U/Pb Zircon Geochronology, Petrology, and Structural Geology of the Crystalline Rocks of the Southernmost Sierra Nevada and Tehachapi Mountains, Kern County, California

Citation

Sams, David Bruce (1986) U/Pb Zircon Geochronology, Petrology, and Structural Geology of the Crystalline Rocks of the Southernmost Sierra Nevada and Tehachapi Mountains, Kern County, California. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/C883-H765. https://resolver.caltech.edu/CaltechETD:etd-07212008-101544

Abstract

Field mapping, petrography, U/Pb zircon geochronology, and Rb/Sr geo-chemistry on the crystalline rocks of the southernmost Sierra Nevada and Tehachapi Mountains north of the Garlock fault have 1) generated a structural, geo-chemical, and geochronological framework; 2) demonstrated a continuation of Sierran plutonic and metasedimentary rocks into the Tehachapi Mountains; 3) indicated that the region, in particular the gneiss complex of the Tehachapi Mountains, represents the deepest exposed levels of the Sierra Nevada batholith; 4) placed constraints on possible mixing models between upper mantle and meta-sedimentary components to generate the observed geochemical signatures of the rocks; and 5) resolved a major mid-Cretaceous deformation event.

The main crystalline rocks of the study area are the rocks of the Bear Valley Springs intrusive suite and the gneiss complex of the Tehachapi Mountains. The Bear Valley Springs suite is a mid-Cretaceous tonalite batholith complex with coeval gabbroic intrusives. The gneiss complex of the Tehachapi Mountains consists dominantly of early-Cretaceous orthogneiss, with subordinate paragneiss and local domains having granulite affinities. The orthogneisses are dominantly tonalitic in composition, with significant layers and domains of granodioritic to granitic and lesser dioritic to gabbroic gneiss. Quartz-rich metasedimentary rocks and marble constitute the main framework assemblage into which the plutonic rocks were emplaced. Field relations demonstrate assimilation of metasedimentary material into the orthogneisses and magma mixing between mafic, tonalitic, and anatectic granitic material derived from the metasediments.

Crystalline rocks of the region, with the exception of metasedimentary framework rocks, fall into a narrow age range of 90-120 Ma, and exhibit three main age suites. Most samples have zircon populations with systematics indicative of igneous crystallization, with signs of zircon inheritance or entrainment in the vicinity of metamorphic septa. Strongly discordant samples are relatively rare, and include the granodiorite of Claraville (concordia intercepts of 90/1900 Ma), the paragneiss of Comanche Point (108/1450), and a quartzite in the Kings sequence metasedimentary framework rocks (1700 Ma upper intercept).

The rocks in the first age suite (gneiss complex of the Tehachapi Mountains and augen gneiss of Tweedy Creek) exhibit a greater degree of deformation, especially under moderate to high grade conditions. Major deformational fabrics are expressed as gneissic banding, mylonitization, recrystallization, boudinaging, and transposition of internal contacts. Internally and externally concordant zircon systematics of the orthogneisses in this suite indicate igneous crystallization between 110-120 Ma. Discordant zircon systematics suggest entrainment of minor amounts of mid-Proterozoic zircon and/or open system lead loss in response to the 100 Ma magmatic culmination (Bear Valley Springs event).

The second suite, 100±2 Ma Bear Valley Springs intrusive suite (tonalite of Mount Adelaide, tonalite of Bear Valley Springs, hypersthene tonalite of Bison Peak, and metagabbro of Tunis Creek) contains igneous rocks which locally cross-cut the older suite. These rocks have a late-stage deformational fabric shown primarily in the tonalites as pervasive foliation and faint gneissic banding. The zircon systematics of this suite are internally and externally concordant, indicating igneous crystallization ages, with only local evidence of entrainment of mid-Proterozoic zircon. The deformation of the suite was synplutonic, with later phases within the suite lacking significant deformational fabrics. The major deformational fabrics exhibited in the Tehachapi and Bear Valley Springs suites may be the result of the intrusion of the tonalite batholith into the lower crust, and/or the result of intra-arc shearing that was preferentially concentrated in various intrusive bodies.

The third suite, late deformational intrusive rocks, consists of units which cross-cut deformational features in both the older suites. These youngest rocks are themselves slightly to nondeformed. The members in the suite have ages of 90 Ma (granodiorite of Claraville), 93 Ma (tonalite stock at Tweedy Creek), and 94 Ma (pegmatite dike at Comanche Point).

