Journal of Petrology Advance Access published online on June 19, 2008
Journal of Petrology, doi:10.1093/petrology/egn030
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Dynamic Magma Systems, Crustal Recycling, and Alteration in the Central Sierra Nevada Batholith: the Oxygen Isotope Record
1Geology Department, Pomona College, Claremont, CA 91711, USA
2Department of Geology and Geophysics, University of Wisconsin, Madison, WI 53706, USA
3Science Division, Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA
4Department of Geology, University of Kansas, Lawrence, KS 66045, USA
Received January 6, 2007; Revised typescript accepted May 27, 2008
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONGEOLOGICAL BACKGROUNDSAMPLE SELECTION, PREPARATION,...RESULTSDISCUSSIONCONCLUDING REMARKSSUPPLEMENTARY DATAREFERENCES |
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Values of
18O of zircon from the central Sierra Nevada batholith (SNB), California, yield fresh insight into the magmatic evolution and alteration history of this classic convergent margin batholith. Direct comparison of whole-rock and zircon (Zrc) 18O provides evidence for modest (0.5
), but widespread, alteration, which has complicated interpretation in previous whole-rock 18O studies. Four discrete belts of 18O values are recognized in the central Sierra. A small belt of plutons with relatively low 18O(Zrc) values (5·2–6·0) intrudes the foothills, with a sharp increase of 18O revealing the concealed Foothills Suture; high 18O(Zrc) values (7·0–8·5) dominate the rest of the western SNB. East of the axis of the Sierra, 18O is distinctly lower (6·75–5·75), and decreases monotonically to the Sierra Crest. A sharp 1 increase of 18O in the eastern Sierra reveals a second crustal boundary, with the fourth belt hosted in high-18O North American crust in the White Mountains and Owens and Long Valleys. Correlated O, Sr, and Pb isotope ratios reveal differences in magma generation between the western and eastern Sierra. The western Sierra experienced massive crustal recycling, with substantial melting and mobilization of accreted oceanic and volcanic arc rocks; crustal contamination affects many western SNB plutons. In contrast, the eastern Sierra was dominated by voluminous recycling of the lithospheric mantle and lower crust, with minimal crustal contamination. Batholith-wide shifts in 18O occur between pulses of Cretaceous magmatism that may be linked to tectonic reorganizations of magma sources. Within intrusive suites, 18O may be unchanged (Tuolumne); increase (Sonora and Whitney); or decrease (Sequoia and John Muir) with time. These trends show stable long-lived sources, or those where recycling and contamination may increase or decrease with time. Overall, 18O reveals diverse magma system behavior at a range of scales in the Sierran arc. KEY WORDS: zircon; crustal growth; granitoids; supracrustal; magma systems; Sierra Nevada
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL BACKGROUNDSAMPLE SELECTION, PREPARATION,...RESULTSDISCUSSIONCONCLUDING REMARKSSUPPLEMENTARY DATAREFERENCES |
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The Sierra Nevada batholith (SNB, Fig. 1) has been intensely studied for decades to deduce the petrogenesis of individual plutons and intrusive suites and to understand the integrated history of the arc that formed it (see Bateman, 1992
; Moore, 2000). Isotope studies have been particularly helpful in identifying significant crustal and mantle ‘reservoirs’ that contributed to the diversity of magmas of the SNB (Doe & Delevaux, 1973; Kistler & Peterman, 1973, 1978; DePaolo, 1980, 1981; Masi et al., 1981; Saleeby et al., 1987; Kistler, 1990; Kistler & Ross, 1990; Chen & Tilton, 1991; Kistler & Fleck, 1994; Lee et al., 2000; Ducea, 2001; Ratajeski et al., 2001; Wenner & Coleman, 2004; Lackey et al., 2005, 2006). Reservoirs include sub-arc mantle, subducted ocean crust, Phanerozoic sedimentary rocks, and Precambrian continental crust. The relative contributions of these reservoirs to SNB magmas are largely unknown. For example, studies of mantle xenoliths show considerable geochemical heterogeneity in the sub-arc mantle (Mukhopadhyay & Manton, 1994; Ducea, 1998, 2001, 2002; Lee et al., 2000), which suggests that the lithospheric mantle was heavily contaminated during subduction in the Jurassic and Triassic. Also, the amount of recycling of young rocks in the arc is uncertain. Recent experimental studies have reproduced characteristic major and trace element patterns of SNB granitoids by partially melting gabbroic compositions at pressure, temperature, and fluid conditions inferred for the magma-generating regions of the arc (Ratajeski et al., 2005; Sisson et al., 2005). These findings suggest significant remelting of mantle-derived basaltic sources at the base of the arc.
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Oxygen isotopes are well suited to clarifying ambiguity of magma source and contamination processes in the SNB. Low-temperature fractionation of 18O/16O, especially in the presence of water, imparts unambiguous
18O ‘supracrustal’ signatures in sedimentary rocks that form at the Earth's surface, or in igneous and metamorphic rocks that are hydrothermally altered there. When such rocks melt following subduction or burial, the 18O of these melts contrasts markedly with the restricted 18O range of the primitive mantle (5·7 ± 0·3; Taylor & Sheppard, 1986).
Despite the utility of oxygen isotopes, their application to study the SNB has long been regarded with skepticism. Masi et al. (1981) conducted the first large-scale oxygen isotope study of the SNB. They found that, unlike the coeval Peninsular Ranges batholith (Fig. 2a; Taylor & Silver, 1978; Silver et al., 1979), the variation of 18O with Sri in the SNB did not define a simple binary mixing pattern. Subsequent studies have shown an increasingly complex data array (Fig. 2b). Furthermore, hydrogen isotope analyses have revealed low D values, –85 and lower, indicating hydrothermal alteration of many samples.
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Our study provides fresh insight into the origin and evolution of magmas in the SNB. We present a comprehensive laser fluorination study of
18O of zircon (Zrc) in the central Sierra (Fig. 3). Zircon was selected for the study because it is highly retentive of magmatic 18O (Valley, 2003; Page et al., 2007). Prior study showed that analysis of 18O(Zrc) in the SNB resolves magmatic 18O in rocks that have undergone subsolidus exchange and hydrothermal alteration (Lackey et al., 2001, 2003, 2005, 2006). In this study, over 100 rocks were analyzed for both 18O(WR) (where WR is whole-rock) and 18O(Zrc); direct comparison of 18O is presented to illustrate the magnitude and frequency of alteration of 18O(WR). Also, we detail how whole-rock SiO2 content and 18O(Zrc) allow calculation of magmatic 18O(WR). These findings elucidate the scale and nature of magma systems in the arc.
