Advances in field observations and experimental petrology on anatectic products have motivated us to investigate the geochemical consequences of accessory mineral dissolution and nonmodal partial ...melting processes. Incorporation of apatite and monazite dissolution into a muscovite dehydration melting model allows us to examine the coupling of the Rb-Sr and Sm-Nd isotope systems in anatectic melts from a muscovite-bearing metasedimentary source. Modeling results show that (1) the Sm/Nd ratios and Nd isotopic compositions of the melts depend on the amount of apatite and monazite dissolved into the melt, and (2) the relative proportion of micas (muscovite and biotite) and feldspars (plagioclase and K-feldspar) that enter the melt is a key parameter determining the Rb/Sr and
87Sr/
86Sr ratios of the melt. Furthermore, these two factors are not, in practice, independent. In general, nonmodal partial melting of a pelitic source results in melts following one of two paths in ε
Nd-
87Sr/
86Sr ratio space. A higher temperature, fluid-absent path (Path 1) represents those partial melting reactions in which muscovite/biotite dehydration and apatite but not monazite dissolution play a significant role; the melt will have elevated Rb/Sr,
87Sr/
86Sr, Sm/Nd, and ε
Nd values. In contrast, a lower temperature, fluid-fluxed path (Path 2) represents those partial melting reactions in which muscovite/biotite dehydration plays an insignificant role and apatite but not monazite stays in the residue; the melt will have lower Rb/Sr,
87Sr/
86Sr, Sm/Nd, and ε
Nd values than its source. The master variables controlling both accessory phase dissolution (and hence the Sm-Nd system), and melting reaction (and hence the Rb-Sr systematics) are temperature and water content. The complexity in Sr-Nd isotope systematics in metasediment-derived melts, as suggested in this study, will help us to better understand the petrogenesis for those granitic plutons that have a significant crustal source component.
Low‐angle subduction of oceanic lithosphere may be an important process in modifying continental lithosphere. A classic example is the underthrusting of the Farallon plate beneath North America ...during the Laramide orogeny. To assess the relevance of this process to the evolution and composition of continental lithosphere, the mantle stratigraphy beneath the Mojave Desert was constrained using ultramafic xenoliths hosted in Plio‐Pleistocene cinder cones. Whole‐rock chemistry, clinopyroxene trace element and Nd isotope data, in combination with geothermometry and surface heat flow, indicate kilometer‐scale compositional layering. The shallow parts are depleted in radiogenic Nd (ɛNd = −13 to −6.4) and are interpreted to be ancient continental mantle that escaped tectonic erosion by low‐angle subduction. The deeper samples are enriched in radiogenic Nd (ɛNd = +5.7 to +16.1) and reveal two superposed mantle slices of recent origin. Within each slice, compositions range from fertile lherzolites at the top to harzburgites at the bottom: the latter formed by 25–28% low‐pressure melt depletion and the former formed by refertilization of harzburgites by mid‐ocean‐ridge‐basalt‐like liquids. The superposition and internal compositional zonation of the slices preclude recent fertilization by Cenozoic extension‐related magmas. The above observations imply that the lower Mojavian lithosphere represents tectonically subcreted and imbricated lithosphere having an oceanic protolith. If so, the lherzolitic domains may be related to melting and refertilization beneath mid‐ocean ridges. The present Mojavian lithosphere is thus a composite of a shallow section of the original North American lithosphere underlain by Farallon oceanic lithosphere accreted during low‐angle subduction.
The Rand and Sierra de Salinas schists of southern California were underplated beneath the southern Sierra Nevada batholith and adjacent Mojave‐Salinia region along a shallow segment of the ...subducting Farallon plate in Late Cretaceous time. Various mechanisms, including return flow, isostatically driven uplift, upper plate normal faulting, erosion, or some combination thereof, have been proposed for the exhumation of the schist. We supplement existing kinematic data with new vorticity and strain analysis to characterize deformation in the Rand and Sierra de Salinas schists. These data indicate that the schist was transported to the SSW from deep to shallow crustal levels along a mylonitic contact (the Rand fault and Salinas shear zone) with upper plate assemblages. Crystallographic preferred orientation patterns in deformed quartzites reveal a decreasing simple shear component with increasing structural depth, suggesting a pure shear dominated westward flow within the subduction channel and localized simple shear along the upper channel boundary. The resulting flow type within the channel is that of general shear extrusion. Integration of these observations with published geochronologic, thermochronometric, thermobarometric, and paleomagnetic studies reveals a temporal relationship between schist unroofing and upper crustal extension and rotation. We present a model whereby trench‐directed channelized extrusion of the underplated schist triggered gravitational collapse and clockwise rotation of the upper plate.
