Central Sierra Nevada Batholith Research Articles

Reconciling along-strike disparity in slip displacement of the San Andreas fault, central California, USA

Abstract The correlation of the ca. 23 Ma Pinnacles and Neenach volcanic complexes provides the most robust estimate on the timing and magnitude of Neogene right-lateral displacement on the San Andreas strike-slip fault system (California, United States). Displacement of ∼315 km has been applied rigorously along the plate margin to guide reconstruction of offset paleogeographic features. We present new detrital zircon U-Pb geochronology from the La Honda and western San Joaquin basins to document sediment provenance and reevaluate compositional constraints on a hypothesized key cross-fault tie (i.e., Castle Rock–Recruit Pass submarine fan system). Whereas the Upper Oligocene–Lower Miocene Vaqueros Formation of the La Honda basin was likely recycled from or shared a similar southern Sierra Nevada–western Mojave source with the underlying Eocene stratigraphy, we found that the Temblor Formation of the central Temblor Range (e.g., Recruit Pass submarine fan) was derived directly from Late Cretaceous northern Salinian basement. Furthermore, the Carneros Sandstone of the northern Temblor Range had a central Sierra Nevada batholith source that was likely recycled during early Miocene unroofing of the underlying stratigraphy. Conversely, strata of the southwest San Joaquin basin have provenance characteristics that match more closely with those of the La Honda basin. Our data preclude a contiguous Castle Rock–Recruit Pass submarine fan system across the San Andreas fault. These relationships are resolved by restoring the ca. 105–100 Ma basement of the northernmost Salinian block an additional ∼45 km or greater farther south relative to the Sierra Nevada batholith during late Oligocene–early Miocene time. Inconsistency in displacement along the San Andreas fault with the coeval correlation of the Pinnacles–Neenach volcanic complex is reconciled by postdepositional Miocene–Quaternary off-fault NW-SE structural shortening via major thrusts and/or transrotation of the Tehachapi block, in combination with extension of the northern Salinian block. This additional displacement reduces the need for pre–28 Ma slip on the San Andreas or predecessor faults to resolve Cretaceous through Eocene cross-fault relationships and reconciles an early Miocene discrepancy with Pacific–North America relative plate motion. This study highlights the fact that displacement histories of major strike-slip faults are divergent across changing structural domains, and recognition of slip disparities can constrain the magnitude of deformation.

Oxygen isotope ratios in zircon and garnet: A record of assimilation and fractional crystallization in the Dinkey Dome peraluminous granite, Sierra Nevada, California

Abstract The 119 Ma Dinkey Dome pluton in the central Sierra Nevada Batholith is a peraluminous granite and contains magmatic garnet and zircon that are complexly zoned with respect to oxygen isotope ratios. Intracrystalline SIMS analysis tests the relative importance of magmatic differentiation processes vs. partial melting of metasedimentary rocks. Whereas δ18O values of bulk zircon concentrates are uniform across the entire pluton (7.7‰ VSMOW), zircon crystals are zoned in δ18O by up to 1.8‰, and when compared to late garnet, show evidence of changing magma chemistry during multiple interactions of the magma with wall rock during crustal transit. The evolution from an early high-δ18O magma [δ18O(WR) = 9.8‰] toward lower values is shown by high-δ18O zircon cores (7.8‰) and lower δ 18O rims (6.8‰). Garnets from the northwest side of the pluton show a final increase in δ18O with rims reaching 8.1‰. In situ REE measurements show zircon is magmatic and grew before garnets. Additionally, δ18O in garnets from the western side of the pluton are consistently higher (avg = 7.3‰) relative to the west (avg = 5.9‰). These δ18O variations in zircon and garnet record different stages of assimilation and fractional crystallization whereby an initially high-δ18O magma partially melted low-δ18O wallrock and was subsequently contaminated near the current level of emplacement by higher δ18O melts. Collectively, the comparison of δ18O zoning in garnet and zircon shows how a peraluminous pluton can be constructed from multiple batches of variably contaminated melts, especially in early stages of arc magmatism where magmas encounter significant heterogeneity of wall-rock assemblages. Collectively, peraluminous magmas in the Sierran arc are limited to small <100 km2 plutons that are intimately associated with metasedimentary wall rocks and often surrounded by later and larger metaluminous tonalite and granodiorite plutons. The general associations suggest that early-stage arc magmas sample crustal heterogeneities in small melt batches, but that with progressive invigoration of the arc, such compositions are more effectively blended with mantle melts in source regions. Thus, peraluminous magmas provide important details of the nascent Sierran arc and pre-batholithic crustal structure.