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GEOLOGICAL BACKGROUND |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL BACKGROUNDSAMPLE SELECTION, PREPARATION,...RESULTSDISCUSSIONCONCLUDING REMARKSSUPPLEMENTARY DATAREFERENCES |
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The central Sierra Nevada batholith is a collage of plutons and intrusive suites (Fig. 3) with pronounced spatial variation in composition, age, and isotope chemistry. Cretaceous granitoids are exposed over c. 35 000 km2 (Fig. 1). The estimated original thickness of granitic crust in the batholith is 35 km (Fliedner et al., 2000
; Saleeby et al., 2003); thus the Sierran arc must have generated at least 1 x 106 km3 of granitic magma between 120 and 85 Myr ago. The batholith has long been recognized to have lateral (west to east) gradients in bulk composition (Moore, 1959; Bateman, 1992), pluton age (Kistler & Peterman, 1973, 1978; Stern et al., 1981; Chen & Moore, 1982), major and trace element geochemistry (Ague & Brimhall, 1988b; Bateman, 1992), and isotope ratios (Doe & Delevaux, 1973; Kistler & Peterman, 1973, 1978; DePaolo, 1980, 1981; Masi et al., 1981; Saleeby et al., 1987; Kistler, 1990; Kistler & Ross, 1990; Chen & Tilton, 1991; Kistler & Fleck, 1994; Ducea, 2001; Wenner & Coleman, 2004). Besides lateral variations in magma chemistry, recent geochemical studies of xenoliths (Fig. 2a) and deep crustal exposures suggest pronounced vertical zonation in the geochemistry of the batholith as well (Mukhopadhyay & Manton, 1994; Ducea, 1998, 2001, 2002; Lee et al., 2000; Saleeby et al., 2003; Lackey et al., 2005).
Various studies have interpreted transverse changes in SNB geochemistry as resulting from heterogeneity in the pre-batholith lithosphere and sub-arc mantle. The classic geochemical boundary in the SNB, the Sri = 0·706 line (Figs 1 and 3), delineates the inferred boundary between accreted Phanerozoic rocks in the western Sierra and Proterozoic continental lithosphere in the eastern Sierra (Kistler, 1990). A second boundary recognized inboard of the 0·706 line is the ‘PA/NA break’ of Kistler (1990). This break runs through the central Sierra and delineates a cryptic pre-batholithic domain boundary between oceanic ‘Panthalassan’ (Kistler, 1990) lithosphere to the west and North American lithosphere to the east (Fig. 3). The break is based on the location of the 0·706 line, variations of 18O(WR), and the location of offset pendants or plutons with similar lithology, age, and chemistry (Kistler, 1990, 1993).
Intrusive suites in the Sierra are the ‘microcosms’ of the batholith. Suites are often defined from field relations (Bateman, 1992), geochemical, and isotopic zonation (Bateman & Chappell, 1979; Kistler et al., 1986; Bateman, 1992), and similar ages or age progressions in member plutons, such as inward decreasing age in zoned plutonic complexes (Chen & Moore, 1982; Coleman et al., 2004). Because trends in geochemistry and age in some suites mimic trends of those same parameters across the SNB, we conducted detailed sampling in several suites to determine how their magmatic systems relate to the batholith as a whole.
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SAMPLE SELECTION, PREPARATION, AND ANALYSIS |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL BACKGROUNDSAMPLE SELECTION, PREPARATION,...RESULTSDISCUSSIONCONCLUDING REMARKSSUPPLEMENTARY DATAREFERENCES |
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Zircon was concentrated by standard crushing, density, and magnetic separation techniques. Least magnetic fractions were concentrated with a Frantz separator. Most samples were originally collected for geochronology (Chen & Moore, 1979
, 1982; Ratajeski et al., 2001) and thermochronology (House et al., 1997, 1998, 2001; Stockli et al., 2003) studies in the Sierra Nevada and White Mountains. The first author collected samples with ‘1S’ and ‘3S’ prefixes in 2001 and 2003, respectively (see Table 1). Electronic Appendix 1 (available for downloading at http://www.petrology.oxfordjournals.org/) provides details of location, lithological description, and researcher for all samples. Detailed discussion of sampling and mineral separation techniques for the earlier studies is given in the original papers.
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Zircon was prepared for
18O analysis by sequential acid leaching in HNO3, HF, and HCl, as described by Lackey et al. (2006). HF preferentially dissolves radiation-damaged domains from zircon, which are subject to alteration of 18O, but does not affect crystalline zircon which preserves pristine 18O (King & Valley, 2001; Valley, 2003). Handpicking under a binocular microscope further purified zircon separates. For each analysis, comprising tens to hundreds of crystals, zircon was powdered using a boron carbide mortar and pestle before laser fluorination. Powdering creates uniform size and increases fluorination efficiency.
Mineral separates and whole-rock powders were analyzed in the University of Wisconsin Stable Isotope Laboratory by laser fluorination. Isotope ratios were measured on a dual inlet gas source Finnigan MAT 251 mass spectrometer. Whole-rock powders (2–2·5 mg) were analyzed with an airlock sample chamber (Spicuzza et al., 1998). Oxygen was liberated from silicates with BrF5 using a 30W CO2 laser (
= 10·6 µm), then purified cryogenically, passed through hot Hg to remove any residual F2, and finally converted to CO2 with a hot carbon rod (Valley et al., 1995). All analyses were standardized daily based on four or more analyses of UWG-2, Gore Mountain garnet standard, and sample 18O values were corrected to the long-term accepted value of 5·80 for UWG-2. The average raw 18O of UWG-2 for 40 days of analyses (n = 217) in this study is 5·71 ± 0·13 [1SD; standard error (1
= 1SD/(n2) is ±0·009]. Average day-to-day precision of UWG-2 is ± 0·06; daily corrections average 0·08. Average 18O of NBS-28 run on 7 days was 9·45 ± 0·13 after correction, n = 20, and 
(NBS-28–UWG-2) averaged 3·65 ± 0·12, in excellent agreement with the UW laboratory long-term (NBS-28–UWG-2) of 3·70 ± 0·06 (1SD) (Valley et al., 1995). During 9 days of whole-rock analyses using the airlock chamber, UWG-2 averaged 5·67 ± 0·11 (n = 41); replicate analyses of whole-rock powders averaged ±0·11. Replicate analyses give uncertainties of less than ±0·04% for zircon and ±0·09% for quartz.
Major elements and selected trace elements (Rb, Sr, Y, Nb, Zr) were determined for 96 samples by X-ray fluorescence at XRAL Laboratories, Canada. These data and discussion of analytical techniques are provided in Electronic Appendix 2.
Comparison of the 18O values of mineral pairs with equilibrium fractionation factors is used in several sections of the paper to test if minerals have exchanged oxygen, and the degree to which they approach isotopic equilibrium. Isotopic fractionation factors are expressed as
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); quartz–plagioclase = 1· 20 (Clayton et al., 1989); quartz–K-feldspar = 1· 00 (Clayton et al., 1989); quartz–biotite = 2·16 (Chacko et al., 2001); quartz–hornblende = 3·01 by combining the fractionation factors quartz–almandine (Valley et al., 2003) and garnet–hornblende (Kohn & Valley, 1998). |
RESULTS |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL BACKGROUNDSAMPLE SELECTION, PREPARATION,...RESULTSDISCUSSIONCONCLUDING REMARKSSUPPLEMENTARY DATAREFERENCES |
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Oxygen isotope ratios
Zircon
Zircon
18O in the study area ranges from 4·2 to 9·2 (Table 1). Variation of 18O is presented both as a color-coded map of the central Sierra (Fig. 4), and as two east–west transects (Figs 5 and 6) that divide the data approximately between the Fresno (Fig. 5; 36–37°N) and Mariposa (Fig. 6; 37–38°) 1/250 000 sheets (Fig. 1).
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Values span most of the terrestrial range for
18O of igneous zircon in plutonic rocks (Valley et al., 2005); 94% are above the mantle value for zircon (5·3 ± 0·3, 1SD; Valley et al., 1998), and the rest are in or slightly below the mantle range. Comparison of 18O variation between different grain-size fractions of zircon (see Table 1) reveals no consistent pattern of size versus 18O within samples (Lackey, 2005). Intra-pluton 18O variations typically are small, <0·3 (Table 1), and comparable with the range of 18O(Zrc) from volcanic systems with relatively simple zircon crystallization histories. For example, the Bishop Tuff varies by only 0·21 between early and late erupted units (Bindeman & Valley, 2002). The Mount Givens, Bass Lake, and Giant Forest Plutons are zoned in 18O (Figs 3 and 4).