We summarize the post Late Cretaceous regional tectonic evolution of the central California Coast Ranges, west of the San Andreas fault system. The Monterey terrane of North American origin was ...laterally transferred to the Pacific plate via the San Andreas fault. The Monterey terrane is an assembly of three tectonic units, Salinia, Nacimiento and Sierra de Salinas blocks, two of which have been previously identified as separate terranes. These blocks are separated by two regionally important thrust faults: the Sur fault as well as the Salinas shear zone. Based on thermobarometric and thermochronologic constraints and the existence of a common younger cover sequence, these blocks were juxtaposed together after the latest Cretaceous. The Salinian assemblage represents a crustal section through the continental interior side of the Mesozoic California arc and formed during the Late Cretaceous, primarily during a regionally significant magmatic flare-up between 95 and 80 Ma. In the Santa Lucia Range, parts of the arc are exposed to palaeo-depths in excess of 30 km. The Nacimiento and Sierra de Salinas assemblages comprise basement rocks representing Late Cretaceous variants of the Franciscan Complex and are interpreted to be correlative. They represent the lower plate of a regionally important thrust system; the upper plate is the Salinian assemblage, whereas the Sur and Salinas faults are local exposures of the structure. We concur with previous estimates of 150 to 180 km of shortening during a brief time span (<6 my), at a rate of >3 cm/yr. This fault system corresponds to the megathrust of the Farallon subduction beneath North America during the early stages of the regionally extensive episode of shallow subduction (Laramide orogeny). As a result, trench sediment was thrust under North America and tectonically underplated to the lower crust of North America. The Salinas shear zone, in particular, is a ductile expression of shallow subduction; thermobarometry in the upper plate, lower plate and the shear zone itself indicate that this is the fossil subduction megathrust originating at depths of ∼35 km. The entire system collapsed extensionally soon after the trench sediment was underthrust, possibly because of the lack of strength of the lower plate. Arc magmatism in the upper plate ceased at the onset of underplating. This regional example illustrates the significance of tectonic underplating in shallow subduction systems. Accretion-related trench sediment was shuffled from the trench to the sub-arc region of the upper plate, but not recycled into the mantle. This process requires that the subduction megathrust be located solely within the North American crust. This geometry requires a sudden migration of the subduction interface toward the arc and may apply to other regional examples, including the modern shallow subduction of the Cocos plate beneath southern Mexico. The tectonically underplated trench sediment undergoes regional, Barrovian metamorphism, after initially following a high-pressure/low-temperature path. Moreover, the shear zone marking the fossil intracrustal megathrust was subject to granulite-facies metamorphism and limited partial melting.
Salinia, as originally defined, is a fault-bounded terrane in westcentral California. As defined, Salinia lies between the Nacimiento fault on the west, and the Northern San Andreas fault (NSAF) and ...the main trace of the dextral SAF system on the east. This allochthonous terrane was translated from the southern part of the Sierra Nevada batholith and adjacent western Mojave Desert region by Neogene-Quaternary displacement along the SAF system. The Salina crystalline basement formed a westward promontory in the SW Cordilleran Cretaceous batholithic belt, relative to the Sierra Nevada batholith to the north and the Peninsular Ranges batholith to the south, making Salinia batholithic rocks susceptible to capture by the Pacific plate when the San Andreas transform system developed. Proper restoration of offsets on all branches of the San Andreas system is a critical factor in understanding the Salinia problem. When cumulative dextral slip of 171 km (106 mi) along the Hosgri-San Simeon-San Gregorio-Pilarcitos fault zone (S-N), or dextral slip of 200 km (124 mi) along the Hosgri-San Simeon-San Gregorio-Pilarcitos-northern San Andreas fault system, is added to the cumulative dextral slip of 315-322 km (196-200 mi) along the main trace of the SAF north of the San Emigdio-Tehachapi mountains, central California, there is a minimum amount of cumulative dextral slip of 486 km (302 mi) or a maximum amount of cumulative dextral slip of 522 km (324 mi) along the entire SAF system north of the Tehachapi Mountains. When these sums are compared with the offset distance (610-675 km or 379-420 mi) between the batholithic rocks associated with the Navarro structural discontinuity (NSD) in northern California, and those in the 'tail' of the southern Sierra Nevada granitic rocks in the San Emigdio-Tehachapi mountains, central California, a minimum deficit of from ∼100 km (∼62 mi) to a maximum deficit of ∼189 km (∼118 mi) is needed to restore the crystalline rocks associated with the NSD with the crystalline terranes within the San Emigdio and Tehachapi mountains - the enigma of Salinia. Two principal geologic models compete to explain the enigma (i.e. the discrepancy between measured dextral slip along traces of the SAF system and the amount of separation between the Sierra Nevada batholithic rocks near Point Arena in northern California and the Mesozoic and older crystalline rocks in the San Emigdio and Tehachapi mountains in southern California). (i) One model proposes pre-Neogene (>23 Ma), Late Cretaceous or Maastrichtian (<ca. 71 Ma) to early Palaeocene or Danian (ca. 66 Ma) sinistral slip of 500-600 km (311-373 mi) along the Nacimiento fault and of the western flank of Salinia from the eastern flank of the Peninsular Ranges (sinistral slip but in the opposite sense to later Neogene (<23 Ma) dextral slip along and within the SAF system. (ii) A second model proposes that the crystalline rocks of Salinia comprise a series of 100 km- (60 mi-) scale allochthonous (extensional) nappes that rode southwestward above the Rand schist-Sierra de Salinas (SdS) shear zone subduction extrusion channels. The allochthonous nappes are from NW-SE: (i) Farallon Islands-Santa Cruz Mountains-Montara Mountain, and adjacent batholithic fragments that appear to have been derived from the top of the deep-level Sierra Nevada batholith of the western San Emigdio-Tehachapi mountains; (ii) the Logan Quarry-Loma Prieta Peak fragments that appear to have been derived from the top of a buried detachment fault that forms the basement surface beneath the Maricopa sub-basin of the southernmost Great Valley; (iii) The Pastoria plate-Gabilan Range massif that appears to have been derived from the top of the deep-level SE Sierra Nevada batholith; and (iv) the Santa Lucia-SdS massif, which appears to be lower batholithic crust and underlying extruded schist that were breached westwards from the central to western Mojave Desert region. In this model, lower crustal batholithic blocks underwent ductile stretching above the extrusion channel schists, while mid- to upper-crustal level rocks rode southwestwards and westwards along trenchward dipping detachment faults. Salinian basement rocks of the Santa Lucia Range and the Big Sur area record the most complete geologic history of the displaced terrane. The oldest rocks consist of screens of Palaeozoic marine metasedimentary rocks (the Sur Series), including biotite gneiss and schist, quartzite, granulite gneiss, granofels, and marble. The Sur Series was intruded during Cretaceous high-flux batholithic magmatism by granodiorite, diorite, quartz diorite, and at deepest levels, charnockitic tonalite. Local nonconformable remnants of Campanian-Maastrichtian marine strata lie on the deep-level Salinia basement, and record deposition in an extensional setting. These Cretaceous strata are correlated with the middle to upper Campanian Pigeon Point (PiP) Formation south of San Francisco. The Upper Cretaceous strata, belonging to the Great Valley Sequence, include clasts of the basement rocks and felsic volcanic clasts that in Late Cretaceous time were brought to a coastal region by streams and rivers from Mesozoic felsic volcanic rocks in the Mojave Desert. The Rand and SdS schists of southern California were underplated beneath the southern Sierra Nevada batholith and the adjacent Salinia-Mojave region along a shallow segment of the subducting Farallon plate during Late Cretaceous time. The subduction trajectory of these schists concluded with an abrupt extrusion phase. During extrusion, the schists were transported to the SW from deep- to shallow-crustal levels as the low-angle subduction megathrust surface was transformed into a mylonitic low-angle normal fault system (i.e. Rand fault and Salinas shear zone). The upper batholithic plate(s) was(ere) partially coupled to the extrusion flow pattern, which resulted in 100 km-scale westward displacements of the upper plate(s). Structural stacking, temporal and metamorphic facies relations suggest that the Nacimiento (subduction megathrust) fault formed beneath the Rand-SdS extrusion channel. Metamorphic and structural relations in lower plate Franciscan rocks beneath the Nacimiento fault suggest a terminal phase of extrusion as well, during which the overlying Salinia underwent extension and subsidence to marine conditions. Westward extrusion of the subduction-underplated rocks and their upper batholithic plates rendered these Salinia rocks susceptible to subsequent capture by the SAF system. Evidence supporting the conclusion that the Nacimiento