Thermal evolution of the Sierra Nevada batholith, California, and implications for strain localization

The Sierra Nevada batholith (California, USA) hosts multiple shear zones of different ages and different styles of deformation. In this study we present new data syntheses and maps of U-Pb zircon and hornblende and biotite Ar age distributions through the batholith in order to examine the temporal and thermal settings under which contractional and transpressional shear zones arose. These maps highlight the localization of intrabatholithic shear zones at the boundaries between swaths of some of the oldest and youngest plutons, and help to distinguish deformation styles in the southern and central Sierran arc. We also present new ^(40)Ar/^(39)Ar ages and crystallization and deformation temperatures from along the Kern Canyon fault system in the southern part of the batholith, and contrast these new constraints with previously published thermochronological conditions for shear zones to the north. The transpressional proto–Kern Canyon fault was continuously active from ca. 95 to 85 Ma. Deformation temperatures along the fault increase by ∼100 °C from north to south, following the trend of increasing pluton emplacement pressures. These observations, in conjunction with a steep cooling path for the southeastern section of the batholith east of the proto–Kern Canyon fault, support previous interpretations that rapid exhumation in the southernmost part of the batholith followed the arrival and low-angle subduction of an oceanic plateau (the Shatsky Rise conjugate). We suggest that local forces such as these triggered mid-Cretaceous shear zone development in the southern Sierra Nevada batholith, while shear zones in the central part of the batholith, which record the transition from 100–90 Ma compression to 90–80 Ma transpression, were triggered by changing kinematic patterns in regional subduction forcing. This idea is supported by our thermochronologic color contour maps, which reveal an east-west–trending older (and colder) swath within the batholith that may have prevented the more northern shear zones from propagating southward. We contrast the cooling paths of regions within the southernmost part of the batholith, and propose a two-part deformation history in which (1) the 95–85 Ma ductile proto–Kern Canyon fault initiated in the southeast as a compressional structure that transitioned to transpressional, and (2) the 85–75(?) Ma ductile to brittle Kern Canyon–White Wolf fault initiated in the southwest, accommodated extrusion of shallowly subducted accretionary material and thinning of the overlying batholith, and then overprinted the proto–Kern Canyon fault along its northern end. Shear zone activity within the central Sierra Nevada batholith can thus be distinguished by location: in the central zone deformation arose from regional forces, and in the southern zone deformation was triggered by local forces.

Detrital zircon geochronology of Neoproterozoic–Lower Cambrian passive-margin strata of the White-Inyo Range, east-central California: Implications for the Mojave–Snow Lake fault hypothesis

Correlation of lithotectonic packages across major transcurrent structures is critical to understanding the tectonic evolution of the North American continental margin. Detrital zircon geochronology of uppermost Proterozoic to Lower Paleozoic miogeoclinal strata from the White-Inyo Mountains permits evaluation of: (1) the age and provenance of these metasediments and (2) a model for truncation of the passive margin along a postulated large-magnitude Cretaceous dextral shear zone, i.e., the Mojave–Snow Lake fault. U-Pb ages of detrital zircons from the Neoproterozoic Wyman Formation, the uppermost Proterozoic Reed Dolomite (Hines Tongue Member), and clastic strata of the Lower Cambrian Deep Springs, Campito (Montenegro Member), Poleta, and Harkless formations reflect ultimate derivation from the adjacent 1.7–pre-1.8 Ga Mojavia terrane and/or 1.7–1.8 Ga Yavapai continental basement, with subsidiary sources in both the ca. 1.4 Ga Yavapai-Mazatzal anorogenic granitoids and the >2.5 Ga North American craton, and a small proportion of 1.0–1.3 Ga grains, most likely reworked from Grenville clastic wedge deposits. Detrital zircon age spectra from the Lower Cambrian Andrews Mountain Member of the Campito Formation are unique in comparison with the remainder of the studied section, containing a major age peak centered at ca. 1.1 Ga and a subsidiary Lower Cambrian age peak, permitting calculation of a ca. 527 ± 12 (2σ) Ma maximum depositional age. These features, in addition to abundant detrital magnetite-ilmenite grains, reflect a distal source for these rocks, most likely from the ca. 1.1 Ga Pikes Peak batholith and/or the Midcontinent rift and ca. 0.53 Ga bimodal intrusions of the Oklahoma-Colorado aulacogen. In terms of zircon age distribution, allochthonous metamorphic pendants in the Snow Lake terrane of the central Sierra Nevada batholith are most similar to those of stratigraphically equivalent units in the Death Valley region and, to lesser degrees, the White-Inyo section, and the Mojave Desert region. Given the similarity and relative proximity of Death Valley facies assemblages to the Snow Lake terrane, we suggest that the latter was not transported northward from the Mojave Desert region and instead represents footwall assemblages of a late Early to early Middle Jurassic low-angle normal fault system, probably along the outer transform-truncated margin of the Last Chance thrust stack. This model implies a few tens of kilometers of offset, in contrast to the hundreds of kilometers required by the Mojave–Snow Lake fault hypothesis.