Whole-rocks
Measured 18O(WR) across the central Sierra varies from 6 to 11·7, with one value as low as –0·5 (Table 1). The range of 18O(WR) values is larger than previously reported in the central Sierra (Masi et al., 1981; Kistler, 1990). Overall, this study adds considerably to the coverage of 18O(WR) in the Fresno sheet (Fig. 5b), and doubles the data for the Mariposa sheet (Fig. 6b), which increases the known range of 18O(WR).
Quartz
Values of 18O(Qtz) were analyzed from rocks throughout the central Sierra. The range of 18O(Qtz) is 8·8–12·0. Distribution of oxygen isotopes between coexisting quartz and zircon yields apparent temperatures of 575–650°C, which are significantly lower than are expected for magmatic crystallization (850–650°, Fig. 7). Similar 18O(Qtz–Zrc) disequilibrium is recognized in the Idaho batholith (King & Valley, 2001) and is explained by closed-system resetting of 18O(Qtz) during sub-solidus cooling, owing to relatively fast oxygen diffusion in quartz (Farver & Yund, 1991; Sharp et al., 1991), which yields a lower closure temperature (Dodson, 1973) for oxygen exchange in quartz than in zircon.
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Oxygen isotope mapping of the Central Sierra Nevada
High sampling density, homogeneity of
18O in most plutons, and thorough mapping allow us to present the results as a detailed isotopic map of the central SNB (Fig. 4). This representation both shows regional 18O variations and resolves isotopic zoning of intrusive suites with detail beyond simple contouring of 18O. Except where 18O varies by >0·5, values were averaged for plutons having multiple samples. An overarching pattern emerges with four north–south running belts of rocks having 18O(Zrc) that is greater or less than 6·5 (inset, Fig. 4). Nonetheless, there is substantial heterogeneity within the belts, including on-strike changes of 18O (see ‘Regional 18O belts’ below). |
DISCUSSION |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL BACKGROUNDSAMPLE SELECTION, PREPARATION,...RESULTSDISCUSSIONCONCLUDING REMARKSSUPPLEMENTARY DATAREFERENCES |
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Relating
18O(Zrc) to 18O(WR)Correlation with SiO2
To use zircon as a benchmark for evaluating the degree of contamination or sub-solidus alteration, the fractionation of
18O(Zrc) and 18O(WR) at magmatic temperatures must be calibrated. Overall, measured 18O(WR) correlates with 18O(Zrc), but 18O(WR) is more variable and 0·5–3·0 higher (Fig. 8a; Table 1). Values of 18O(WR) positively correlate with SiO2 content and 18O(Zrc) has a slight positive correlation. The high variance in Fig. 8a reflects differences of magmatic 18O throughout the central Sierra Nevada.
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Values of
18O(Zrc) are independent of wt % SiO2 in the Tuolumne Intrusive Suite, with one outlier, whereas 18O(WR) is positively correlated and varies by over 2·0 from 57 to 76 wt % SiO2 (Fig. 8a). Mafic magmas undergoing closed-system differentiation typically see 18O(WR) increase by 0·5 for each 10% increase in SiO2 (Taylor & Sheppard, 1986); however, results from the Tuolumne suite indicate that 18O(WR) in calc-alkaline systems increases by 
1· 0 for each 10% increase of SiO2. In general, increasing SiO2 in calc-alkaline igneous rocks correlates with increasing modal abundance of high-18O minerals such as quartz and feldspar, and with decreasing amounts of lower 18O minerals such as amphibole, biotite, and magnetite. The Tuolumne suite exemplifies how differentiation creates variability of 18O(WR) in a pluton or co-genetic suites, even if minerals have constant 18O, and why zircon best tracks magmatic 18O.
Various approaches can be used to predict equilibrium fractionations (18O) between igneous rocks, melts, and their various constituent minerals. Valley et al. (1994) used correlation of 18O(WR–Zrc) in four model rock compositions of varying SiO2 content to derive an approximate relationship of 18O(WR–Zrc) as a function of wt % SiO2. By assuming all minerals were in equilibrium at 800°C, they calculated whole-rock 18O values that were mass balanced for the mode of each mineral in a rock. Two recent studies have used normative mineralogy to calculate 18O(melt–mineral) values (Appora et al., 2003; Zhao & Zheng, 2003). This approach uses the principles of the Garlick Index (Garlick, 1966), which considers the degree of polymerization in a magma. Felsic magmas contain greater amounts of Al and Si, stabilizing minerals whose crystal chemistry favors stronger oxygen bonding, and therefore higher 18O/16O ratios. The disadvantage of using normative mineralogy includes omission of hydrous phases, poorly known fractionation factors for some normative phases, and mismatch between normative mineralogy and actual rock modes.
18O(WR–Zrc) in the Sierra Nevada
Modeling of 18O(WR–Zrc) uses a compilation of 297 SNB rocks ranging from gabbro to granite that the US Geological Survey analyzed for mineral modes and SiO2 (see Electronic Appendix 3). The silica content of each rock is directly related to 18O(WR–Zrc), which is calculated from the modal abundance and the equilibrium fractionation factors for quartz, K-feldspar, plagioclase, biotite, hornblende, and zircon (see ‘Methods’). Solidus temperatures appropriate for magma compositions ranging from gabbro to high-silica granite were chosen (Fig. 8c), with the justification that both solidus and zircon saturation temperatures decrease with increasing SiO2, such that 18O(WR–Zrc) represents different temperatures across a range of compositions. Recent Ti-in-zircon thermometry of a representative suite of these rocks confirms higher average Ti-in-zircon temperatures in diorites and gabbros than in peraluminous granites (Fu et al., 2008). Solidus temperatures were assumed to have decreased linearly from 900°C (gabbro, PH2O = 3·0 kbar, 45 wt % SiO2) to 650°C (high-silica granite, PH2O = 3·0 kbar, 78 wt % SiO2). Calculated 18O(WR–Zrc) values show a strong (r2 = 0·96) positive correlation with SiO2 (Fig. 8c). A linear fit yields the following equation:
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18O(WR) values to be calculated based on measured 18O(Zrc) and whole-rock silica content. Possible sources of error on calculated 18O come from uncertainty in SiO2 content and 18O(Zrc). If SiO2 and 18O(Zrc) uncertainties are 0·5% and 0·1, respectively, the error on the calculated 18O(WR) is ±0·12.
Comparison of measured and calculated 18O(WR) values shows that many Sierran rocks deviate from the predicted values (Fig. 8d). To estimate the number of rocks that are hydrothermally altered, the average analytical precision (0·12, 1SD, Fig. 8d) of 18O(WR) analyses was used as a cutoff. In cases where calculated and measured 18O(WR) values vary by greater than ±0·12 of the calculated WR uncertainty and 3SD of analytical precision (>0·36), the 18O(WR) is considered altered. By these criteria, approximately 28% of the 18O(WR) values in the central Sierra are altered. Many values deviating by less than 0·36 are probably altered as well; thus 28% is a minimum estimate. Textural indicators of alteration include widespread sericitization of feldspar and chlorite and epidote veins and overgrowths.
