In contrast to the much-studied central and southern Sierra Nevada, relatively little is known about the growth and petrogenesis of the batholith in its northern reaches, making it difficult to evaluate range-wide, spatiotemporal trends in batholithic development and the regional extent of eclogite root production and/or loss. New U-Pb ages from northern Sierra plutons reveal a shift between the age of Cretaceous magmatism recorded in the northern Sierra and the timing of an apparent flare-up in the main batholith, indicating that: (1) the northern batholith was more spatially dispersed and emplaced into regions beyond the modern topographic range, and (2) the Cretaceous high-flux event may have occurred over a longer period of time than previously suggested. Relative to the southern Sierra, Nd and Sr isotopic signatures in northern plutons are more primitive, mimicking the predominantly juvenile nature of the terranes into which the plutons are built. Despite differences in isotopic character, however, major and trace element trends are remarkably similar between northern plutons and the rest of the batholith, suggesting that emplacement into juvenile and/or oceanic lithosphere does not inhibit the generation of evolved, arc-type magmatic products. Northern plutons have relatively high La/Yb and Sr/Y and steep rare-earth element patterns, with small to no Eu anomalies. Taken together, these trends are interpreted to indicate deep processing of magmas in equilibrium with a feldspar-poor, amphibolite-rich residue, containing modest amounts of garnet. It is therefore likely that the northern Sierra Nevada batholith was emplaced into relatively thick crust and developed a dense mafic to ultramafic root. Because it is not seismically imaged today, we posit that the root was subsequently lost, perhaps in response to encroachment of proto-Cascade arc volcanism.
The Sierra Nevada, one of the largest batholiths in North America, strikes NNW and can be traced ∼600 km along the eastern spine of California, varying in width between ∼80 and 120 km. The batholith comprises mainly subduction-related intermediate to felsic intrusive rocks and associated metamorphic screens and pendants (Bateman and Wahrhaftig, 1966; Evernden and Kistler, 1970; Ducea, 2001; Saleeby et al., 2003). Although the Sierra Nevada is commonly classified as a single, exhumed continental mega-plutonic complex, major lateral (W to E) changes in the age (Kistler and Peterman, 1973, 1978; Stern et al., 1981; Chen and Moore, 1982), composition (Moore, 1959; Bateman, 1992), and isotopic signature (Doe and Delevaux, 1973; DePaolo, 1980, 1981; Masi et al., 1981; Saleeby et al., 1987a; Kistler, 1990; Kistler and Ross, 1990; Chen and Tilton, 1991; Kistler and Fleck, 1994; Ducea, 2001; Lackey et al., 2005, 2008) of the batholith have long been recognized. These changes are largely attributed to the flaring up and migration of magmatism across heterogeneous crustal and upper mantle domains.
In addition to significant transverse changes, the Sierra Nevada also changes along strike in several important ways: (1) plutonic rocks are volumetrically more significant in the central and southern Sierra Nevada (between ∼35 °N and 37.5 °N; Fig. 1), comprising nearly the entire exposed arc in those portions of the range; (2) Eocene–Miocene clastic sedimentary and volcanic rocks are preserved in the northern Sierra Nevada (N of 38 °N), whereas they are conspicuously absent in other parts of the range (Fig. 1); (3) the estimated depths of pluton emplacement become shallower toward the north (Ague and Brimhall, 1988; Pickett and Saleeby, 1993; Ague, 1997; Nadin and Saleeby, 2008); (4) the basement into which the batholith is built changes from mainly late Proterozoic continental and transitional lithosphere in the south to a tapestry of accreted Phanerozoic belts in the north (Kistler, 1990; Fig. 1); and (5) the southern and central part of the batholith had a documented dense root that has recently delaminated from the base of the crust (e.g., Ducea and Saleeby, 1998b; Zandt et al., 2004), whereas the development and/or loss of such a root in the northern Sierra is unclear (Frassetto et al., 2011).
Although a wealth of information exists about the main batholithic body of the Sierra Nevada in its southern and central extents, much less is known about the age and petrogenesis of granitic plutons in the north. Because of the north-south changes enumerated above, we examined the geochronologic, geochemical, and isotopic character of northern plutons with the aim of (1) describing their petrology and emplacement history, (2) identifying range-wide spatial and temporal patterns in batholith genesis, and (3) identifying fundamental differences, if any, between magma sources regions in the north and other parts of the range. This work was conducted as part of a larger, interdisciplinary project—the Sierra Nevada Earthscope Project (SNEP)—aimed at resolving the nature and complexity of the Sierran lithosphere. Of particular interest to principal investigators were: the presence or absence of a dense residue at the base of the felsic crust, the distribution of that residue through both time and space, and its influence on recent volcanism and surface deformation. In light of the greater context of this work, the isotopic and geochemical data presented herein are interpreted with an eye toward understanding the evolution of the lower crust and mantle lithosphere in the northern Sierra Nevada.
