PETROGENESIS OF A MIDDLE JURASSIC “HIGH SIERRA”-TYPE INTRUSION: THE SACATAR INTRUSIVE COMPLEX, KERN PLATEAU, SOUTHEASTERN SIERRA NEVADA, CA

Unlike the Klamath Mountains of Cal Barnes’ fame, the record of Jurassic Sierra Nevada arc magmatism is largely obscured by Late Cretaceous plutons. However, a relatively complete record of Jurassic arc magmatism is exposed on the Kern Plateau (~36°N latitude) where zircon U-Pb data indicate that Jurassic magmas intruded as two suites: a large, intermediate to felsic intrusion mapped as the Middle Jurassic Sacatar granodiorite (ca. 185 - 162 Ma; N=13); and small pods and ring dikes of latest Jurassic Summit Igneous Complex (ca. 151 - 146 Ma; N=15), similar in age and orientation to the Independence dike swarm. The Tübatulabal hypabyssal-volcanic unit (N=6) spans emplacement of both suites: Tübatulabal A corresponds in age and εHf_i with the Sacatar, Tübatulabal B with the Summit, and together indicate upper crustal emplacement of Jurassic magmas in the Kern Plateau. The Sacatar unit varies from quartz diorite to granite; the Summit is a bimodal gabbro-granite suite. Whole-rock compositions within the high-K calc-alkaline Sacatar vary from 51 to 73 wt. % SiO₂, overlapping compositional trends of the latest Jurassic Independence dike swarm and the neighboring Late Cretaceous Mt. Whitney intrusive complex. We propose including coeval, adjacent NW-elongate plutons, bracketed by the Middle Jurassic Redrock Meadows and Grasshopper Flats granitoids, in a compositionally zoned, ~1500 km² Sacatar Intrusive Complex, comparable to Cretaceous complexes such as the Fine Gold (>3100 km² intruded over ca. 19 m.y.) and Mt. Whitney (~1200 km² over ca. 5 m.y.).

Enriched mantle derivation of eastern Sierra magmas was initially proposed to explain whole-rock Sr_i-εNd_i values in Late Cretaceous High Sierra intrusive complexes. Although initial Sr-Nd ratios are generally enriched in the Summit-Sacatar systems, zircon εHf_i values require sourcing in a depleted mantle reservoir beneath the southeastern Sierra. Coupled Sr_i-εNd_i-εHf_i indicate that differentiation within the deep Laurentian crust produced enriched mafic magmas, and additional mid-crustal differentiation produced voluminous intermediate-felsic magmas. Upper crustal Triassic-Early Jurassic extension migrated deeper in the Middle Jurassic to evacuate Sacatar magmas from a mid-crustal mush column. Latest Jurassic rifting along the Kern Plateau shear zone liberated bimodal Summit magmas from the lower crustal MASH zone.