Geology and petrotectonic significance of crystalline rocks of the southernmost Sierra Nevada, California

Abstract

Crystalline rocks of the southernmost Sierra Nevada north of the Garlock fault consist primarily of Early Cretaceous orthogneisses with subordinate paragneiss, and a mid-Cretaceous tonalite batholith complex with coeval gabbroic intrusives. Quartz-rich metasedimentary rocks and marble constitute the main framework into which the plutonic rocks were emplaced. The orthogneisses are predominantly tonalitic in composition with significant layers and domains of granodioritic to granitic, and dioritic to gabbroic gneiss. Field relations, isotopic Rb/Sr, O, and zircon U/Pb data demonstrate assimilation of metasedimentary material into the orthogneisses and magma mixing between mafic, tonalitic, and anatectic granitic magma derived from the metasedimentary material. The orthogneisses yield a family of internally, and externally concordant zircon ages, indicating igneous crystallization between 110 and 120 m.y.b.p. A bulk-rock Rb/Sr isochron determined on the orthogneiss zircon sample suite yields a similar age of 117 ± 5 m.y. The tonalite batholith complex lies generally to the northeast of the gneiss complex with some phases intruding or grading into the gneisses. The batholith complex along with small (5-km-scale) cumulate gabbro bodies intruded into the gneisses yields numerous, mainly concordant, zircon ages of 100 ± 2 m.y. A series of late to postdeformational intrusives yield zircon ages of 90-95 m.y ., thus putting a tight constraint on the cessation of ductile deformation under high-temperature conditions. The orthogneisses exhibit a banding which developed under igneous to hot subsolidus conditions by the transposition of magmatic segregations and/or igneous mineral assemblages. Blastomylonitic fabrics commonly overprint such banding and represent continued or superimposed deformation compared to tonalitic to mafic varieties, perhaps as the result of bulk material properties. Rocks of the batholith complex have a late-stage deformational fabric shown primarily in the tonalites as pervasive foliation and faint gneissic banding. Much of the tonalite batholith complex was deformed under igneous conditions, but late phases lack significant deformation features. Some gab bros and diorites show deep-level (6-8 kb) postcrystallization autometamorphic garnet growth. The major deformational fabrics exhibited in these suites of rocks may be related to intrusion of the tonalite batholith into the lower crust, and/or the result of intra-arc shearing that was preferentially concentrated in selected intrusive bodies. Alternatively, the deformational fabrics may represent mid- to lower-crustal flow and perhaps detachment related to the overall dynamics of batholithic magmatism. Crystalline rocks of the southernmost Sierra Nevada represent the deepest exposed levels of the composite Sierra Nevada batholith. Several petrologic relations suggest a depth of formation corresponding to about 8 kb pressure. At such crustal levels, igneous and metamorphic processes and products cannot be clearly separated. The southwestern margin of the 100-m.y .-old tonalitic batholithic complex may represent the tilted floor zone of the batholith which originally spread laterally over its gneissic substrate. A number of high-grade metamorphic features produced in this environment appear to have formed on a retrogressing P/T trajectory upon descent from solidus conditions. Rapid uplift of the crystalline rocks occurred prior to late Eocene time, perhaps in response to major thrust tectonics documented throughout southern California. Crystalline rocks of the southernmost Sierra Nevada provide an excellent opportunity to study deep-level Phanerozoic batholithic processes, which in this case operated by the addition of mantle-derived magma that interacted with and reconstituted previously tectonically assembled crustal fragments into new sialic crust. The central Sierra Nevada exposes volcanic and epizonal levels of the mid-Cretaceous batholithic belt, and progressively deeper rocks are exposed southward. Thus, the southern half of the range offers a down-plunge view of the composite batholith, with the southernmost rocks sampling perhaps as deep as 20-25 km.

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