The Sierra Nevada batholith formed as a result of long-lived subduction of the Pacific plate beneath western North America (Dickinson, 1981). Magmatism was in general continuous from ca. 248 to 80 Ma, but most active during episodic events in the Late Jurassic (165–145 Ma) and Late Cretaceous (100–85 Ma), building a thick Mesozoic crustal column—at least in the central and southern Sierra—composed of ∼35 km of batholithic rocks (Pickett and Saleeby, 1993; Ducea, 2001). In general, plutons making up the batholith tend to become younger, more silicic, and more voluminous toward the east. Unlike in the central and southern stretches of the Sierra Nevada, where batholithic rocks comprise the vast majority of the crust, the northern Sierra Nevada batholith is a loosely assembled patchwork of quartz diorite, granodiorite, and tonalite plutons. In the north, plutons are more variable in their composition and size, collectively comprising less than 50% of the exposed surface geology north of latitude 38.5 °N.
Prebatholithic Rocks of the Northern Sierra Nevada
Granitoid rocks of the northern Sierra are intruded across range-parallel, accretionary tectonostratigraphic packages of Paleozoic and Mesozoic metamorphic rocks, which are juxtaposed along Late Jurassic and older faults (Saleeby, 1982; Edelman and Sharp, 1989; Snow and Scherer, 2006). We have grouped these packages into a western, Panthalassan (oceanic) belt and a central, North American (continental) belt, after the two different lithosphere types defined by Kistler (1990) (Fig.1). The western belt of Panthalassan affinity is composed mainly of Jura–Triassic metavolcanic arc rocks and associated Upper Jurassic accretionary metasediments. Basement rocks to this belt are primarily mid-ocean ridge basalt (MORB)–affinity Paleozoic ophiolitic rocks (Saleeby, 1982; Sharp, 1988; Ernst et al., 2008). Rocks of the North American belt consist primarily of the early Paleozoic Shoo Fly Complex, which contains mostly deep marine siliciclastic rocks deposited in proximity to a continental source (e.g., Girty et al., 1996; Harding et al., 2000). Although derived from a continental source, the Shoo Fly and associated assemblages are floored by oceanic or transitional lithosphere, as inferred from their relatively juvenile isotopic nature.
In the southern Sierra Nevada, the western boundary of the Precambrian continental margin is geochemically outlined by the classic 87Sr/86Sri = 0.706 isopleth, which bisects the west-central part of the batholith (Kistler and Peterman, 1978; Kistler, 1990) (Fig. 1). Eastward transition into older continental lithosphere is evidenced by more evolved isotopic signatures in the batholith (Chen and Tilton, 1991; Sisson et al., 1996; Coleman and Glazner, 1998) and the presence of ancient, continent-derived miogeoclinal rocks in metamorphic screens and pendants (Saleeby and Busby, 1993). To the north, that boundary becomes less well defined and the zone that is inferred to separate the more juvenile, accreted metamorphic belts from the ancient continental margin is generally thought to lie well east of the Sierra Nevada batholith (Davis et al., 1978). We present new geochemical and isotopic data from the northern Sierra, which is used to elucidate the influence of strikingly different prebatholithic lithospheric conditions on the magmatic development of the batholith.
SAMPLE SELECTION, PREPARATION, AND ANALYSIS
Plutons were sampled from 20 locations along two range-perpendicular transects in the northern Sierra Nevada: a northerly transect at the latitude of the Yuba River (∼39.25 °N), and another at the approximate latitude of the Consumnes River (∼38.5 °N) (Fig. 1). Only samples with no visible weathered surfaces and/or secondary minerals were crushed and pulverized to a coarse-sand size. The bulk of the crushed material was processed for zircon separation using standard density and magnetic techniques, and small aliquots were set aside and powdered in an Al2O3-lined shatter box for whole-rock geochemical and isotopic analysis. Zircons separated from each sample were picked and mounted, along with Sri Lanka and R33 standards, in 2.5 cm epoxy discs. U-Pb zircon geochronology data were collected via laser ablation–multicollector–inductively coupled plasma mass spectrometry (LA-MC-ICPMS) (GV Isoprobe) at the Arizona LaserChron Center (see Gehrels et al., 2008, for instrumentation and methodology details). Sample information, location, and U-Pb age data are summarized in Table 1. Individual U-Pb measurements and concordia plots are provided in the Supplemental File1. Some samples (G06 and G10, for example) show a high scatter in individual zircon U-Pb ages. These are attributed to analytical error, as opposed to meaningful geologic heterogeneity, due to the fact that: (1) zircon age standards analyzed in the same session show similar scatter, and (2) there is no observed relationship between calculated age and U concentration, U/Th, or any other geologic indicator.
Major and trace element analyses were performed at the GeoAnalytical Laboratory in the School of Earth and Environmental Sciences at Washington State University using X-ray fluorescence (XRF) (Johnson et al., 1999) and ICPMS. Major and trace element data are summarized in Table 2. Isotopic analyses were performed at the University of Arizona via isotope-dilution–thermal ionization mass spectrometry (ID-TIMS), using both an automated VG Sector multicollector instrument with adjustable 1011 Ω Faraday collectors and a Daly photomultiplier (Otamendi et al., 2009) and a VG Sector 54 instrument (Ducea, 2002) (see Appendix for details of Sm-Nd and Rb-Sr isotopic measurements). All isotope data are summarized in Table 3.