Zircon U‐Pb geochronology of the Mount Givens Granodiorite: Implications for the genesis of large volumes of eruptible magma

AbstractThe Mount Givens Granodiorite, a large pluton in the central Sierra Nevada batholith, California, is similar in area to zoned intrusive suites yet is comparatively chemically and texturally homogenous. New zircon U‐Pb geochronology indicates that the pluton was constructed over at least 7 Ma from 97.92 ± 0.06 Ma to 90.87 ± 0.05 Ma. Combining the new geochronology with the exposed volume of the pluton yields an estimated magma flux of <0.001 km3/a. The geochronologic data are at odds with the previously speculated links between plutons such as the Mount Givens Granodiorite and large‐volume homogeneous ignimbrites (often termed monotonous intermediates). Existing data indicate that large plutons accumulate at rates of ≤0.001 km3/a, 1–2 orders of magnitude less than fluxes calculated for dated monotonous intermediates. If monotonous intermediates are remobilized, erupted plutons accumulated at rates comparable to dated examples, they should preserve a record of zircon growth of up to 10 Ma. Alternatively, the long history of zircon growth recorded in plutons may be erased during the processes of reheating and remobilization that precede supervolcano eruption. However, zircon dissolution modeling, based on hypothetical temperature‐time histories for preeruptive monotonous intermediates, indicates that rejuvenation events would not sufficiently dissolve zircon. We suggest that eruptions of monotonous intermediates occur during high magmatic flux events, leaving little behind in the intrusive rock record, whereas low fluxes favor pluton accumulation.

Petrogenesis of Sierra Nevada plutons inferred from the Sr, Nd, and O isotopic signatures of mafic igneous complexes in Yosemite Valley, California

Mafic complexes in the central Sierra Nevada batholith record valuable geochemical information regarding the role mafic magmas play in arc magmatism and the generation of continental crust. In the intrusive suite of Yosemite Valley, major and trace element compositions of the hornblende-bearing gabbroic rocks from the Rockslides mafic complex and of the mafic dikes in the North America Wall are compositionally similar to high-alumina basalt. Of these rocks, two samples have higher Ni and Cr abundances as well as higher eNd values than previously recognized for the intrusive suite. Plagioclase crystals in rocks from the North America Wall and the Rockslides have prominent calcic cores and sharply defined sodic rims, a texture commonly associated with mixing of mafic and felsic magmas. In situ analyses of 87Sr/86Sr in plagioclase show no significant isotopic difference from the cores to the rims of these grains. We propose that the high 87Sr/86Sr (~0.7067) and low eNd (~−3.4) of bulk rocks, the homogeneity of 87Sr/86Sr in plagioclase, and the high δ18O values of bulk rocks (6.6–7.3 ‰) and zircon (Lackey et al. in J Petrol 49:1397–1426, 2008) demonstrate that continental crust was assimilated into the sublithospheric mantle-derived basaltic precursors of the mafic rocks in Yosemite Valley. Contamination (20–40 %) likely occurred in the lower crust as the magma differentiated to high-alumina basalt prior to plagioclase (and zircon) crystallization. As a consequence, the isotopic signatures recorded by whole rocks, plagioclase, and zircon do not represent the composition of the underlying lithospheric mantle. We conclude that the mafic and associated felsic members of the intrusive suite of Yosemite Valley represent 60–80 % new additions to the crust and include significant quantities of recycled ancient crust.

