ASSESSING CRUSTAL INFLUENCE ON SILICIC CORDILLERAN ARC VOLCANISM IN THE MESOZOIC: WHOLE ROCK AND ZIRCON TRACE ELEMENT INSIGHTS

The southwestern Cordilleran arc of California includes spatially vast volcanic and plutonic expressions of voluminous silicic magmatism throughout the Mesozoic era. Geochemical analyses of the arc’s volcanic members provide insight into crustal processes affecting magmas during ascent. Structural, tectonic, and stratigraphic studies of volcanic regions complement geochemical characterizations of the rocks and help develop theories about temporal change in the overlying lithosphere of the subduction zone. Zircons crystallized in erupted volcanic rocks remain an archive of original melt compositions even in areas that have undergone significant metamorphism. Previous zircon trace element studies of the Sierran and Mojave volcanic centers of the Mesozoic have established distinct patterns in zircons including: 1) stronger cerium enrichment during the Jurassic compared to the Triassic, 2) increased partitioning of thorium as the Jurassic progressed to the Early Cretaceous, and 3) decreasing scandium/ytterbium ratios with increasing uranium/ytterbium ratios through the continental arc’s Mesozoic history. This study investigates silicic volcanics along the strike of the magmatic arc and hypothesizes chemical indications of a greater role for mantle melting and decreased crustal input as the Mesozoic progressed. Similarly, trace element geochemistry of zircons in late-Jurassic volcanics records crustal thinning as compared to earlier volcanic units, which likely encountered a coherent crustal “lid” during their ascent, and thus would have been subjected to greater crustal contributions during zircon fractionation.