U-Pb zircon ages of plutons from the northern Sierra Nevada record the presence of a Late Jurassic arc, active between 167 and 145 Ma, and a mid-Cretaceous arc, with magmatism continuous between 125 and 90 Ma. The distribution of ages presented here is similar to range-wide estimates of magmatic flux, in that both reveal pronounced magmatic episodes in the Jurassic and in the Cretaceous, but they differ in that: (1) the Jurassic event is apparently more significant in the north; and (2) the distribution of ages from the northern Sierra is offset from the apparent intrusive flux curve for the greater Sierra Nevada, as defined by Ducea (2001) (Fig. 2). Although the similarities and differences between these trends are significant, it is important to note that magmatic rocks in general comprise a much smaller proportion of the crust in the northern Sierra Nevada and that much of which is presumably underlain by granitic rocks in the higher, eastern reaches of the range, is presently covered by younger volcanic and clastic deposits (e.g., Saucedo and Wagner, 1992; Busby and Putirka, 2009) (Fig. 1). Thus, the ages we present may not accurately reflect the relative volumes of magmatic pulses in this part of the range. For example, a comparison of the age histogram from this study with the estimated age-area distribution of previously dated plutons from the northern Sierra Nevada (between 39 °N and 40 °N; Cecil et al., 2010) suggests that the large ca. 165 Ma age peak is overrepresented.
In addition to revealing temporal variability (periods of high and low magmatic flux), U-Pb zircon ages are also spatially variable. In both the northern and southern transects, Cretaceous plutons become systematically younger to the east (Fig. 3). This is similar to trends previously reported in the southern and central Sierra Nevada (Chen and Moore, 1982; Nadin and Saleeby, 2008), the Coast Mountain batholith of British Columbia (Gehrels et al., 2009), and the Peninsular Ranges (Silver et al., 1979; Silver and Chappell, 1988). Jurassic plutons are restricted to the western parts of both transects, and their ages have no apparent migratory trends. A single Late Devonian age (370 Ma) from the Bowman Lake pluton was also measured, and is in good agreement with previously published zircon ages from the same composite batholith (Saleeby et al., 1987b; Hanson et al., 1988). Because its petrogenetic history is not related to that of the other Mesozoic intrusions, it is not considered in subsequent sections.
Major and Trace Element Geochemistry
In this and subsequent sections and figures, northern Sierra plutons from this study are compared with a range-wide suite of Jurassic and Cretaceous intrusive rocks, compiled from the western North American volcanic and intrusive rock database (NAVDAT; see Fig.4 caption for a list of references). For discussions of elemental and isotopic geochemistry, the northern Sierra Nevada plutons investigated in this study are divided into a Jurassic and a mid-Cretaceous group, based on the distribution of ages presented. Both groups are similar compositionally, but differ in some of their geochemical and/or isotopic trends. In general, Jurassic intrusions tend to have greater compositional heterogeneity, showing greater scatter and less well defined trends, both geochemically and isotopically.
Plutons of the northern Sierra are calc-alkaline to slightly calcic and form a trend that is nearly identical to all other Sierra Nevada intrusive rocks when plotted as Na2O + KO-CaO versus SiO2 (Fig. 4A). With the exception of few high SiO2 (>70%) samples, northern Sierra plutons are metaluminous to mildly peraluminous (Fig. 4B). With the exception of K2O, most major oxides in both age groups form linear arrays and are negatively correlated with silica content, which varies between 54 and 78 wt% SiO2, trending toward higher values in mid- to Late Cretaceous rocks (Fig. 5). In both Jurassic and Cretaceous plutons, FeO is higher than that of most Sierra Nevada intrusions, across all values of SiO2. Trace element compositions are generally characterized by enrichments in large ion lithophile elements (LILE) and strong depletions in Nb and Ti (Fig. 6). Jurassic plutons show greater depletion in high field strength elements (HFSE), which is reflected in higher average Ba/Th ratios (average Jurassic Ba/Th = 235; average Cretaceous Ba/Th = 65).
Like most major and trace element patterns, chondrite-normalized, rare-earth element (REE) patterns are also similar to those of the greater Sierra Nevada batholith (Figs. 7A and 7B). Rare-earth element patterns show enrichment in the light rare-earth elements (LREE) (up to >100× chondrite) and relative depletion in heavy rare-earth elements (HREE), which have flat to slightly concave trends. La/Yb ratios range from 5 to 35, but are slightly higher (steeper REE slope) and more homogeneous in mid-Cretaceous plutons. Most samples have small to no Eu anomalies, with the exception of rare high SiO2 intrusions, which have large negative anomalies (Fig. 7C).
Sr and Nd Isotopes
Strontium and neodymium isotope systematics of northern Sierra Nevada plutons were investigated in order to identify any spatial and temporal patterns in magmatic source and to estimate the degree to which magmas interacted with older, preexisting crust. Initial 87Sr/86Sr ratios in most plutons range between 0.7047 and 0.7059, with the notable exceptions of samples G01 and G02, which have more primitive ratios (0.7029 and 0.7034, respectively). G01 and G02 are from Jurassic plutons emplaced into Panthalassan lithosphere of the northwestern Sierra Nevada foothills, the significance of which will be discussed in subsequent discussion sections. Additionally, samples from high SiO2 intrusions have low Sr, high Rb/Sr ratios, and high 87Sr/86Sri ratios (0.7070–0.7081). Initial ɛNd values for most northern Sierra plutons are negative and vary between –7.5 and –2.9 and show expected negative correlation with initial Sr ratios (Fig. 8). Low 87Sr/86Sri samples (G01 and G02) have correspondingly high ɛNd(i) values (–0.3–2.7). High SiO2 (>75%) rocks with high 87Sr/86Sri (indicated by asterisks in Fig. 8), however, have ɛNd(i) values within the normal range. These anomalously high 87Sr/86Sri values likely reflect minor contamination by crustal components, such as Calaveras Formation wall rocks (Fig. 8), or, potentially, seawater-altered metabasalts. Given the exceptionally low Sr concentrations in these rocks (<49 ppm), only very small degrees of contamination are necessary to greatly elevate 87Sr/86Sri ratios.