Magma accumulation rates and thermal histories of plutons of the Sierra Nevada batholith, CA

Zircon U–Pb geochronology results indicate that the John Muir Intrusive Suite of the central Sierra Nevada batholith, California, was assembled over a period of at least 12 Ma between 96 and 84 Ma. Bulk mineral thermochronology (U–Pb zircon and titanite, 40Ar/39Ar hornblende and biotite) of rocks from multiple plutons comprising the Muir suite indicates rapid cooling through titanite and hornblende closure following intrusion and subsequent slow cooling through biotite closure. Assembly of intrusive suites in the Sierra Nevada and elsewhere over millions of years favors growth by incremental intrusion. Estimated long-term pluton assembly rates for the John Muir Intrusive Suite are on the order of 0.001 km3 a−1 which is inconsistent with the rapid magma fluxes that are necessary to form large-volume magma chambers capable of producing caldera-forming eruptions. If large shallow crustal magma chambers do not typically develop during assembly of large zoned intrusive suites, it is doubtful that the intrusive suites represent cumulates left behind following caldera-forming eruptions.

Processes involved during incremental growth of the Jackass Lakes pluton, central Sierra Nevada batholith

The Jackass Lakes pluton (JLP), located in the central Sierra Nevada batholith, is a 98 Ma composite intrusion that preserves field, structural, and petrologic evidence of how incrementally emplaced plutons grow and evolve both spatially and temporally. In contrast to many other Sierra Nevada batholith intrusions, the compositional and textural diversity found within the JLP allows individual increments to be easily discerned in the field. Previous work has resulted in two different incremental emplacement models for the JLP. The first states that the JLP was emplaced and assembled via vertical diking or sheeting, some downward return flow along the margins of the pluton, and local stoping. The second involves replenishment by mafic sheets that may represent originally subhorizontal floors of an evolving magma chamber. We present new data and a model suggesting that the JLP (1) contains multiple, irregularly shaped intrusions of both felsic and mafic material that do not represent dikes or paleofloors; (2) magma increments were extensively mingled, both between and within intrusions; (3) records evidence of magma mixing locally and possibly at the intrusion scale; (4) has not been tilted; (5) has magmatic mineral fabrics that record superimposed regional strain, not emplacement-related strain; (6) preserves large metavolcanic pendants representing subhorizontal roof contacts; and (7) was emplaced by ductile deformation of its host rocks, return flow, and widespread stoping of older host rock and its internal increments. This model, based on field, structural, strain, thermobarometric, and petrologic analyses, elucidates that the JLP construction is considerably more complex spatially and temporally than previous models suggest, and highlights processes involved during incremental emplacement of plutons.

Processes involved during incremental growth of the Jackass Lakes pluton, central Sierra Nevada batholith

Geosphere, April 2010, v. 6, p. 130-159, doi:10.1130/GES00224.1, Plate 2 - PDF file of full map seen in Fig. 3, which includes more detailed unit descriptions and structural data plotted on map in lower-hemisphere equal-area stereonets.

Processes involved during incremental growth of the Jackass Lakes pluton, central Sierra Nevada batholith

Geosphere, April 2010, v. 6, p. 130-159, doi:10.1130/GES00224.1, Plate 2 - PDF file of full map seen in Fig. 3, which includes more detailed unit descriptions and structural data plotted on map in lower-hemisphere equal-area stereonets.

Dynamic Magma Systems, Crustal Recycling, and Alteration in the Central Sierra Nevada Batholith: the Oxygen Isotope Record

Values of δ18O 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) δ18O provides evidence for modest (0.5‰), but widespread, alteration, which has complicated interpretation in previous whole-rock δ18O studies. Four discrete belts of δ18O values are recognized in the central Sierra. A small belt of plutons with relatively low δ18O(Zrc) values (5·2–6·0‰) intrudes the foothills, with a sharp increase of δ18O revealing the concealed Foothills Suture; high δ18O(Zrc) values (7·0–8·5‰) dominate the rest of the western SNB. East of the axis of the Sierra, δ18O is distinctly lower (6·75–5·75‰), and decreases monotonically to the Sierra Crest. A sharp 1‰ increase of δ18O in the eastern Sierra reveals a second crustal boundary, with the fourth belt hosted in high-δ18O 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 δ18O occur between pulses of Cretaceous magmatism that may be linked to tectonic reorganizations of magma sources. Within intrusive suites, δ18O 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, δ18O reveals diverse magma system behavior at a range of scales in the Sierran arc.