fault is principally a megathrust includes: (i) shear planes of the Nacimiento fault zone in the westcentral Coast Ranges locally dip NE at low angles. (ii) Klippen and/or faulted klippen are locally present along the trace of the Nacimiento fault zone from the Big Creek-Vicente Creek region south of Point Sur near Monterey, to east of San Simeon near San Luis Obispo in central California. Allochthonous detachment sheets and windows into their underplated schists comprise a composite Salinia terrane. The nappe complex forming the allochthon of Salinia was translated westward and northwestward ∼100 km (∼62 mi) above the Nacimiento megathrust or Franciscan subduction megathrust from SE California between ca. 66 and ca. 61 Ma (i.e. latest Cretaceous-earliest Palaeocene time). Much, or all, of the westward breaching of the Salinia batholithic rocks likely occurred above the extrusion channels of the Rand-SdS schists; following this event, the Franciscan Sur-Obispo terrane was thrust beneath the schists, perhaps during the final stages of extrusion in the upper channel. Later, the Sur-Obispo terrane was partially extruded from beneath the Salinia nappe terrane, during which time the upper plate(s) underwent extension and subsidence to marine conditions. Attenuation of the Salinia nappe sequence during the extrusion of the Franciscan Complex thinned the upper crust, making the upper plates susceptible to erosion from the top of the Franciscan Complex near San Simeon, where it is now exposed. In the San Emigdio Mountains, the relatively thin structural thickness of the upper batholithic plates made them susceptible to late Cenozoic flexural folding and disruption by high-angle dip-slip faults. The ∼100 km (∼62 mi) of westward and northwestward breaching of the Salinia batholithic rocks above the Rand-SdS channels, and the underlying Nacimiento fault followed by ∼510 km (∼320 mi) of dextral slip from ∼23 Ma to Holocene time along the SAF system, allow for the palinspastic restoration of Salinia with the crystalline rocks of the San Emigdio-Tehachapi mountains and the Mojave terrane, resolving the enigma of Salinia.
We provide data on the geochemical and isotopic consequences of nonmodal partial melting of a thick Jurassic pelite unit at mid-crustal levels that produced a migmatite complex in conjunction with ...the intrusion of part of the southern Sierra Nevada batholith at ca. 100 Ma. Field relations suggest that this pelitic migmatite formed and then abruptly solidified prior to substantial mobilization and escape of its melt products. Hence, this area yields insights into potential mid-crustal level contributions of crustal components into Cordilleran-type batholiths. Major and trace-element analyses in addition to field and petrographic data demonstrate that leucosomes are products of partial melting of the pelitic protolith host. Compared with the metapelites, leucosomes have higher Sr and lower Sm concentrations and lower Rb/Sr ratios. The initial 87Sr/86Sr ratios of leucosomes range from 0.7124 to 0.7247, similar to those of the metapelite protoliths (0.7125-0.7221). However, the leucosomes have a much wider range of initial eNd values, which range from -6.0 to -11.0, as compared to -8.7 to -11.3 for the metapelites. Sr and Nd isotopic compositions of the leucosomes, migmatites, and metapelites suggest disequilibrium partial melting of the metapelite protolith. Based on their Sr, Nd, and other trace-element characteristics, two groups of leucosomes have been identified. Group A leucosomes have relatively high Rb, Pb, Ba, and K2O contents, Rb/Sr ratios (0.15values, which range from -6.0 to -11.0, as compared to -8.7 to -11.3 for the metapelites. Sr and Nd isotopic compositions of the leucosomes, migmatites, and metapelites suggest disequilibrium partial melting of the metapelite protolith. Based on their Sr, Nd, and other trace-element characteristics, two groups of leucosomes have been identified. Group A leucosomes have relatively high Rb, Pb, Ba, and K2O contents, Rb/Sr ratios (0.15 less than Rb/Sr less than 1.0), and initial eNd values. Group B leucosomes have relatively low Rb, Pb, Ba, and K2O contents, Rb/Sr ratios (<0.15), and initial eNd values. The low Rb concentrations and Rb/Sr ratios of the group B leucosomes together suggest that partial melting was dominated by water-saturated or H2O-fluxed melting of quartz + feldspar assemblage with minor involvement of muscovite. Breakdown of quartz and plagioclase with minor contributions from muscovite resulted in low Rb/Sr ratios characterizing both group A and group B leucosomes. In contrast, group A leucosomes have greater contributions from K-feldspar, which is suggested by: (1) their relatively high K concentrations, (2) positive or slightly negative Eu anomalies, and (3) correlation of their Pb and Ba concentrations with K2O contents. It is also shown that accessory minerals have played a critical role in regulating the partitioning of key trace elements such as Sm, Nd, Nb, and V between melt products and residues during migmatization. The various degrees of parent/daughter fractionations in the Rb-Sr and Sm-Nd isotopic systems as a consequence of nonmodal crustal anatexis would render melt products with distinct isotopic signatures, which could profoundly influence the products of subsequent mixing events. This is not only important for geochemical patterns of intracrustal differentiation, but also a potentially important process in generating crustal-scale as well as individual pluton-scale isotopic heterogeneities.PUBLICATION ABSTRACT