Flare-ups and Migration of the Magmatic Arc
Plutons of the northern Sierra Nevada are mainly calc-alkaline, subduction-related products that were generated episodically during intrusive events in the Middle to Late Jurassic and the mid-Cretaceous. Although there are plutons of this general age in the main body of the batholith to the south, the distribution of ages reported here is offset from recognized major, range-wide apparent high-flux events (Ducea, 2001; Fig. 2). There is an observed paucity of post–100 Ma plutons in the north compared with great volumes of batholithic material emplaced into the central and southern Sierra during the mid- to Late Cretaceous flare-up (ca. 100–85 Ma). Likewise, the observed pulse of northern magmatism between ca. 130 and 100 Ma (Fig. 2) is not as prevalent in the southern and central parts of the batholith. This could be explained by a southward shift in magmatism through time, although intermediate arc-related basement rocks that are early to mid-Cretaceous in age have been identified in well cores from the eastern Great Valley at central to southern Sierra latitudes (Saleeby, 2007). Alternatively, the age distribution presented here, together with new insights from the Great Valley subsurface, could be indicating that the Cretaceous flare-up event was less punctuated and occurred over a longer period of time than previously estimated (Fig. 2).
The apparent north-south difference in the timing of Cretaceous high-volume magmatism could be due in part to the covering of younger plutons in the northeastern Sierra by late Cenozoic volcanic and clastic deposits. It is more likely, however, to be due to the widening of the batholith to the north. In the southernmost Sierra Nevada (35.5 °N), the width of the exposed batholith is ∼60 km, and it widens gradually along strike to the north such that at the latitudes of this study, it is roughly 100 km, from outcroppings of Jura–Cretaceous plutons in the western foothills to exposed granites east of the range crest. In the north, the locus of Late Cretaceous (post–100 Ma) magmatism coeval with large intrusive suites of the eastern Sierra Nevada (e.g., the Sonora Pass, Tuolumne, John Muir, and Mount Whitney suites) is shifted to the northeast, up to 200 km east of the modern range crest (Smith et al., 1971; Barton et al., 1988; Van Buer and Miller, 2010).
Because of its age, presumed volumetric significance (covering an area of >1000 km2), geochemistry, and zonation pattern, the Sahwave batholith of NW Nevada is considered part of the greater Sierra Nevada batholith, suggesting that it is continuous across the modern divide and into Nevada (Van Buer and Miller, 2010). Even accounting for maximum amounts of Cenozoic extension across northern California and northwestern Nevada (∼35%; Surpless et al., 2002; Colgan et al., 2006a, 2006b), the eastern extent of the magmatic Sierra in the north is far greater than it is in south, stretching the batholith to ∼180 km in width at 40 °N. Subsurface sampling of the north-central Great Valley basement near Sutter Buttes (Fig. 1) reveals that it is underlain by mafic and intermediate batholithic products (Williams and Curtis, 1977). Similar rocks have been observed in cores from other parts of the eastern Great Valley and, where dated, yield early Cretaceous ages ranging from ca. 140 to 130 Ma (Saleeby, 2007). This further increases the overall dispersion of the northern Sierra Mesozoic batholith to ≥200 km. However, the width of the arc at any given 10 My time period was probably not more than 60 km, similar to the modern frontal arcs of the Andes (e.g., Mamani et al., 2010).
The presence of Cretaceous Sierran magmatism both east and west of the modern topographic range helps account for the apparent reduction in batholithic volume in the northern Sierra Nevada, while highlighting the difference between voluminous, spatially-concentrated magmatism in the south and dispersed magmatism in the north. Because of the northward widening of the batholith, the apparent rate of eastward migration of Cretaceous magmatism in the north is also greater (Fig. 3). In the central Sierra, Chen and Moore (1982) estimate eastward propagation of the arc at 2.7 km/Ma, whereas in the northern Sierra, that estimate increases to 4.8 km/Ma. The widening of the batholith to the north coincides with the widening and dispersal of the Mesozoic thrust belt in the retroarc. At the latitudes of the southern Sierra Nevada (∼35 °N–37.5 °N), the eastern Sierra thrust belt is confined to a narrow belt of folds and mylonitic thrusts along the eastern margin of the batholith (Dunne et al., 1978, 1983; Walker et al., 1990; Dunne and Walker, 1993). This narrow belt transitions to a wider belt to the north, where the westernmost zone of Mesozoic deformation, the Luning-Fencemaker fold and thrust belt, defines a broader zone of mainly thin-skinned imbricate thrusts (Wyld, 2002; DeCelles, 2004). This pattern suggests that the delivery of continental lithosphere components from the foreland region into the magma source region by retroarc thrusting was more vigorous in the south, potentially driving a more spatially concentrated pattern of magma genesis and resulting composite batholith growth.
