex (eastern domain) contains fossiliferous marbles potentially correlative with Stikine strata, and primitive Nd-Sr values reported for Stikine
rocks by Samson et al. (1989). There is no evidence for the input of older,
reworked continental crust in any Stikine rocks in the study area, suggesting that, much like in the case of the Alexander terrane, they were
produced either by the wholesale melting of juvenile Stikine lower crust
or by direct melting of the mantle with variable contributions from terrane
components.
Between the outboard Banks Island and Alexander terranes and the
inboard Stikine terrane, there is a zone of structural deformation delineated by a regionally extensive belt of mid-Cretaceous thrust faults
(e.g., Rubin et al., 1990) and by the Coast shear zone (e.g., Rusmore et
al., 2005; Hollister and Andronicos, 2006), which is characterized by
marked heterogeneity in Hf signatures (Fig. 8). Initial εHf values within
this zone range from +1.5 to +11.6, and these are interpreted to represent the imbrication of juvenile Alexander and Stikine terranes with
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Hf isotopes of the Coast Mountains batholith
continental-margin rocks of the Yukon-Tanana terrane. This is consistent
with the geologic evidence for protracted and large-scale displacement
along these structures.
It is important to note that there is very little interaction of plutons
with country rocks at the present level of exposure, which represents
emplacement depths of ~10–25 km (Butler et al., 2001, 2006). Furthermore, new geochemical data from the Coast Mountains batholith indicate that plutons were generated and emplaced at depths greater than
35 km (Girardi et al., 2008). The fact that the Hf isotopic signatures
of the plutons appear to be tracking with the country rock assemblages
therefore raises interesting questions about the nature of the terranes at
depth and the structural boundaries that separate them. Lateral changes
in Hf isotopes imply marked heterogeneity in the magma source regions;
the correlation between Hf signatures and the known isotopic character
of country rocks indicates the presence of those or similar assemblages
at depth. The assertion that presently exposed metamorphic rocks could
be present at melt-generation depths is corroborated by high δ18O values
in quartz from the same plutons (Wetmore and Ducea, 2011).
Relationships between Crustal Terranes of the Coast Mountains
Batholith
Discernible shifts in Hf isotopic values in plutons are interpreted to
reflect boundaries between discrete crustal terranes (Fig. 9). This implies
Central CMB at ca. 50 Ma
To Wrangellia/trench
Kitkatla
fault
X
| RESEARCH
that structures controlling those boundaries act as through-going crustal
barriers restricting magma migration and/or that the crustal boundaries
have remained steeply dipping through time. For example, the thrusting
and crustal thickening of mid-Cretaceous age along a regional belt in the
axial Coast Mountains batholith (Rubin et al., 1990) have not affected
the magma source regions in adjacent parts of the batholith. This is evidenced in pluton ages in the eastern domain, which range from 151 to
55 Ma, but which have no associated change in Hf behavior. This is also
observed in individual samples with zircon inheritance. For example, sample MT05-135, an Eocene pluton located on the eastern periphery of the
Coast shear zone, has inherited Jurassic and Early Cretaceous cores and
magmatic rims with measured ages of 55 Ma (Fig. 10). There is very little
change, however, in εHf(t) values across those zircon domains, despite the
fact that a major period of deformation and shortening between 101 and
85 Ma occurred in the zone immediately adjacent to where this pluton was
intruded (Rubin et al., 1990) (Fig. 10). Although thrusting has imbricated
various terranes along the axial belt, the zone of imbrication is apparently
thick skinned and restricted to a narrow region (~20-km-wide swath).
Thrusting may have been thin skinned and low angle along the margins of
the belt at shallow crustal levels, e.g., above the present 10–25 km levels
of exposure.
Many of the structural and/or stratigraphic relationships between the
crustal terranes discussed here are unclear because the original contacts
between those terranes have been commonly overprinted by more recent
Coast
shear
zone
Shames
normal fault
X′
20
30
No t t o s c a l e
?
Moho
40
?
Subducting slab
εHf (t) of plutons
+10 to +13
+6 to +9
+2 to +6
<+2
Crustal terranes
Banks Island
Alexander
Stikine
Yukon-Tanana
Underplating of
primitive
mantle magmas
No t t o s c a l e
Depth (km)
10
Juvenile, mantle-derived basalts
Zones of melting and assimilation
km
of preexisting crust of varying age
and isotopic composition
0
50
25
Vertically exaggerated; surface and Moho topography estimated
Figure 9. Schematic cross section of the Coast Mountains batholiths (CMB) at the latitude of the Douglas channel (line X–X′
in Fig. 2) at ca. 50 Ma, modified after Gehrels et al. (2009). Crustal terranes are inferred on the basis of Hf data (presented
here) and existing Nd-Sr and detrital zircon data (see text). Patterned section of the lower crust is intended to represent
regions of mingling of juvenile, mantle-derived melts (dark-gray areas) and partial melts of the different crustal terranes,
which impart distinctive Hf signatures to the plutons emplaced above. Although there is little evidence to suggest that
evolved, continental-margin rocks of the Yukon-Tanana terrane underlie the crust east of the Coast shear zone, one pluton
from the easternmost part of the Stikine terrane yields a uniquely low εHf(t) value, which could possibly be attributed to
Yukon-Tanana terrane contribution, although the nature of the contact between Yukon-Tanana terrane and Stikine terrane
is ambiguous. Crustal thickness interpreted from Morozov et al. (1998, 2001). Geology and structure are adapted in part
from Wheeler and McFeely (1991) and Gehrels et al. (2009).