We present evidence for a thick (∼100 km) sequence of cogenetic rocks which make up the root of the Sierra Nevada batholith of California. The Sierran magmatism produced tonalitic and granodioritic ...magmas which reside in the Sierra Nevada upper- to mid-crust, as well as deep eclogite facies crust/upper mantle mafic-ultramafic cumulates. Samples of the mafic-ultramafic sequence are preserved as xenoliths in Miocene volcanic rocks which erupted through the central part of the batholith. We have performed Rb-Sr and Sm-Nd mineral geochronologic analyses on seven fresh, cumulate textured, olivine-free mafic-ultramafic xenoliths with large grainsize, one garnet peridotite, and one high pressure metasedimentary rock. The garnet peridotite, which equilibrated at ∼130 km beneath the batholith, yields a Miocene (10 Ma) Nd age, indicating that in this sample, the Nd isotopes were maintained in equilibrium up to the time of entrainment. All other samples equilibrated between ∼35 and 100 km beneath the batholith and yield Sm-Nd mineral ages between 80 and 120 Ma, broadly coincident with the previously established period of most voluminous batholithic magmatism in the Sierra Nevada. The Rb-Sr ages are generally consistent with the Sm-Nd ages, but are more scattered. The 87Sr/86Sr and 143Nd/144Nd intercepts of the igneous-textured xenoliths are similar to the ratios published for rocks outcroping in the central Sierra Nevada. We interpret the mafic/ultramafic xenoliths to be magmatically related to the upper- and mid-crustal granitoids as cumulates and/or restites. This more complete view of the vertical dimension in a batholith indicates that there is a large mass of mafic-ultramafic rocks at depth which complement the granitic batholiths, as predicted by mass balance calculations and experimental studies. The Sierran magmatism was a large scale process responsible for segregating a column of ∼30 km thick granitoids from at least ∼70 km of mainly olivine free mafic-ultramafic residues/cumulates. These rocks have resided under the batholith as granulite and eclogite facies rocks for at least 70 million years. The presence of this thick mafic-ultramafic keel also calls into question the existence of a "flat" (i.e., shallowly subducted) slab at Central California latitudes during Late Cretaceous-Early Cenozoic, in contrast to the southernmost Sierra Nevada and Mojave regions.
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.
We report a new 153 ± 2 Ma SIMS U‐Pb date for zircons from the hypabyssal Guadalupe pluton which crosscuts and contact metamorphoses upper crustal Mariposa slates in the southern Sierra. A ∼950 m ...thick section of dark metashales lies below sandstones from which clastic zircons were analyzed at 152 ± 2 Ma. Assuming a compacted depositional rate of ∼120 m/Myr, accumulation of Mariposa volcanogenic sediments, which overlie previously stranded Middle Jurassic and older ophiolite + chert‐argillite belts in the Sierran Foothills, began no later than ∼160 Ma. Correlative Oxfordian‐Kimmeridgian strata of the Galice Formation occupy a similar position in the Klamath Mountains. We speculate that the Late Jurassic was a time of transition from (1) a mid‐Paleozoic–Middle Jurassic interval of mainly but not exclusively strike‐slip and episodic docking of oceanic terranes; (2) to transpressive plate underflow, producing calcalkaline igneous arc rocks ± outboard blueschists at ∼170–150 Ma, whose erosion promoted accumulation of the Mariposa‐Galice overlap strata; (3) continued transpressive underflow attending ∼200 km left‐lateral displacement of the Klamath salient relative to the Sierran arc at ∼150–140 Ma and development of the apparent polar wander path cusps for North and South America; and (4) then nearly orthogonal mid and Late Cretaceous convergence commencing at ∼125–120 Ma, during reversal in tangential motion of the Pacific plate. After ∼120 Ma, nearly head‐on subduction involving minor dextral transpression gave rise to voluminous continent‐building juvenile and recycled magmas of the Sierran arc, providing the erosional debris to the Great Valley fore arc and Franciscan trench.
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