High-Flux Events in the Northern Sierra Nevada
The northern Sierra Nevada arc was assembled in a non-steady state, with times of higher fluxes separated by magmatic lulls (Fig. 2), similar to the southern Sierra Nevada and most other Cordilleran arcs studied in some detail to date (e.g., Gehrels et al., 2009; Coast Mountains batholith). What stands out for the northern Sierra Nevada transects is the high apparent intrusive flux during the earlier, Jurassic stage of magmatism. This is similar to the Coast Mountains batholith, where the initial stages of magmatism were characterized by high apparent fluxes, intermediate to felsic compositions, and high δ18O (Wetmore and Ducea, 2011), suggesting in that case, that the fertile framework represented by continental miogeoclinal rocks provided an important mass contribution to the magmatic budget.
We suggest a similar scenario for the northern Sierra Nevada, where the arc was built from its inception onto the continental margin, albeit one comprising some accreted (and primitive) terranes. This contrasts with various peri–American island arcs of Jurassic age that are primitive and more mafic in composition (e.g., the Talkeetna arc in Alaska; DeBari and Sleep, 1991) or the Jurassic arc segments found along the western margin of South America in Chile (Oliveros et al., 2007). There are no good estimates of magmatic fluxes for those arcs, but there is every indication that they were built as island arcs away from the continental margin and were subsequently docked. The northern Sierra Nevada example supports a previous suggestion that arcs developing along a continental margin have high magmatic fluxes initially because they are inevitably built on a melt-fertile sequence of passive margin (miogeoclinal) assemblages (Ducea et al., 2010). The later, mid-Cretaceous flare-up in the northern Sierra Nevada and the latest Cretaceous one east of the range, represent more typical mature arc events that formed in response to the overall cyclic nature of foreland deformation, crustal thickening, and magmatism (DeCelles et al., 2009), which are superimposed on the constant baseline flux of hydrous melting from the mantle wedge (Grove et al., 2003).
Emplacement of the Northern Sierra Nevada Batholith into Primitive and Heterogeneous Crust
Strontium and neodymium isotopes from both Jurassic and Cretaceous plutons in the northern Sierra Nevada are variable but trend toward more primitive values than those in the south. The distribution of Sr-Nd signatures from northern plutons is consistent with a two-component mixing of depleted, mantle-derived magmas with evolved metasedimentary wall rocks (Fig. 8). The Jurassic Yuba River (G01) and Pleasant Valley (G02) plutons intruding Panthalassan lithosphere in the western foothills have distinctively primitive 87Sr/86Sri (0.7030–0.7035) and ɛNd(i) (–0.3–+2.7), limiting the reservoir to isotopically juvenile oceanic arc rocks and/or altered MORB. Large, Late Cretaceous intrusive suites of the central and southern Sierra show a greater enrichment in radiogenic strontium than Cretaceous plutons in the northern Sierra and northwest Nevada, indicative of a major along-strike change in magma source, which incorporates older, continental material in the south.
Initial ɛNd and 87Sr/86Sri plotted along range-perpendicular transects reveal a trend toward more evolved isotopic compositions from the foothills east to near the range crest, and a trend toward more primitive values eastward from the range crest (Fig. 9). This is in contrast to the central and southern Sierra, which was emplaced across a presumed boundary between accreted Phanerozoic rocks to the west and Proterozoic continental lithosphere to the east, such that isotopic values evolve consistently eastward toward more continental values (Fig. 9B). New investigation of pendant rocks in the central and southern Sierra indicates that that portion of the batholith was likely emplaced into transitional lithosphere and the western margin of continental lithosphere coincided with the location of the eastern Sierra thrust belt (J. Saleeby, 2011, personal commun.). The close proximity to, and underthrusting of, continental lithosphere from the retroarc imparts the eastward progressive and more evolved isotopic signatures in the south. To the north, the boundary between Panthalassan and continental lithosphere not only becomes more diffuse, but is truncated by strike-slip faults in the backarc (e.g., Oldow, 1983), thereby apparently bending the Sri 0.706 line and imparting an appendage-like shape to the isotope data. Ignoring the ca. 162 Ma Yuba River and Pleasant Valley plutons, there is no correlation between age and isotopic character of plutons, suggesting that observed isotopic variability is controlled by heterogeneity in relatively primitive crustal and upper mantle magma source regions.
Despite clear along-strike changes in the character of magmatic source, as evidenced from changing isotopic values, major and trace element data are remarkably similar along all segments of the Sierran batholith. This can be seen via comparison of major element trends (Figs. 4 and 5), REE element trends (Fig. 7), and selected trace element ratios (Fig. 10). This indicates that the juvenile nature of the lithosphere in the northern Sierra Nevada (its age, chemistry, mineralogy, thickness, etc.) does not preclude the generation of significant volumes of fractionated granitoids. This has been documented in other geologic settings and is perhaps best exemplified by the generation of the great Coast Mountains batholith of British Columbia and southeast Alaska (e.g., Mahoney et al., 2009; Wetmore and Ducea, 2011). The non-dependence of source lithosphere type on granitoid melt production can be seen in plots showing no relationship between Sr/Y and La/Yb, proxies for depth of melting, and initial 87Sr/86Sr, a proxy for type and/or maturity of preexisting lithosphere (Fig. 11). The geochemical similarity of plutons emplaced into fertile continental lithosphere (southern Sierra), an amalgamation of accreted Phanerozoic belts (northern Sierra), and basinal terranes (northwest Nevada; Van Buer and Miller, 2010), attests to along-strike similarity in style of Sierran batholithic petrogenesis.