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CECIL ET AL.
0.05
0.05
300
A
260
0.04
0.04
Pb/235U
0.03
0.03
206
220
0.02
0.02
180
140
CONCLUSIONS
100
60
0.01
0.01
Analyses used for magmatic
age determination
20
0.00
0
0.0
0
0.1
0.1
0.2
0.2
207
18
16
εHf(t)
14
12
B
0.3
0.3
Pb/235U
MT05-135
Depleted mantle
Magmatic
age = 55 Ma
20
10
8
6
Thrust belt development
and crustal thickening
2
0
20
Xenocrystic cores
Mixing of U-Pb/Hf
domains?
4
40
60
80
206
100
120
140
160
180
200
Pb/238U age (Ma)
Figure 10. U-Pb geochronometric and Hf isotopic data for zircons
from sample MT05-135, an Eocene pluton intruded ~15 km east of
the Coast shear zone. (A) U-Pb ages range from 180 to 55 Ma, with
systematically older cores and younger rims, which were used to
interpret the magmatic age. (B) Hf isotope values as a function of
age. Hf values vary between +8.5 and +12.5 and show little change
with zircon age. The string of zircon ages intermediate between
the 125 Ma cores and the 55 Ma rims likely represents mixing of
the two age domains as a result of the 40 μm laser spot size used.
The gray vertical bar represents the proposed timing of major
deformation and thickening along an axial fold-and-thrust belt in
the mid-Cretaceous (Rubin et al., 1990).
deformation. It remains unclear, for example, why the Banks Island continental margin–type assemblage is located outboard of the primitive
Alexander terrane. Relationships among Alexander, Stikine, and YukonTanana terranes are likewise enigmatic. East of the Coast shear zone,
plutonic rocks record uniformly juvenile, Stikine-like εHf(t) signatures.
This is different than relationships inferred to the north, where isotopically evolved rocks of the Yukon-Tanana terrane and associated metamorphic assemblages are observed wedged between primitive Alexander and
Stikine terranes (Samson et al., 1991a). There appears to be an alongstrike change, therefore, in the tectonostratigraphic relationship between
the Yukon-Tanana terrane and Stikine terrane. One notable exception to
this comes from an Eocene pluton (53 Ma) located at the eastern margin
of outcropping batholithic rocks, which has an εHf(t) value of +2.0, i.e.,
258
distinctly more evolved than those from any other portion of the Coast
Mountains batholith. It is possible that this signature reflects a component of evolved continental-margin strata of the Yukon-Tanana terrane
that extends beneath the western Stikine terrane. Although the nature of
the links between Stikine and Yukon-Tanana is unclear, they have been
proposed on the basis of inherited Precambrian zircons in Jurassic Stikine
plutons (Thorkelson et al., 1995), and evolved Nd isotopic characteristics
of Upper Triassic Stikine strata (Jackson et al., 1991).
Plutons making up the west-central Coast Mountains batholith represent ~100 m.y. of continental arc magmatism. In general, the juvenile
character of Hf isotopic data from those plutonic rocks suggests the production of new continental crust derived primarily from mantle sources,
with little recycling of Precambrian continental crust into arc-type melts.
Substantial variation in εHf(t) values of plutons (+1 to +13), and the systematic spatial distribution of those values, however, suggests that Hf
isotopes are tracing heterogeneities in source regions and that those heterogeneities are a function of the influence of different crustal terranes.
From outboard to inboard, discrete crustal panels appear to be composed
of the Banks Island assemblage, the Alexander terrane, and the Stikine
terrane, with the imbrication of thin fragments of Yukon-Tanana terrane
along mid-Cretaceous thrust faults and along the Coast shear zone, and the
possibility of Yukon-Tanana terrane basement beneath Stikine strata in the
easternmost part of the study area.
The juxtaposition of these crustal terranes requires complicated structural and/or stratigraphic relationships between the various terranes,
particularly in the case of the Yukon-Tanana and Stikine terranes. If the
Yukon-Tanana terrane to the north was indeed emplaced outboard of the
Stikine terrane during a transpressive regime, as has been suggested by
Samson et al. (1991a), the inferred structural imbrication of the YukonTanana terrane near the Coast shear zone in our study area could perhaps
represent the pinching out of the Yukon-Tanana terrane to the south. Structural emplacement of Yukon-Tanana terrane rocks outboard of the Stikine
terrane along Cretaceous thrusts and left-lateral faults could explain an
enigmatic quartzite cobble conglomerate located in the south-central part
of our study area, west of the Coast shear zone, which, unlike other local
lithologies, has Archean detrital zircon populations (Boghossian and Gehrels, 2000; Gehrels and Boghossian, 2000). The presence of an isotopically evolved pluton to the east, however, suggests a potential stratigraphic
tie between the Stikine and the Yukon-Tanana terranes. That relationship
remains cryptic due to the uniformity of juvenile Hf signatures in plutonic
rocks presented here and juvenile Nd-Sr signatures of Stikine country
rocks (Samson et al., 1989). Further isotopic work to the east and to the
south of the study could help resolve the extent of Yukon-Tanana terrane
influence in Coast Mountains batholith plutons.
ACKNOWLEDGMENTS
This work was sponsored by National Science Foundation (NSF) award
EAR-0309885 for support of the BATHOLITHS projects, and by EAR0732436 for support of the Arizona LaserChron Center. The authors
thank two anonymous reviewers, whose thoughtful and critical comments
greatly improved the manuscript.
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