Deciphering the source and contamination history of peraluminous magmas using δ18O of accessory minerals: examples from garnet-bearing plutons of the Sierra Nevada batholith

Peraluminous granitoids provide critical insight as to the amount and kinds of supracrustal material recycled in the central Sierra Nevada batholith, California. Major element concentrations indicate Sierran peraluminous granitoids are high-SiO2 (68.9–76.9) and slightly peraluminous (average molar Al2O3/(CaO + Na2O + K2O)=1.06). Both major and trace element trends mimic those of other high-silica Sierran plutons. Garnet (Grt) in the peraluminous plutons is almandine–spessartine-rich and of magmatic origin. Low grossular contents are consistent with shallow (<4 kbar) depths of garnet crystallization. Metasediments of the Kings Sequence commonly occur as wallrocks associated with the plutons, including biotite schists that are highly peraluminous (A/CNK=2.25) and have high whole rock (WR) δ18O values (9.6–21.8‰, average=14.5±2.9‰, n=26). Ultramafic wallrocks of the Kings–Kaweah ophiolite have lower average δ18O (7.1±1.3‰, n=9). The δ18O(WR) of the Kings Sequence is variable from west to east. Higher δ18O values occur in the west, where quartz in schists is derived from marine chert; values decrease eastward as the proportion of quartz from igneous and metamorphic sources increases. Peraluminous plutons have high δ18O(WR) values (9.5–13‰) consistent with supracrustal enrichment of their sources. However, relatively low initial 87Sr/86Sr values (0.705–0.708) indicate that the supracrustal component in the source of peraluminous magmas was dominantly altered ocean crust and/or greywacke. Also, plutons lack or have very low abundances (<1% of grains) of inherited zircon (Zrc) cores. Average δ18O(Zrc) is 7.9‰ in peraluminous plutons, a higher value than in coeval metaluminous plutons (6–7‰). Diorites associated with peraluminous plutons also have high δ18O(Zrc), 7.4–8.3‰, which is consistent with the diorites being derived from a similar source. Magmatic garnet has variable δ18O (6.6–10.5‰, avg.=7.9‰) due to complex contamination and crystallization histories, evidenced by multiple garnet populations in some rocks. Comparison of δ18O(Zrc) and δ18O(Grt) commonly reveals disequilibrium, which documents evolving magma composition. Minor (5–7%) contamination by high δ18O wallrocks occurred in the middle and upper crust in some cases, although low δ18O wallrock may have been a contaminant in one case. Overall, oxygen isotope analysis of minerals having slow oxygen diffusion and different times of crystallization (e.g., zircon and garnet), together with detailed textural analysis, can be used to monitor assimilation in peraluminous magmas. Moreover, oxygen isotope studies are a valuable way to identify magmatic versus xenocrystic minerals in igneous rocks.

Base- and precious-metal deposits in the Basin and Range of Southern California and Southern Nevada—Metallogenic implications of lead isotope studies

Southern California and southern Nevada contain abundant lead–zinc deposits with strikingly different characteristics. On the west, the Darwin Terrane contains abundant Jurassic and Cretaceous intrusions surrounded by lead–zinc skarn and replacement deposits rich in pyrite and manganese minerals. The Tecopa Terrane is east of the Darwin Terrane and contains some lead deposits that are hosted by the Proterozoic Noonday Dolomite. These lead deposits have no consistent relation to igneous rocks; they contain mainly galena, and are devoid of pyrite and manganese minerals. Other skarn and vein deposits in the Ivanpah and Tecopa districts are more closely associated with igneous rocks. Mississippi Valley type lead–zinc deposits are present still farther to the east in the Goodsprings Terrane in Nevada. These deposits are hosted by breccias formed below the Mississippian–Pennsylvanian unconformity and are unrelated to igneous rocks. Deposits in the Darwin Terrane have lead isotopic signatures that lie along a mantle–sediment mixing line indicating that they formed in a continental arc setting analogous to that for the plutons in the Sierra Nevada batholith [Chen, J.H. and Tilton, G.R., 1991. Application of lead and strontium isotopic relationships to the petrogenesis of granitoid rocks, central Sierra Nevada batholith, California. Geological Society of America Bulletin 103, 439–447]. Encroachment of this continental arc on the North American continent in the eastern part of the Darwin Terrane resulted in a lead isotopic signature that is like that of the strongly contaminated plutons [Chen, J.H. and Tilton, G.R., 1991. Application of lead and strontium isotopic relationships to the petrogenesis of granitoid rocks, central Sierra Nevada batholith, California. Geological Society of America Bulletin 103, 439–447]. Many deposits from the Inyo Mountains on east side of the Owens Valley have lead isotopic signatures that reflect this encroachment. To the east in the Tecopa Terrane, encroachment of the continental arc on the Mojave crust resulted in partial melting of 1.7 Ga amphibolite and granulite facies rocks to produce the plutons and mineral deposits associated with plutons in this terrane. Lead from deposits in this terrane hosted in the Proterozoic Noonday Dolomite and associated rocks have a lead isotope signature that reflects hydrothermal circulation of fluids in the Mojave supracrustal rocks. The boundary between the Darwin Terrane and the Tecopa Terrane lies just west of the Ash Valley–Panamint Range Fault and is bounded by the Towne Fault on the north and the Garlock Fault on the south. Lead isotopic data from the Goodsprings district in southwestern Nevada, east of the Tecopa Terrane, form a 1.45 Ga linear array that is indicative of Mississippi Valley type mineralization. Although we have no independent evidence of the timing of the tectonic events that formed these deposits, it is likely that all formed as a result of the Mesozoic collision of the Panthalassen crust with the North American continent.