Constraints on Depth of Melting and Associated Batholithic Residues
Plutons of the northern Sierra Nevada are interpreted to have been generated through melting of deep crustal sources, based on REE fractionation patterns, the absence of Eu anomalies, and elevated La/Ybnorm and Sr/Y ratios, all of which are discussed in detail below.
Rare-earth element patterns for northern Sierra plutons are moderately steep, characterized by both enrichment in LILE and relative depletion in HREE, and show mildly concave upward “dished” middle-REE depletion (Fig. 7). These trends are consistent with low to moderate degrees of partial melting in equilibrium with a residue rich in amphibole, which preferentially incorporates middle-REE, and modest amounts of garnet. The above described REE trends are evident in both Jurassic and Cretaceous plutons, although REE patterns of Jurassic rocks are more variable and tend to be slightly less steep (average Jurassic La/Ybnorm ≅ 9; average Cretaceous La/Ybnorm ≅ 11). Given that average Gd/Yb ratios (a measure of HREE differentiation) are the same, but La/Sm (a measure of LREE differentiation) are higher in Cretaceous plutons, the observed difference in overall steepness is apparently due to greater LREE enrichment in Cretaceous plutons. This is similar to REE trends for the Peninsular Ranges batholith observed by Gromet and Silver (1987), who document increases eastward and through time in La/Ybnorm and LREE enrichment. The difference in La/Ybnorm between Jurassic and Cretaceous magmatic episodes could be due to a decrease in the degree of partial melting, and/or a change in the mineralogy of the source residua over time.
All rocks, with the exception of the high SiO2 plutons, have very small Eu anomalies (average Eu/Eu* = 0.93; Fig. 7C). Eu2+ can substitute for inter-tetrahedral cations in feldspars and is strongly partitioned by plagioclase (Weill and Drake, 1973) and alkali feldspars (e.g., Ren, 2004). It does not, however, have an affinity for occupying Ca sites in amphibole or clinopyroxene. In fact, partition coefficient patterns of REE between amphibole and silicic melts show negative Eu anomalies (e.g., Sisson, 1994). Thus, the lack of a large Eu depletion signal in Sierran granitoids indicates the generation of granitoid melts in equilibrium with a relatively feldspar-poor source. Some residual feldspar is certainly permissible, however, given the competing effects of feldspar (Eu relatively compatible) and amphibole (Eu relatively incompatible).
In addition to having small to no Eu anomalies, most northern Sierran intrusives have high Sr concentrations (average ≅ 450 ppm), high Sr/Y ratios (up to 75), and low Y concentrations (average ≅ 14 ppm) (Fig. 10). Relative depletion in the HREE, high Sr/Y, high Sr, and low Y are all characteristics of magmas produced at depth from the melting of a subducting mafic slab (Kay, 1978; Defant and Drummond, 1990). They are also characteristic of magmas produced via melting of amphibolite in the deep subarc crust in equilibrium with a garnet + amphibole + clinopyroxene residue (Petford and Atherton, 1996; Stevenson et al., 2005; Mahoney et al., 2009). A plot of Sr/Y against Y concentration in northern Sierra Nevada plutons reveals trends consistent with those from experimental batch partial-melts of source amphibolite with garnet abundance ranging between 7% and 30% (Petford and Atherton, 1996) (Fig. 10). Batholithic residua likely to be amphibole rich, relatively feldspar poor, and to have modest amounts of garnet, are consistent with high pressure (>10 kbar) melting experiments of a basaltic source (e.g., Rapp et al., 1991; Wolf and Wyllie, 1993, 1994). The production of northern Sierra Nevada granodiorites and tonalites is therefore loosely constrained to depths ≥35 km and is interpreted to result from the processing of metabasaltic sources in the lower arc crust.
The Presence and Fate of a Dense Batholithic Root in the North
Studies of the southern Sierra Nevada suggest that it was composed of a thick (35–40 km; Pickett and Saleeby, 1993, 1994; Ducea, 2001) column of felsic and intermediate rocks underlain by a dense, garnet- and clinopyroxene-rich residue (Ducea and Saleeby, 1996, 1998a; Saleeby et al., 2003). This deep residue, on the basis of its mineralogy, was termed an “eclogitic” root (Saleeby et al., 2003), also informally known as “arclogite.” Fundamental changes between lower crustal and upper mantle xenoliths entrained in Miocene basalts and those entrained in Plio–Quaternary basalts, provide evidence that such an eclogitic root existed at the base of the felsic batholith in the Miocene, but was subsequently lost from beneath the southeastern Sierra by ca. 3.5 Ma (Ducea and Saleeby, 1996, 1998a, 1998b; Farmer et al., 2002; Saleeby et al., 2003). Foundering of the eclogite root accounts for the present-day modest crustal thickness (∼35 km) of the southern Sierra Nevada (Park et al., 1995; Jones et al., 1994; Zandt et al., 2004), and its replacement by hot asthenosphere has been called upon to explain recent uplift and high topography in the southern Sierra (Jones et al., 2004) and Cenozoic basaltic volcanism (Manley et al., 2000; Farmer et al., 2002). The continued attachment of the root beneath the west-central Sierra Nevada has likewise been associated with tectonic subsidence there (Saleeby and Foster, 2004). As such, the development and removal of an eclogitic root has played a fundamental role in the evolution of the central and southern Sierra. Much less understood in this context, however, is the northern Sierra, which may have had a much different tectonic history, as inferred from the different ages and distribution of plutons and the lower and more subdued topography. Indeed, a major focus of SNEP was to image the lithospheric structure of the northern Sierra Nevada and to learn more about the potential development and/or loss of a high-density batholithic root there.