Inspection of Fig. 8d shows that most of the measured 18O values showing alteration are higher than their expected magmatic value. Closed-system sub-solidus exchange in a rock will result in shifts in 18O among the minerals in a rock relative to one another, but the overall 18O(WR) will remain unchanged. Alteration causing a shift in 18O(WR) typically results from sub-solidus exchange with externally derived fluids, possibly at a wide range of temperatures and fluid compositions. The positive shift of 18O(WR) observed for most samples is inconsistent with infiltration of meteoric water; however, three samples with low-18O(WR) values, 2–5, confirm some sub-solidus infiltration of heated meteoric water in the eastern Sierra (Fig. 8d). The samples come from areas of the eastern Sierra where igneous barometry indicates shallow (<2 kbar) depths of crystallization (Ague & Brimhall, 1988a; Ague, 1997). Hydrogen isotope data from these same areas confirm infiltration of meteoric water as D(WR) values are commonly less than –85 (Godfrey, 1962; Masi et al., 1981). In addition, samples from the peraluminous Grant Grove pluton have measured 18O(WR) values that are 1–2 greater than predicted (Fig. 8d). These samples are from the margin of the pluton and record contamination by high-18O wall-rocks after zircon crystallized (Lackey et al., 2006).
Although generally slight, alteration is enough to obscure isotopic zoning in plutons or intrusive suites and can hide detail such as 18O discontinuities. Figures 5a, b and 6a, b show direct comparison of measured and calculated 18O(WR) across the SNB. In these two transects, additional calculated 18O values are shown, although they are less precise because SiO2 was not measured for those samples and was estimated from published SiO2 data (see Electronic Appendix 4). This direct comparison along transects shows local zones of alteration. For instance, the White Mountains have both low and variable measured 18O, and calculated values that are higher and less variable, suggesting additional areas of meteoric water infiltration, one of which is confirmed below.
Two case studies distinguishing magmatic and alteration history
The Barcroft pluton
In the Jurassic Barcroft pluton, northern White Mountains (Fig. 3), pervasive secondary mineralization and a wide range of 18O(WR) values (6–10) indicate hydrothermal alteration of the pluton (Ernst & Rumble, 2003). Three widely spaced zircon samples in the pluton show that magmatic 18O was in fact very homogeneous: 7·60 ± 0·08 (Fig. 9a). Using the zircon data from this study and measured SiO2 of Ernst & Rumble (2003), magmatic 18O(WR) is calculated to vary less than 1 in the pluton. All measured 18O(WR) values deviate over 1 from calculated values, and three of four measured values are lower than calculated (Fig. 9a). Although none of the 18O(WR) values that Ernst & Rumble (2003) reported is exceptionally low, the tendency toward lower measured values confirms some meteoric water infiltration during hydrothermal alteration of the Barcroft pluton.
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Saddlebag Lake meta-rhyolite
A Triassic rhyolite in the Saddlebag Lake pendant, east of the Tuolumne Suite (Fig. 3), has a complex magmatic, alteration, and metamorphic history. The rhyolite has a typical
18O(Zrc) value of 6·2. In contrast, the 18O values of the whole-rock and quartz phenocrysts are 11· 6 and 12·6, respectively; both are elevated well above typical igneous values and confirm alteration (Fig. 9b). If the original SiO2 content of the rhyolite was 70 wt %, then the calculated 18O(WR) is c. 8·3 (Fig. 9b), much lower than the measured value.
Metamorphic garnet in the meta-rhyolite yields another constraint on its alteration history. Because garnet has slow oxygen diffusion (Couglan, 1990; Vielzeuf et al., 2005), like zircon, it records the 18O of a rock at the time it crystallizes. For Fe–Mn garnets, equilibrium fractionation with zircon is 0 at magmatic temperatures (Valley et al., 2003). Therefore, the 9 garnet is not equilibrated with the 6·2 zircon. As such, a chronology of events affecting the rhyolite is: (1) the rhyolite was erupted; (2) hydrothermal alteration elevated 18O(WR); (3) Cretaceous metamorphism of the altered rhyolite led to garnet crystallization and re-equilibration of all phenocrysts except zircon.
Regional 18O belts
Zone 1: Foothills belt
Mantle-like 18O(Zrc) values occur in and around mafic ring dike complexes in the western Sierran foothills (Fig. 4, inset). This low-18O domain abruptly changes to high-18O rocks 20 km into the batholith (Fig. 5a). The extent of interaction between basement rock and magmas in this belt is unclear, although detailed O, Sr, and Nd isotope investigation of the ring dike complexes indicates a nonradiogenic, depleted mantle source with melting at pluton–wall-rock contacts causing some localized contamination (Clemens-Knott, 1992). Mantle-like 18O(Zrc) values appear to rule out widespread contamination; however, the average 18O(WR) of the wall-rocks, a mélange of the Kings–Kaweah ophiolite belt (Saleeby, 1992; Saleeby & Busby, 1993), is 7·1 ± 1·3 (Lackey et al., 2006). Thus, there is minimal magma–wall-rock isotopic contrast, which would obscure source or contamination contributions from melting of ophiolitic rocks.
Zone 2: Fine Gold–Sequoia belt
In the Fine Gold Suite 18O decreases from west to east, and is negatively correlated with Sri (Figs 4 and 6a); rocks with high 18O(Zrc), up to 8·7, have low Sri (0·704). The pattern of these isotope ratios requires substantial input of hydrothermally altered ocean crust or volcanic arc sediments in the western parts of the Fine Gold suite (e.g. Fig. 2a). In ocean crust, circulation of heated water increases the 18O(WR) of upper pillow basalt and sheeted dike sections considerably from mantle values (altered 10, unaltered 5·4–6·0; McCulloch et al., 1980; Muehlenbachs, 1998). Low Sri values result because 87Sr/86Sr is only modestly elevated from mantle values by alteration (McCulloch et al., 1980; Muehlenbachs, 1998). Furthermore, low Sri indicates that the rocks are relatively young because insufficient time has passed for significant ingrowth of 87Sr. Similar 18O and Sri would be expected for arc volcanic rocks, such as greywacke, that have been hydrothermally altered (Magaritz & Taylor, 1976).
East of the Fine Gold rocks, 18O and Sri show positive correlation in a series of plutons that lead up to the PA/NA break (Figs 3 and 6). In the Fresno Sheet, rocks of Zone 2 initially have high 18O and sub-0·706 Sri values (Fig. 5; 38–55 km), indicating sources with juvenile supracrustal rock, but less than the Fine Gold Suite. Eastward increasing 18O and Sri in the belt, into the Sequoia region, mimics the pattern in plutons east of the Fine Gold Suite, with the PA/NA break again marking the transition to lower 18O.
Overall, the geochemistry of the plutons in Zone 2 reflects the influence of the exposed wall-rocks. For instance, rocks of the Fine Gold Suite with the highest 18O are in direct contact with volcaniclastic rocks of the Jurassic–Triassic island arc belt and outboard accretionary sequence of the Sierran foothills (Fig. 4; Snow & Scherer, 2006). These rocks have the exact isotopic composition, with low Sri (0·704–0·707; Kistler & Peterman, 1973) and high 18O(WR) (8–14; Böhlke & Kistler, 1986), to produce the local contamination patterns in the Fine Gold Suite. Negative correspondence of Sri and 18O on the east side of the Fine Gold Suite suggests differences of magma source and contamination (Fig. 6). Permo-Triassic chert–argillite mélange of the Calaveras Complex is in fault contact with the Jurassic–Triassic metamorphic rocks belt near the 0·706 line (Fig. 4). Pendants and septa show southeastern continuation of both metamorphic belts to their termination near the boundary of the Mariposa and Fresno sheets (Fig. 4). Therefore the Bass Lake Tonalite intrudes across the fault contact into both metamorphic belts. Because Calaveras rocks contain greater amounts of continental sediments than rocks to the west, eastward increases of Sri in the Bass Lake Tonalite indicate greater proportions of Calaveras Complex rock in magma sources or as a contaminant (Fig. 6a and b). Decreasing 18O may reflect greater depleted mantle contributions to the east or lower degrees of partial melting of refractory chert and argillite-rich Calaveras rocks. Minor partial melting, especially involving labile phases such as micas, can radically shift magmatic Sri without markedly changing 18O. Thus, the 0·706 line may reflect crustal architecture as well as wall-rock fertility.
