THE VIEW FROM THE FOREARC: A SURVEY OF SIERRA NEVADA MAGMATISM USING GREAT VALLEY DETRITAL ZIRCON TRACE ELEMENT GEOCHEMISTRY (Invited Presentation)

The well-characterized Sierra Nevada magmatic arc offers an unparalleled opportunity to understand continental arc magmatism. However, present bedrock exposure provides an incomplete record that is dominated by Cretaceous plutons, making it challenging to decipher details of older magmatism and the dynamic interplay of plutonism and volcanism. Moreover, the forearc detrital record includes abundant zircon formed during apparent magmatic lulls, suggesting that understanding the long-term history of arc magmatism requires integrating plutonic, volcanic, and detrital records. We present trace element geochemistry of detrital zircon from the Great Valley forearc basin to survey Sierra Nevadan arc magmatism through Mesozoic time. We analyzed 250 previously-dated detrital zircon grains from 7 sandstone samples deposited ca. 145-80 Ma along the length of the forearc basin. Samples include volcanogenic, arkosic, and mixed compositions, with volcanic lithic grains comprising 39-100% of total lithic grains. Detrital zircon trace element geochemistry is largely consistent with arc derivation, and shows similar geochemical ranges between samples, regardless of location along strike of the forearc basin, depositional age, or sandstone composition. Comparison of our detrital zircon data with zircon trace element data from the eastern Sierra Nevada demonstrates geochemical asymmetry across the arc that is persistent through time. Comparison of forearc and retroarc detrital zircon trace element data shows that detritus from the western Early Cretaceous arc did not reach the retroarc region during middle Cretaceous time, consistent with the presence of a topographic divide within the Cretaceous arc. We also identified a minor group of Jurassic detrital zircon grains with oceanic geochemistry that may have provenance in the Coast Range ophiolite (CRO) and/or ophiolitic rocks of the western Sierra Nevada metamorphic belt. The Jurassic continental arc fully overlaps in time with the CRO, requiring that any model for CRO formation must also account for an active continental arc to the east. Our results demonstrate that detrital zircon geochemical data can enhance understanding of a well-characterized arc, and may prove an effective means to survey an arc that is inaccessible and therefore poorly characterized.