In terms of the exposure of batholithic rocks within the modern topographic range, the northern Sierra Nevada appears to have volumetrically less material than the main batholith to the south. Xenolith studies, as well as experimental studies of partial melts derived from hydrous mafic lithologies, indicate that only magmatic arcs producing large volumes and thick (>25 km) columns of felsic material will generate gravitationally unstable roots (e.g., Ducea, 2002). Similarly, it has been suggested that sourcing of melts in oceanic lithosphere reduces the potential for generating garnet-rich lithologies (e.g., Ducea and Saleeby, 1998a). It could be argued, therefore, that the northern batholith was less likely to have developed a dense residual root. The lack of lower crustal and/or mantle xenoliths in the northern Sierra Nevada prevents direct sampling of the batholith at depth there. However, diagnostic trace elements evidence presented here for deep processing of magmas in the northern Sierra implies that: (1) the crust was reasonably thick (>>35 km) in the Mesozoic, and (2) a dense mafic residue was produced during batholith generation. Thus, despite the patchy exposure of northern Sierra plutons and their emplacement into relatively juvenile lithosphere, it is likely that the bulk of the crustal column there was composed of a thick section of intermediate to felsic batholithic products floored by a dense residual root. This is in contrast to proposed estimates of thin crust during Mesozoic time in northwestern Nevada, based on limited Cenozoic crustal extension there (Colgan et al., 2006a, 2006b).
Seismic receiver-function analysis of crust and upper mantle structure in the Sierra Nevada reveals a bright, “delamination” Moho at ∼30 km depth in the southern and eastern parts of the range, which is interpreted to be tectonic in origin and generated by a large step in wave speed between relatively homogeneous continental crust and asthenosphere below (Zandt et al., 2004; Frassetto et al., 2011). This delamination Moho is thought to result from the foundering of dense, negatively buoyant lithosphere and its replacement by asthenosphere at shallow levels. The Moho dips westward, becoming deep and significantly weaker, in seismic expression, beneath the central and western portions of the Sierra Nevada. Areas where the Moho appears weak correlate well with a tomographically imaged, high-speed anomaly at depth (Gilbert et al., 2008; Reeg et al., 2008; Schmandt and Humphreys, 2010). Together, these observations have been interpreted as a dense root, which is attached to the base of the Sierran crust and presently in the process of foundering into the mantle. This explains the high-speed anomaly at depth, as well as the weak Moho, since the crustal root appears seismically fast and similar to the mantle with which it is in contact. Seismic imaging (e.g., Zandt et al., 2004; Schmandt and Humphreys, 2010; Frassetto et al., 2011), together with heat-flow measurements (Saltus and Lachenbruch, 1991) and studies of xenoliths (Ducea and Saleeby, 1996, 1998a, 1998b) and young volcanics (Manley et al., 2000; Farmer et al., 2002), form a coherent picture of evolving lithosphere through time and space in the southern and eastern Sierra Nevada region.
Conversely, the crustal structure and lithospheric evolution of the northern Sierra Nevada has remained ambiguous. Large-scale tomographic studies of the western United States reveal the absence of dense, over-thickened crust in the northern Sierra Nevada region (e.g., Yang et al., 2008). A recent study using receiver-function analysis (Frassetto et al., 2011) shows a continuous Moho at ∼35 km depth beneath the northern Sierra Nevada that dips shallowly westward and appears stepped (deeper to the east) at the approximate eastern margin of the metamorphic foothills belt. Although brighter than the weak Moho recognized in the central and western Sierra, the amplitude of the northern Moho is not as high as that of the delamination Moho to the south and east, and has been interpreted as resulting from extension and volcanism associated with the westward encroachment of the Basin and Range (Frassetto et al., 2011).