Another example of wall-rock influence on magma chemistry in the Zone 2 high-18O belt concerns the Kings Sequence, which contaminates magmas throughout the Sequoia region (Fig. 4; Ague & Brimhall, 1988b; Kistler, 1990; Chen & Tilton, 1991). Kings Sequence rocks and some from the Calaveras Complex host the Strongly Contaminated and Reduced plutons of Ague & Brimhall (1988b). These plutons, including several that are peraluminous (Fig. 3), intrude Kings Sequence rocks (Fig. 3). Ague & Brimhall (1988b) showed increased Fe/(Fe + Mg) in biotite in these rocks and proposed that graphite-rich Kings Sequence rocks led to magmas being relatively reduced, 0–2 log fO2 units below QFM (quartz–fayalite–magnetite). High 18O(Zrc) > 7·5, but relatively low Sri (0·704–0·707), in these plutons shows that they contain high proportions of young supracrustal rocks in their sources; comparison of pluton margin and interior geochemistry and mineralogy reveals localized melting and contamination by the Kings Sequence (Lackey et al., 2006).
Zone 3: Sierra Crest and eastern Sierra
East of Zones 1 and 2 there is a monotonic decrease in 18O toward the Sierra Crest (Fig. 5, 55–104 km; Fig. 6, 85–130 km). Many of the values in the eastern Sierra fall in the range of mantle 18O(Zrc) values (5·3 ± 0·3, Valley et al., 1998). Calculated 18O(WR) values for granodiorites and granites in this belt are 7–8. These relatively low values agree with studies proposing that significant portions of the late Cretaceous SNB were generated directly from melting of enriched (Sri = 0·706; 
Nd = –4·5) lithospheric mantle beneath the eastern Sierra (Coleman et al., 1992; Coleman & Glazner, 1997). Although pendants of metavolcanic and metasedimentary rock are exposed in the eastern Sierra (Fig. 3), 18O values do not indicate significant magmatic incorporation of these or other crustal rocks.
Zone 4: Owens Valley–White Mts. belt
Triassic, Jurassic, and Cretaceous plutons in the Owens Valley, Long Valley re-entrant, and White Mountains typically have 18O(Zrc) > 6·5 (Fig. 4). Values of Sri in these rocks often exceed > 0·709 (Fig. 6c), indicating magma sources containing aged sediments or crust. Wall-rock contamination effects are pronounced in many of the solitary plutons in the White Mountains that intrude Proterozoic to Paleozoic continental-derived sedimentary rocks. Values of Sri and Nd at the margin and cupola regions of the plutons are more crustally influenced (Sri > 0·710, Nd < –9·0) than their interiors (Sri < 0·706, Nd > –1· 5; Ernst et al., 2003), indicating ‘veneers’ of contamination.
Crustal boundaries
Projection of lateral changes of 18O in the SNB shows mostly gradual changes (Figs 5 and 6); however, localized, sharp, step-function changes of 18O suggest steeply dipping trans-crustal breaks. Abrupt lateral shifts in 18O suggest that magmas generated on either side of a break inherit the 18O signature of the terrane in which they form, with a steep geometry of the terrane boundary preventing significant averaging of values across the break (see Peck et al., 2004). Below we discuss two significant lithospheric breaks that are resolved by 18O(Zrc), and the position of the PA/NA break.
Foothills Break
The abrupt increase of 1· 5 in 18O(Zrc) that separates Zones 1 and 2 in the western Fresno sheet (20 km, Fig. 5a), is not discernible from 18O(WR) or Sri (Fig. 5), but may be the geochemical expression of the Foothills suture (Saleeby, 1992), which is poorly exposed. Saleeby (1992) proposed northward projection of the Foothills suture into the Bass Lake Tonalite, but the limited extent of Zone 1 low-18O rocks to the north means that the break terminates before the tonalite. Greater wall-rock heterogeneity to the north and different magmatic style may conceal the break.
Eastern Sierra Break
The sharp 1 west-to-east increase of 18O(Zrc) defining the boundary between Zones 3 and 4 (Figs 4 and 6a, 128 km), defines a feature in the eastern Sierra and Owens and Long Valleys that has long been inferred to be a major crustal boundary (Saleeby et al., 1986; Kistler, 1993). This discontinuity corresponds to Intrabatholithic Break 3 (IBB3) of Saleeby et al. (1986), and the Tinemaha Fault of Stevens et al. (1997). This boundary was originally recognized by displacement of Jurassic and Triassic rocks along a lineament that intersects a major right-lateral offset in the 0·706 line in Long Valley (Fig. 4). The boundary is also inferred from structural and stratigraphic relationships where SW-trending Neoproterozoic to Triassic sedimentary shelf sequences in the White–Inyo Mountains are juxtaposed against the NW-trending SNB (Stevens et al., 1997). A steeply dipping geometry for this break favors tectonic truncation as the mode of origin, and the expression of the 18O step in Triassic and younger plutons means that the boundary is at least pre-Jurassic. In addition, recent dating of dikes offset in the Owens Valley indicates that the 65 km of dextral offset along this break occurred since 83 Ma (Kylander-Clark et al., 2005).
PA/NA Break
Values of 18O(Zrc) decrease gradually at the PA/NA break (Figs 5a and 6a) and generally correspond to its originally proposed location (Fig. 4). The lack of a sharp step in 18O suggests a diffuse transition, rather than a trans-crustal break; yet changes in isotopic gradients, and the preponderance of data from the high- to low-18O zones that define it, indicate that this is a major lithospheric discontinuity. Potentially, the integrated effects of lithospheric mantle differences as well as an upper crustal overprint have blurred the break; however, such complex controls cannot be evaluated with certainty. Although the PA/NA break remains enigmatic, 18O(Zrc) values show that some adjustments of the position of the PA/NA break are needed.
In the Fresno sheet, we propose two refinements. First, the trace of the break appears to be west of the Mineral King pendant, rather than through it (Fig. 4). Whereas Kistler (1993) proposed linking the PA/NA break to faults in the Mineral King pendant, the low 18O(Zrc) of the pluton to the west of the pendant (granodiorite of Castle Creek) suggests that the break is 10 km to the west (Fig. 4). Whereas the Mineral King pendant has lithologies similar to the Kings Sequence, volcanic rocks in the pendant have affinities with eastern terranes (Saleeby & Busby, 1993). The second adjustment is a 5 km eastward shift of the break at the Boyden Cave pendant (Fig. 4). The pendant clearly juxtaposes rocks from the Kings Sequence and the Goddard terrane to the east; however, refinement based on 18O may better reflect the deep orientation.
