Insights from the Alabama Hills into Mesozoic Magmatism and Tectonics in Eastern California

Abstract

New zircon U-Pb ages for the Alabama Hills Granite in Owens Valley, eastern California, range from 103 to 102 Ma, nearly 20 Ma older than previously published zircon ages. The data preclude previously implied links between the pluton and the adjacent Late Cretaceous Mount Whitney Intrusive Suite. Geochronologic and isotopic data indicate a connection between the Alabama Hills Granite and leucogranites to the northwest on the Sierra Nevada crest, as well as a pluton to the southeast in the Coso Range. We refer to these units as the Kearsarge plutons. The suite was intruded from 103 to 100.5 Ma with S87r/S86ri=0.7045 to 0.7060 and εNdi=−2.4 to -4.5 and has distinctive enrichments in high field strength, middle, and heavy rare earth elements, as well as negative Eu anomalies and high Y/Sr. We suggest the Alabama Hills block could not have moved more than 10 km dextrally relative to the Sierra Nevada batholith since the Middle Jurassic and is thus a suitable piercing point for offsets across Owens Valley. The Kearsarge plutons, like other markers on either side of Owens Valley, support approximately 65–75 km of dextral offset across the valley. The suite’s location east of other middle Cretaceous suites, coupled with its mantle-like isotope geochemistry, suggests it could represent backarc magmatism, perhaps controlled by preexisting shear zones or fractures.