Given the geochemical evidence supporting the development of a dense, mafic batholithic residue beneath the northern Sierra, we propose that the observed northern Moho is the result of sharpening due to post-Mesozoic removal of that residue. We accept, however, that this assertion is problematic in the absence of volcanism attributable to convective removal of the lithosphere and replacement by hot asthenosphere and/or geomorphic signals that can be clearly related to a specific tectonic event or events. Although marked incision of river channels in the central and northern Sierra (e.g., Wakabayashi and Sawyer, 2001; Stock et al., 2004) indicate relative base-level changes in the Pliocene, it is not clear whether or not such base-level changes were generated by a climatic or a tectonic perturbance. It is also not clear what the triggering mechanism for root loss would have been. The northern Sierra Nevada batholith is more dispersed and likely less voluminous than the southern part of the batholith. It is plausible that instead of a singular, large plug of dense material at the base of the felsic crust, there developed multiple, but smaller lenses of mafic-ultramafic cumulates. Given appropriate thermal and rheological conditions not uncommon in arc settings, it is possible for those cumulates to be convectively removed shortly after they are formed (Jull and Kelemen, 2001). These downwellings would have occurred in Mesozoic time and may not require a dramatic, wholesale overturning of the upper mantle, potentially explaining the lack of delamination-driven geomorphic and volcanic features in the northern Sierra region. Alternatively, the northern Sierra Nevada could have developed a dense eclogitic root similar to that proposed for the southern Sierra, but it was thermally destabilized by eruptive events of the proto–Cascade arc at ca. 15 Ma, which initiated along the eastern margin of the northern Sierra Nevada batholith (Busby et al., 2008a, 2008b). It is conceivable that proto–Cascade arc volcanism weakened the lithosphere, leading to a delamination event, the formation of the Sierra Nevada microplate (Busby et al., 2008b; Busby and Putirka, 2009), and eruption of the Table Mountain latite at ca. 10.4 Ma (e.g., Pluhar et al., and references therein). In such a scenario, modification of the northern Sierra Nevada landscape and/or volcanism resulting from foundering of the dense root could be attributed to Basin and Range extension and the development of the ancestral Cascades.
The data presented here afford new insights into the petrogenetic and tectonic development of the northern segment of the Sierra Nevada batholith. Documentation of the geochemical and isotopic character of a regional suite of northern plutons allows for the identification of spatial and temporal variations in a long-lived continental magmatic arc system. New U-Pb geochronology data argue for the widening and dispersal of the batholith to the north, which has important tectonic implications for the spatial concentration of magmatic products and their related residues. A preponderance of early to mid-Cretaceous pluton ages further suggests that the documented Cretaceous flare-up event was less punctuated and longer-lived than previously estimated. Principal findings, based on new major and trace element chemistry, and Nd-Sr isotopes, are that plutons of the northern Sierra Nevada were sourced from—and emplaced into—Phanerozoic oceanic and juvenile lithospheric terranes, but are nonetheless geochemically very similar to those emplaced into less heterogeneous and more evolved lithosphere in the southern Sierra. This suggests that younger and/or thinner lithosphere does not preclude the development of thick crust or dense residual phases. Perhaps most interesting is the recognition that northern plutons have clear geochemical signatures consistent with equilibration of magmas at depth (>35 km) with amphibole + garnet residua. On this basis, we propose that the northern Sierra Nevada developed a dense, “arclogitic” root similar to the one sampled by Miocene lower crustal xenoliths in the southern Sierra, even though the greater spatial distribution of plutons in the north implies a more dispersed and thinner root mass there. Because the crust is only ∼35 km thick today and is underlain by a seismically strong Moho, we suggest that the northern dense root was convectively recycled back into the mantle, perhaps in response to the initiation of proto–Cascade arc volcanism and the initiation of the Sierra Nevada microplate.
Concentrations of Rb, Sr, Sm, and Nd were determined by isotope dilution, with isotopic compositions determined on the same spiked runs. An off-line manipulation program was used for isotope-dilution calculations. Typical runs consisted of acquisition of 100 isotopic ratios. The mean result of ten analyses of the Rb standard NRbAAA performed during the course of this study is: 85Rb/87Rb = 2.61199 ± 20. Fifteen analyses of the Sr standard Sr987 yielded mean ratios of: 87Sr/86Sr = 0.710285 ± 7 and 84Sr/86Sr = 0.056316 ± 12. The mean results of five analyses of the Sm standard nSmb performed during the course of this study are: 148Sm/147Sm = 0.74880 ± 21, and 148Sm/152Sm = 0.42110 ± 6. Fifteen measurements of the La Jolla Nd standard were performed during the course of this study. The standard runs yielded the following isotopic ratios: 142Nd/144Nd = 1.14184 ± 2, 143Nd/144Nd = 511853 ± 2, 145Nd/144Nd = 0.348390 ± 2, and 150Nd/144Nd = 0.23638 ± 2. The Sr isotopic ratios of standards and samples were normalized to 86Sr/88Sr = 0.1194, whereas the Nd isotopic ratios were normalized to 146Nd/144Nd = 0.7219. The estimated analytical ±2σ uncertainties for samples analyzed in this study are: 87Rb/86Sr = 0.35%, 87Sr/86Sr = 0.0011%, 147Sm/144Nd = 0.2%, and 143Nd/144Nd = 0.0010%. Procedural blanks averaged from five determinations were: Rb 6 pg, Sr 110 pg, Sm 2.7 pg, and Nd 5.0 pg.
The authors thank Victor Valencia for help in the University of Arizona LaserChron laboratory. We are grateful to Craig Jones and an anonymous reviewer for their thorough and thoughtful reviews, which greatly improved the manuscript. This research was supported by National Science Foundation (NSF) awards EAR-0606967 (Continental Dynamics Program) to Ducea, EAR-0732436 for support of the Arizona LaserChron Center (Gehrels), and by the George and Betty Moore Foundation, Caltech Tectonics Observatory Number 171 (Saleeby and Cecil).