North of the Mount Givens pluton, in the Mariposa sheet, the original PA/NA break strikes NW and joins the 0·706 line; however, high- and low-18O domains are continuously juxtaposed to the north (Fig. 4). If the bulbous westward protrusions of the Mount Givens pluton and Tuolumne Intrusive Suite are ignored as distortional effects (Fig. 4), the 18O-defined PA/NA break projects directly through Yosemite to the offset 0·706 line in the Sonora Intrusive Suite (Fig. 4). Whereas Kistler (1993) favored the joining of the PA/NA break and 0·706 line to include the Melones fault as a northern extension of the PA/NA break, Seleeby et al. (1986) proposed that a straight intrabatholithic break, labeled IBB2 in Fig. 4, continues from the PA/NA trace into Yosemite (Kistler, 1993). The pattern of 18O supports the northward trend into Yosemite.
Direct comparison of O, Sr, and Pb isotopes
Chen & Tilton (1991) reported Sr and Pb isotope ratios on a subset of samples analyzed in this study from the Fresno sheet (see Chen samples in Fig. 3). Therefore, correlation of O, Sr, and Pb isotope ratios can be evaluated for a representative suite of rocks across the SNB (Fig. 10). Plots depict Sr and Pb isotope variation relative to 18O of zircon (Fig. 10a–d) and whole-rocks (Fig. 10e–h) to evaluate the effect of differing silica among samples on isotopic correspondence. The samples are divided into western and eastern samples; western samples have Sri < 0·706; eastern samples occur east of the PA/NA break. A third group of samples are from the Sequoia region (Fig. 3).
Oxygen and strontium isotopes
Co-variation of 18O(Zrc) with Sri is geographically distinct for the western and eastern Sierra (Fig. 10a). Calculated 18O(WR) comparisons yield more scatter but similar trends (Fig. 10e). The array of 18O and Sri for western samples intersects a mantle value at the low-Sri end (Clemens-Knott, 1992) and trends toward an evolved (high-Sri) reservoir; overall there is resemblance to the Peninsular Ranges trend (e.g. Fig. 2b). One sample from the margin of the Grant Grove pluton lies well off the western trend, recording contamination typical of the Sequoia region (Fig. 10a).
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Eastern samples also exhibit a distinct correlation, with a less steep positive slope, forming a flat array of high Sri values throughout the eastern Sierra. Both low- and high-
18O plutons indicate considerable heterogeneity in the crustal content of these magmas, with low 18O values reflecting old lithospheric mantle (e.g. Coleman & Glazner, 1997); high 18O values record melting of crust with variable proportions of supracrustal-derived rock.
Sequoia region rocks have distinctly higher Sri values relative to the eastern and western series, but the data are more scattered (Fig. 10a). The variable 18O and Sri of this sample group exemplifies the widespread but heterogeneous contamination by the Kings Sequence.
Oxygen and lead isotopes
Lead isotope ratios positively correlate with 18O(Zrc) and western and eastern trends are distinct (Figs 10b–d). Calculated whole-rock 18O again shows more scatter, but, nevertheless, consistent patterns (Figs 10f–h). All initial Pb isotope ratios (206Pb/204Pb; 207Pb/204Pb; 208Pb/204Pb) become more radiogenic with increasing 18O(Zrc). The lowest 18O values of both eastern and western samples tend to have similar initial 206Pb/204Pb and 207Pb/204Pb, close to the primitive mantle value at 100 Ma (Chen & Tilton, 1991). Chen & Tilton (1991) suggested that eastern samples have higher 208Pb/204Pb because U/Pb was lower than Th/Pb in the source rocks, an indication of preferential extraction of U by fluid escape during granulite-facies metamorphism.
The increasing variability of initial Pb isotope composition with 18O in western samples produces a ‘fan-like’ spread of the data, which suggests a common low-18O mantle reservoir input (Fig. 10b–d), but highlights source or contamination complexity in high-18O rocks that is not evident from Sri (Fig. 10a). Most western samples document input of aged crust or sediments thereof; however, a group of four samples from the westernmost rocks change little in initial Pb, whereas 18O(Zrc) ranges from 6·0 to 7· 0; these same samples trend toward increasing Sri (Fig. 10a). Not surprisingly, the O–Pb–Sr systematics of these samples are consistent with incorporation of altered ocean crust or arc volcanic rocks. Elevated Sri values but low initial Pb values show decoupling of these radiogenic isotope systems during supracrustal alteration of the source rocks.
Sequoia samples are also scattered with respect to 207Pb/204Pb and 208Pb/204Pb (Fig. 10c and d); 207Pb/204Pb values overlap considerably with those for western and eastern rocks (Fig. 10c). The highly variable 18O–Pb pattern, and the fact that many samples fall between the western and eastern trends, suggests a diverse source and contamination history for this group of plutons.
Large-scale magma systems
Positive correlation of O, Pb, and Sr isotopes indicates both mantle and crustal inputs in the western and eastern Sierra magmas. Each sample defining these trends is a complex integration of source characteristics and contamination, magma mixing, differentiation, and assimilation, obscuring the origin. Nevertheless, correlations for eastern SNB samples are reasonably strong in most cases and distinct in slope from western SNB rocks. This is a remarkable pattern considering that samples in each group vary considerably in age, up to several million years, and location within their respective domains (Fig. 3). Therefore, we hypothesize that two distinct magma systems operated.
It is widely acknowledged that different ‘styles’ of magmatism occur in the western and eastern Sierra (e.g. Chen & Tilton, 1991; Coleman & Glazner, 1997; Tikoff & de Saint Blanquat, 1997; Wenner & Coleman, 2004), and the Peninsular Ranges batholith bears some similarity (Gromet & Silver, 1987; Tulloch & Kimbrough, 2003; Lee et al., 2007). The isotopic patterns imply unique controls on isotope ratios on each side of the batholith that permitted largely continuous trans-batholith gradients in major and minor element chemistry (e.g. Bateman, 1992).
Differing lithosphere compositions in the SNB undoubtedly provide a major control on the isotopic composition of the magmas (Kistler, 1990); yet to average out heterogeneity of crust and mantle reservoirs and produce well-defined isotopic trends requires a long-lived and stable source region, such as is envisaged by the melting, assimilation, storage, and homogenization (MASH) model of Hildreth & Moorbath (1988). Ducea (2001, 2002) and Saleeby et al. (2003) adopted the MASH model to explain magmatic averaging that results in complementary geochemistry between batholithic crust and xenoliths thought to represent residues of melting at the base of the arc crust. Annen et al. (2006) proposed the existence of deep crustal ‘hot zones’ beneath large magma systems in which the geochemical character of the magmas is largely imparted in the source but textural diversity is acquired in the crust during crystallization. Clearly, other models may be applicable, but the MASH and ‘hot zone’ models are apt in that they permit large-scale magma systems that could support long-term geochemical connectivity, but allow for progressive emplacement of texturally diverse plutons.
In addition to basement composition effects, the two magma systems probably varied in depth of melting. In particular, eastern Sierra 18O values indicate mostly mantle sources. This finding concurs with a growing consensus of studies invoking direct melting of the lithospheric mantle and remelting of underplated juvenile magmas as the origin of most magmas in the eastern Sierra (Coleman & Glazner, 1997; Wenner & Coleman, 2004; Ratajeski et al., 2005; Sisson et al., 2005). Broadly characterized, this magma system was driven by massive infracrustal recycling at a deep locus of melting. Any crustal melting that did happen would involve lower crust composed of orthogneiss or amphibolite metamorphosed to granulite facies (Chen & Tilton, 1991); increases of 18O from melting of such rocks would be minimal (Kistler, 1990).