Field mapping and petrography have shown a southward continuation of Sierran plutonic and metasedimentary framework rocks to the region of Tejon Creek. The plutons show a constant age spread and overall composition throughout the region, with a greater degree of solidus to hot sub-solidus deformation exhibited southward. The metamorphic septa have a higher grade, and are more strongly deformed southwards, becoming migmatitic. The southern margin of the tonalite of Bear Valley Springs consists of a gradational contact with the hypersthene tonalite of Bison Peak, which is believed to represent the floor or conduit phase of the batholith. Along its southwestern margin, the tonalite of Bear Valley Springs grades into the gneiss complex of the Tehachapi Mountains through a region of tonalitic gneiss that appears to be derived through the mixing of tonalitic magmas and migmatitic melts produced from paragneiss components in the gneiss complex. Paleomagnetic and structural restoration of the southwestern margin of the tonalite indicates that it may represent the uptilted floor of the batholith that originally spread out over its gneissic substrate.

The crystalline rocks of the southernmost Sierra Nevada represent the deepest exposed levels of the Sierra Nevada batholith. Saleeby and others (1986a) indicate a continual increase in depth of exposure from the central to southern part of the batholith. Elan (1985) shows metamorphic conditions of 3.0 kb and 700°C in the south-central Sierras, while Sharry (1981b) has suggested that parts of the gneiss complex have a deep-seated (8 kb) origin with rapid late-Cretaceous uplift. Granulitic nodules of similar character to parts of the gneiss complex have been described by Domenick and others (1983) as originating from a similar depth beneath the central Sierra. Gneissic granitoids have numerous lenses of mafic to ultramafic cumulates showing igneous crystallization under granulite facies conditions. The domains of "granulite" in the gneiss complex of the Tehachapi Mountains are believed to be hot, relatively dry zones in a crystallizing and deforming batholithic complex. Magmatic epidote-bearing tonalites and late stage sub-solidus autometamorphic garnet growth are further indicators of a deep (≥6 kb) level of origin for the region.

The "granulites" (metagabbro of Tunis Creek and hypersthene tonalite of Bison Peak) are interpreted to be of an igneous origin. Evidence for this interpretation consists of: relict olivine grains and cumulate textures; foliation believed to be the result of igneous flow; zoned plagioclase necessitating the presence of a magma; tonalites that contain epidote that is interpreted to be of magmatic origin; δ¹⁸O and Rb/Sr isotopic values in the igneous range; abundance of retro-grade but paucity of prograde mineral reactions; gradational contacts between plutonic units; and observed intrusive contacts. Pyroxene within the "granulites" is believed to be of a pyrogenic origin. The rocks typically have a retrograde assemblage that consists of olivine → orthopyroxene and pyroxene → amphibole. The mineral assemblages all point to a downward P-T path.

Simple two-component mixing models have been constructed for samples from the southernmost Sierra Nevada, and involve incorporation of partial to complete melts of metasedimentary material into "primitive" upper mantle orogenic mafic magmas prior to crystallization. The two possible end-members are the quartzite-paragneiss of Comanche Point and the hypersthene tonalite of Bison Peak-metagabbro of Tunis Creek. Initial ⁸⁷Sr/⁸⁶Sr correlates directly with δ¹⁸O, and generally correlates inversely with Sr content for most of the samples. Simple isotopic mixing models indicate incorporation of up to 33% metasedimentary material in the granitic rocks, and up to 15% in the tonalites, with younger and more easterly samples requiring a larger metasedimentary component. The non-correlation of Sr_o with Sr content for some of the Pastoria Creek samples indicates an oceanic-affinity source with little interaction with continental crustal material. A number of samples appear to require a third, probable lower continental crustal and/or oceanic crustal-upper mantle component that may have a Paleozoic age.

Based on Rb/Sr and K/Ar age systematics, the region was uplifted in a regional cooling event at ~85 Ma perhaps as part of regional thrusting event(s) in southern California. The crystalline rocks were subsequently exposed and unconformably overlapped by Eocene marine sediments. Paleomagnetic data suggest about 45-60° of clockwise rotation between 80 and 16 Ma for the southern end of the Sierras, possibly as the result of the thrusting event responsible for the regional uplift.

Saleeby and others (1986c) have suggested that the lower crust beneath the Sierra Nevada batholith is comprised in part by granulitic and mafic intrusive rocks. Experimental studies by Christensen and Fountain (1975) also suggest the presence of granulites in the lower continental crust. The interpretation that the study area represents the deepest exposed level of the southernmost Sierra Nevada batholith leads to the implication that granulitic-affinity rocks comprise the lower part of the continental crust. Therefore, this study provides some degree of confirmation to the aforementioned hypotheses.

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