Experimental and geochemical evidence for derivation of the El Capitan Granite, California, by partial melting of hydrous gabbroic lower crust

Partial melting of mafic intrusions recently emplaced into the lower crust can produce voluminous silicic magmas with isotopic ratios similar to their mafic sources. Low-temperature (825 and 850°C) partial melts synthesized at 700 MPa in biotite-hornblende gabbros from the central Sierra Nevada batholith (Sisson et al. in Contrib Mineral Petrol 148:635–661, 2005) have major-element and modeled trace-element (REE, Rb, Ba, Sr, Th, U) compositions matching those of the Cretaceous El Capitan Granite, a prominent granite and silicic granodiorite pluton in the central part of the Sierra Nevada batholith (Yosemite, CA, USA) locally mingled with coeval, isotopically similar quartz diorite through gabbro intrusions (Ratajeski et al. in Geol Soc Am Bull 113:1486–1502, 2001). These results are evidence that the El Capitan Granite, and perhaps similar intrusions in the Sierra Nevada batholith with lithospheric-mantle-like isotopic values, were extracted from LILE-enriched, hydrous (hornblende-bearing) gabbroic rocks in the Sierran lower crust. Granitic partial melts derived by this process may also be silicic end members for mixing events leading to large-volume intermediate composition Sierran plutons such as the Cretaceous Lamarck Granodiorite. Voluminous gabbroic residues of partial melting may be lost to the mantle by their conversion to garnet-pyroxene assemblages during batholithic magmatic crustal thickening.

Mafic magmatic enclaves and mafic rocks associated with some granitoids of the central Sierra Nevada batholith, California: nature, origin, and relations with the hosts

The calc-alkaline granitoids of the central Sierra Nevada batholith are associated with abundant mafic rocks. These include both country-rock xenoliths and mafic magmatic enclaves (MME) that commonly have fine-grained and, less commonly, cumulate textures. Scarce composite enclaves consist of either xenoliths enclosed in MME, or of MME enclosed in other MME with different grain size and texture. Enclaves are often enclosed in mafic aggregates and form meter-size polygenic swarms, mostly in the margins of normally zoned plutons. Enclaves may locally divert schlieren layering. Mafic dikes, which also occur in swarms, are undisturbed, composite, or largely hybridized. In central Sierra Nevada, with the exception of xenoliths that completely differ from the other rocks, host granitoids, mafic aggregates, MME, and some composite dikes exhibit a bulk compositional diversity and, at the same time, important mineralogical and geochemical (including isotopic) similarities. MME and host granitoids display distinct major and trace element compositions. However, strong correlations between MME–host granitoid pairs indicate interactions and parallel evolution of MME and enclosing granitoid in each pluton. Identical mafic mineral compositions and isotopic features are the result of these interactions and parallel evolution. Mafic dikes have broadly the same major and trace element compositions as the MME although variations are large between the different dikes that are at distinctly different stages of hybridization and digestion by the host granitoids. The composition of the granitoids and various mafic rocks reflects three distinct stages of hybridization that occurred, respectively, at depth, during ascent and emplacement, and after emplacement. The occurrence and succession of hybridization processes were tightly controlled by the physical properties of the magmas. The sequential thorough or partial mixing and mingling were commonly followed by differentiation and segregation processes. Unusual MME that contain abundant large crystals of hornblende resulted from disruption of early cumulates at depth, whereas those richer in large crystals of biotite were formed by disruption of late mafic aggregates or schlieren layerings at the level of emplacement. MME and host granitoids are considered cogenetic, because both are hybrid rocks that were produced by the mixing of the same two components in different proportions. The felsic component was produced by partial melting of preexisting crustal materials, whereas the dominant mafic component was probably derived from the upper mantle. However, in the lack of a clear mantle signature, the origin of the mafic component remains questionable.