Except for the Foothills belt, which represents the nascent stages of magmatism in the Sierran arc, greater crustal melting, recycling, and contamination, including abundant melting of supracrustal rocks, defines the western magma system. The high-18O magmas of the Fine Gold–Sequoia belt, and evidence of wall-rock controls on 18O are consistent with a shallow locus of magma generation that melted and reworked the crust considerably. The high content of greywacke and hydrous, disaggregated ophiolite lithologies present in the basement rocks of the western Sierra potentially proved more fertile for melting and facilitated recycling (Montel & Vielzeuf, 1997; Vielzeuf & Schmidt, 2001).
Small-scale magma systems: plutons and intrusive suites
Beyond evidence for two large-scale magmatic domains in the SNB, we observe intriguing spatial and temporal patterns of 18O variation within intrusive suites and individual plutons. Examples of these smaller-scale systems are discussed below and we evaluate the causes of their isotopic diversity and implications for magma sources, contamination, and batholith construction.
Mount Givens Granodiorite
The Mount Givens Granodiorite exhibits a gradual 1 increase in 18O(Zrc) on its west side (Fig. 11a) where the pluton abuts the older Dinkey Creek Granodiorite (102 Ma, Tobisch et al., 1995) at the PA/NA boundary (Fig. 6a). Compared with zircon, the calculated 18O(WR) blurs this pattern because inward changes in lithology from a tonalitic margin to a granitic interior result in highly variable calculated 18O(WR) values (Fig. 11b). Because textures and mineral proportions vary considerably (Bateman, 1992; McNulty et al., 2000), as does the range of U–Pb zircon ages (87·9–92·8 Ma), the pluton appears to be constructed by multistage growth (McNulty et al., 2000). The trend of 18O(Zrc) suggests that the western margin of the pluton has a geochemical affinity with the PA domain, whereas the 18O of the younger eastern areas reflects the NA domain. Similarly, the Eagle Peak pluton, just to the west of the Mount Givens pluton (Fig. 4), shows zoning in O, Sr, and Nd isotopes that hints at variable contamination of the magma as it ascended along the PA/NA break (Hill et al., 1988).
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Sequoia Intrusive Suite
The Sequoia Intrusive Suite (Fig. 3) exhibits distinct
18O zonation. The outer and largest pluton, the Giant Forest Granodiorite, has the highest 18O(Zrc) values of the suite and greater overall variation in 18O; the interior granites of Big Meadows and Weaver Lake have lower average 18O values that are less variable (Fig. 12a). Average Sri also decreases inward in the suite: Giant Forest = 0·70799 ± 0·00204, n = 6; Big Meadows = 0·70733 ± 0·00011, n = 2; Weaver Lake = 0·70657 ± 0·00032, n = 4 (Chen & Tilton, 1991; Wenner & Coleman, 2004). Thus, the margin of the suite exhibits greater supracrustal contamination. In detail, samples from the Giant Forest Granodiorite have higher 18O in the southern half of the pluton where it abuts the Kings Sequence rocks of the Sequoia pendant (Fig. 3, Table 1). Such localized effects indicate upper crustal contamination and explain the 18O heterogeneity in the pluton. The Giant Forest pluton appears to have shielded the ‘core’ of the suite from contamination. Therefore, the lower isotope ratios of the interior plutons better represent the suite's source composition.
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Sierra Crest Suites
Evolution of magmatic
18O(Zrc) from oldest to youngest units is apparent in some plutonic suites exposed in the Sierra Crest (Fig. 12b–d). Relatively constant 18O(Zrc) values occur in the Tuolumne Intrusive Suite (Fig. 12b), but others show progressive 18O changes (e.g. Sonora, John Muir, Fig. 12c and d). In addition, 18O is often homogeneous within each pluton of a suite, but varies between plutons.
The John Muir Suite
Plutons of the John Muir Suite (JMS, Fig. 3) display intriguing spatial and temporal trends of 18O(Zrc). A subset of the JMS, the Mono Pass Suite, is approximately coeval with the Tuolumne Intrusive Suite, and includes, from oldest (95·3 Ma) to youngest (87· 0 Ma), the gabbro of Rock Creek, Lake Edison Granodiorite, Round Valley Peak Granodiorite, and Mono Creek Granite (Gaschnig et al., 2006). The Lamarck Granodiorite (94–92 Ma, J. Gracely, personal communication) and the Evolution Basin Alaskite (94–92 Ma, Wenner & Coleman, 2004) are exposed to the south of the Mono Pass Suite, and a new U–Pb zircon age for the Inconsolable Granodiorite (95 Ma, J. Gracely, personal communication) suggests that it may be a low-silica end-member of the southern JMS.
Values of 18O(Zrc) are reported for all the plutons of the JMS except the Rock Creek gabbro and Evolution Basin Alaskite (Table 1). Relative to pluton age, 18O(Zrc) decreases with age in the two oldest plutons. It displays considerable variation in the Lake Edison pluton; 18O(Zrc) is highest in the Round Valley Peak Granodiorite, and decreases in the Mono Creek Granite (Fig. 12d). Except for the Lake Edison Granodiorite, most plutons are relatively homogeneous in 18O. Variation of 18O vs SiO2 for John Muir samples also reveals two parallel trends of decreasing 18O with increasing SiO2 (Fig. 12e). One trend, the ‘southern trend’ is defined by the two southern plutons (Inconsolable and Lamarck), whereas the Round Valley Peak, Mono Creek, and some of the Lake Edison samples define the ‘northern trend’.
To explain the offset 18O trends in the JMS, we propose that the southern and northern trend plutons were produced from two distinct sources. The Lake Edison Granodiorite, with its wide range of 18O, suggests a hybrid pluton produced by mixing between the two sources. Such behavior supports models for magma mixing as the origin of some granodiorites in the Sierra Nevada (Sisson et al., 1996; Wenner & Coleman, 2004). In addition, higher 18O in the older plutons indicates that the early formed magmas contained higher proportions of recycled supracrustal rocks than later magmas. Decreasing 18O in the younger plutons is consistent with exhaustion of fusible crustal rocks in the magma sources, increased mantle contributions, or possibly conditioning of magma conduits (Scoates & Frost, 1996).
Temporal patterns of 18O
Considering changes in 18O(Zrc) over time for all rocks gives a more complete sense of the evolving arc magma system (Fig. 13a). In addition, magma production rates deduced from measurement of pluton area (Fig. 13b) reveal that the Cretaceous magma system experienced several episodic magma ‘pulses’. These pulses are resolved at finer time-scales than the magmatic ‘flare-ups’ of Ducea (2001), and they correspond to changes in 18O.
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The first magmatic pulse, marked by increasing
18O, resembles the Peninsular Ranges trend, and shows more supracrustal input with time. This period includes the transition from magmatism in the Foothills belt to the Fine Gold Suite and western Sequoia region. Subsequent pulses at 103–102 Ma and 99–97 Ma in the western and axial potions of the SNB saw widening of the range of 18O to both higher and lower values. A temporary decrease in magma production at around 95 Ma (Fig. 13b) led to voluminous magmatism in the eastern SNB. The pronounced downward shift of 18O(Zrc) emphasizes the difference of the eastern Sierra magma system.
