Paper No. 29-4
Presentation Time: 8:30 AM-5:00 PM
U-PB GEOCHRONOLOGY AND HF ISOTOPIC SYSTEMATICS OF METAMORPHIC AND DETRITAL ZIRCON DOMAINS IN THE SCHIST OF SIERRA DE SALINAS, CENTRAL CALIFORNIA COAST RANGES
The Late Cretaceous schist of Sierra de Salinas (SdS) of the central California Coast Ranges represents trench assemblages derived from the Sierra Nevada batholith. The SdS is the only exposure of the family of California schists that include the Pelona, Orocopia, and Rand in which significant domains of recrystallized zircon are recognized. This study employs U-Pb geochronology and Hf isotopic study on zircon domains to clarify the timing and conditions of zircon recrystallization. Rim-core U-Pb dating was done on two SdS samples, which revealed: 1) four grains with Cretaceous rims (87-93 Ma) and Proterozoic cores (~1.3 to 1.7 Ga) and 2) nineteen grains with Cretaceous rims (79-100 Ma) and cores (84-140 Ma). Three zircon populations were targeted for Hf isotopic analysis: 1) 1.0-1.7 Ga grains, which yield generally positive εHf (t) values of +9.4 to +0.3, suggesting crustal additions derived from relatively juvenile sources, most likely ultimately derived from the Mojave Desert or prebatholithic wallrocks; 2) 80-155 Ma grains, which show a trend of decreasing εHf (t) with time from ~+12.5 to -8.3, most likely due to an increasing proportion of crustal melting in the Sierra Nevada; and 3) <80 Ma grains, yielding a wide range of εHf (t) between +12.4 to -33.3. The ~77-81 Ma zircons are critical in this analysis because they are either: 1) detrital and reflect derivation from a Late Cretaceous igneous terrane with Precambrian rocks; or 2) metamorphic and thus related to formation of the schist. Further studies are required to resolve the issue and these could focus on mapping CL and BSE textures before U/Pb analysis.