Magma Mixing and Cretaceous Crustal Growth: Geology and Geochemistry of Granites in the Central Sierra Nevada Batholith, California

Major- and trace-element data suggest that some granites exposed in the Sierra Nevada batholith represent appropriate end members in a mixing process that generated voluminous granodiorites. Geochemical trends in a range of rock types indicate that the highest-silica granites have been variably modified by partial melting and/or fractional crystallization. Trends in initial Sr, Nd, and common Pb isotope ratios reveal little variation within a wide range of rock types in the same suite, but significant spatial and isotopic variation among individual suites of rocks. These observations are consistent with the interpretation that granites in the Sierra are generated by partially melting juvenile, mantle-derived diorites and mixed with penecontemporaneous mantle melts. With the exception of one spatially limited suite in this study, there is little evidence that ancient continental crust was involved in the generation of the central Sierra Nevada batholith. Instead, isotopic trends from east to west in this study indicate that the majority of granites in the central Sierra Nevada are likely juvenile crust. These observations may dramatically increase estimates of crustal growth rates in the western United States during the Mesozoic.

Production and loss of high‐density batholithic root, southern Sierra Nevada, California

Eclogites are commonly believed to be highly susceptible to delamination and sinking into the mantle from lower crustal metamorphic environments. We discuss the production of a specific class of eclogitic rocks that formed in conjunction with the production of the Sierra Nevada batholith. These high‐density eclogitic rocks, however, formed by crystal‐liquid equilibria and thus contrast sharply in their petrogenesis and environment of formation from eclogite facies metamorphic rocks. Experimental studies show that when hydrous mafic to intermediate composition assemblages are melted in excess of 1 GPa, the derivative liquids are typical of Cordilleran‐type batholith granitoids, and garnet + clinopyroxene, which is an eclogitic mineralogy, dominate the residue assemblage. Upper mantle‐lower crustal xenolith suites that were entrained in mid‐Miocene volcanic centers erupted through the central Sierra Nevada batholith are dominated by such garnet clinopyroxenites, which are shown further by geochemical data to be petrogenetically related to the overlying batholith as its residue assemblage. Petrogenetic data on garnet pyroxenite and associated peridotite and granulite xenoliths, in conjunction with a southward deepening oblique crustal section and seismic data, form the basis for the synthesis of a primary lithospheric column for the Sierra Nevada batholith. Critical aspects of this column are the dominance of felsic batholithic rocks to between 35 and 40 km depths, a thick (∼35 km) underlying garnet clinopyroxenite residue sequence, and interlayered spinel and underlying garnet peridotite extending to ∼125 km depths. The peridotites appear to be the remnants of the mantle wedge from beneath the Sierran arc. The principal source for the batholith was a polygenetic hydrous mafic to intermediate composition lower crust dominated by mantle wedge‐derived mafic intrusions. Genesis of the composite batholith over an ∼50 m.y. time interval entailed the complete reconstitution of the Sierran lithosphere. Sierra Nevada batholith magmatism ended by ∼80 Ma in conjunction with the onset of the Laramide orogeny, and subsequently, its underlying mantle lithosphere cooled conductively. In the southernmost Sierra‐northern Mojave Desert region the subbatholith mantle lithosphere was mechanically delaminated by a shallow segment of the Laramide slab and was replaced by underthrust subduction accretion assemblages. Despite these Laramide events, the mantle lithosphere of the greater Sierra Nevada for the most part remained intact throughout much of Cenozoic time. A pronounced change in xenolith suites sampled by Pliocene‐Quaternary lavas to garnet absent, spinel and plagioclase peridotites, whose thermobarometry define an asthenosphere adiabat, as well as seismic data, indicate that much of the remaining sub‐Sierran lithosphere was removed in Late Miocene to Pliocene time. Such removal is suggested to have arisen from a convective instability related to high‐magnitude extension in the adjacent Basin and Range province and to have worked in conjunction with the recent phase of Sierran uplift and a change in regional volcanism to more primitive compositions. In both the Mio‐Pliocene and Late Cretaceous lithosphere removal events the base of the felsic batholith was the preferred locus of separation.