In arc settings, magmatism, heat flow, and development of MASH domains or ‘hot zones’ should facilitate greater crustal recycling with time. A secular change to increased recycling of supracrustal rocks is demonstrated globally in a study of 18O of igneous zircon spanning most of Earth's magmatic history (Valley et al., 2005), and increasing ‘supracrustal oxygen’ with time is seen during magmatic refinement of the Gondwana supercontinent (Kemp et al., 2006) and Superior craton (Moser et al., 2008). In the SNB, individual magmatic pulses generally record greater crustal recycling with time (Fig. 13), including the eastern Sierra where 18O(Zrc) increases slightly. Some short-lived pulses (e.g. 103–102 Ma) are isotopically homogeneous, suggesting insufficient time for the onset of melting of crustal material. Overall, the SNB demonstrates that arcs may not record the same supracrustal oxygen increase as occurs during long-term crustal growth, but that such patterns are resolvable at the time-scale of individual magmatic episodes.
Evolution of the Sierran arc
We now integrate spatial and temporal patterns of SNB magma geochemistry and present a tectonic model in the form of a scheme portraying three critical times during construction of the batholith (Fig. 14).
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Following the Nevadan Orogeny, the future Sierra was underlain by basement oceanic terranes accreted to North America and overlying sedimentary cover sequences (Fig. 14a). Subduction and underplating, or underthrusting in the Triassic and Jurassic probably contaminated the sub-arc lithospheric mantle. The eastern Sierra inherited Proterozoic North American crust and complementary lithospheric mantle, as shown by the
18O break recorded in Jurassic and Triassic plutons that stitch across the eastern Sierra Break (Fig. 14a), and the patterns of supracrustal contamination in these and Cretaceous plutons. Subduction of the Farallon Plate initiated major magmatism by 120 Ma and produced significant batholith volume by 110 Ma (Figs 13a and 14b). Largely depleted mantle sources produced the plutons in the Foothills Zone, with minimal crustal interaction. The Fine Gold Suite, however, documents the onset of massive supracrustal recycling. Increased involvement of the wall-rocks of the Calaveras Complex, Kings Sequence, and cryptic contributions from altered ocean crust and Proterozoic rocks led to more complex isotopic signatures in magmas towards the PA/NA break.
The later stages of magmatism in the SNB highlight the role of tectonic reconfigurations in magma evolution. The deformation field of the Sierran arc is believed to have switched from contraction to transpression and dextral shearing because of changing convergence of the Farallon Plate in the late Cretaceous (Tobisch & Cruden, 1995; Tobisch et al., 1995; Tikoff & de Saint Blanquat, 1997; Fig. 14c). Recent work on the proto-Kern Canyon fault shows evidence for intra-arc shortening at 95 Ma in response to this convergence switch (Nadin & Saleeby, 2008). Shortening potentially suppressed voluminous magmatism, as is seen in the lull at 95 Ma (Fig. 13b). However, post 95 Ma transpression established a positive feedback between magmatism and deformation, facilitating emplacement of the Sierra Crest magmas (Glazner, 1991; Tikoff & de Saint Blanquat, 1997). Shear-enhanced magma emplacement of eastern Sierra magmas possibly facilitated intrusion from MASH zone sources into the upper crust, thereby reducing crustal contamination. Likewise, transpressional conduits may have affected the constancy of 18O seen within some units. Homogeneous 18O in some plutons in the Tuolumne Suite emplaced over 2–3 Myr (Coleman et al., 2004) favors enduring magma sources and conduits. Spatially variable 18O in plutons such as the Mount Givens indicates heterogeneous magmas, possibly the resulting of switching of sources (and conduits) during deformation, varying amounts of contamination, or magma mixing in evolving magma plumbing systems. In the southern Sierra, greater recycling of crustal rocks with time (Lackey et al., 2005) corresponds to increased ductile deformation, including both vertical and lateral transport of wall-rocks in response to elevated geothermal gradients (Saleeby et al., 2003). Thus, tectonic ‘re-shuffling’ in the arc, such as downward flow of wall-rocks, or intra-arc thrusting (Fig. 14c), could possibly have rejuvenated magma sources with crustal rocks.
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CONCLUDING REMARKS |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL BACKGROUNDSAMPLE SELECTION, PREPARATION,...RESULTSDISCUSSIONCONCLUDING REMARKSSUPPLEMENTARY DATAREFERENCES |
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The data presented in this study and by Lackey et al. (2005
, 2006) put in place a modern and comprehensive oxygen isotope evaluation of the Sierra Nevada batholith. Values of 18O(Zrc) provide clear records of changing magmatic 18O in the batholith, and calculation of 18O(WR) values using zircon as a magmatic benchmark can be used to circumvent widespread resetting of the 18O of whole-rocks and single minerals such as quartz. Moreover, examples from the Barcroft pluton and Saddlebag Lake meta-rhyolite illustrate how 18O analysis of a refractory mineral such as zircon is a powerful means to deconvolve magmatic and metamorphic histories previously unobtainable from pervasively altered rocks.
Recognition of distinct belts of 18O values reveals both transverse and longitudinal domains within the batholith and a markedly more complex system than in the coeval Peninsular Ranges batholith. Large-scale pre-batholith boundaries resolved by 18O mapping better define the locations of previously inferred terrane boundaries, including cryptic boundaries such as the Foothills Break. Furthermore, the pre-batholith arrangement of basement and supracrustal rocks, structural reconfiguration during the evolution of the batholith, and different magma source depths, led to the overall patterns of 18O. Lastly, the discrete magma systems that we resolve at different scales in time and space, from batholith to pluton scale, emphasizes the dynamic nature of the Sierran arc.
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SUPPLEMENTARY DATA |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL BACKGROUNDSAMPLE SELECTION, PREPARATION,...RESULTSDISCUSSIONCONCLUDING REMARKSSUPPLEMENTARY DATAREFERENCES |
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Supplementary data for this paper are available at Journal of Petrology online.
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ACKNOWLEDGEMENTS |
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All isotope analyses were completed by J.S.L. in the University of Wisconsin Stable Isotope Laboratory with support from DOE 93ER14389 and NSF EAR99-02973 and EAR02-07340. A UW Dean Morgridge Distinguished Graduate Fellowship, GSA and Sigma Xi grants, and the UW Department of Geology and Geophysics Weeks Fund supported portions of this work. Pomona College Geology Department faculty research funds provided valuable support for publication of this study. We thank Mike Spicuzza for assistance with stable isotope analysis, Brian Hess for making thin sections and grain mounts, and Clark Johnson and Brian Beard for the use of the rock crushing and mineral separation laboratories. Clark Johnson, Brad Singer, Elizabeth King, William Peck, Ilya Bindeman, Cory Clechenko, Aaron Cavosie, and Tom Lapen have added valuable discussion of portions of this research. We thank Martha House for many of the samples. Jason Saleeby, Kent Ratajeski, and Basil Tikoff are thanked for their helpful discussions and samples. Drew Coleman, Rich Gaschnig, and John Gracely, at UNC, kindly provided unpublished U–Pb ages and helpful discussions of the geochronology of the eastern Sierra. Ron Kistler graciously shared unpublished isotope data. Hans Hinke assisted in the field. David Graber and Jan van Wagtendonk helped us obtain permits to sample in Sequoia, Kings Canyon, and Yosemite National Parks. Detailed and encouraging reviews by Allen Glazner, Tom Sisson, Ron Frost, and Charlotte Allen helped improve this paper.
*Corresponding author. Telephone: (909) 621-8677. Fax: (909) 621-8552. E-mail: JadeStar.Lackey{at}pomona.edu
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