The California Arc: Thick Granitic Batholiths, Eclogitic Residues, Lithospheric-Scale Thrusting, and Magmatic Flare-Ups

NOVEMBER 2001, GSA TODAY ABSTRACT Recent geological and geophysical data show that a significant fraction of the crust (~33 km) in the central Sierra Nevada batholith is granitic, requiring that the batholith be underlain by a significant residual mass prior to Cenozoic extension. Although batholith residua are commonly thought to be granulites, xenolith data indicate that eclogite facies residues were an important part of the California arc at depth. The arc was continuously active for >140 m.y., yet most surface and/or shallow crustal magmatism took place via two short-lived episodes: one in the Late Jurassic (160–150 Ma), and a second, more voluminous one in the Late Cretaceous (100–85 Ma). These magmatic flare-ups cannot be explained solely by increases in convergence rates and magmatic additions from the mantle. Isotopic data on xenoliths and midcrustal exposures suggest that North American lower crustal and lithospheric mantle was underthrusted beneath accreted rocks in the arc area. The Late Cretaceous flare-up is proposed to be the result of this major west dipping–lithospheric scale thrusting, an event that preceded flare-up by ~15–25 m.y. I suggest that the central part of the arc shut off at ~80 Ma because the source became melt-drained and not because of refrigeration from a shallowly subducting slab.

Magnetic fabric evidence for conduit‐fed emplacement of a tabular intrusion: Dinkey Creek Pluton, central Sierra Nevada batholith, California

Anisotropy of magnetic susceptibility measurements in the Cretaceous Dinkey Creek Pluton (DCP), central Sierra Nevada batholith, reveal a well‐defined structural pattern and zonation in bulk susceptibility (K). Outward decrease in K within the pluton is due to a decrease in ferrimagnetic contribution, corresponding to a compositional zoning from tonalite to granite. Magnetic fabrics in the pluton formed during supersolidus flow of a crystal‐rich magma, except in the eastern domain where subvertical magnetic lineations and NW striking magnetic foliations are attributed to deformation associated with a regional shear zone. Magnetic foliations in the central domain of the DCP define a NNW trending synformal structure, passing structurally upward into a dome beneath a small roof pendant, interpreted to represent different structural levels in a zone of magma upwelling. Arcuate foliation trajectories in the pluton's SW lobe resemble deformed passive markers in two‐dimensional, NW to SW, horizontal, channel flow of non‐Newtonian magma. Analysis of the fabric pattern in cross‐section estimates the preerosion thickness of the lobe to between 915 and 3660 m. The DCP was emplaced as a tabular body with vertical sides and a gently inclined roof. Magma with increasingly more mafic compositions entered the chamber continuously or episodically via a NNW trending conduit. Space for the pluton was created by vertical inflation, probably accommodated by depression of its floor. These results suggest that the mid crust to upper crust of magmatic arcs may be constructed by vertical stacking of tabular granitic plutons with high width/thickness ratios like the DCP.

Microgranitoid enclave swarms in granitic plutons, central Sierra Nevada, California

Microgranitoid enclaves are common in granitic plutons worldwide, occurring individually and in homogeneous or heterogeneous swarms. Three plutons in the central Sierra Nevada batholith contain swarms with mostly heterogeneous suites of enclaves in the intermediate composition range, and occur in a number of two-dimensional shapes, specifically as dikes, small rafts, lenses, pipe/vortices and large massive shapes. Swarms are characterized by various features, including the nature of their boundary with the host, their planar or non-planar character, internal geometry, density of enclave packing, presence or absence of schlieren and crystal aggregates, and axial ratios and degree of preferred alignment of enclaves. We propose that heterogeneous enclave swarms form by one, or some combination of, the following mechanisms: (1) velocity-gradient sorting parallel or normal to the flow, (2) gravitational sorting or (3) break-up of heterogeneous dikes. Common sites where enclave swarms form include pluton margins or internal viscosity walls, within fractures, and near the pluton roof.