THE SUBSTANTIAL CONNECTION BETWEEN RHYOLITES AND GRANODIORITE PLUTONS OF TRIASSIC AGE IN THE SIERRA NEVADA, CA

The basement rocks of Sierra Nevada, California, are thought to have formed by the collision of two arcs, one oceanic and the other a continental arc built off the North American continental shelf. The Saddlebag Lake pendant (SLP), in the eastern Sierra Nevada is one of the larger remnants of the eastern continental arc. The purposes of this study are to determine the tectonic setting of Triassic explosive volcanism in SLP, and to use rare earth element (REE) and other immobile element abundances to relate these volcanic rocks to nearby plutons. In SLP, the basal volcanic section includes well-preserved tuffs that range in age from 219 to 232 Ma with interlayered deformed conglomerates and mafic breccias. The tuffs are classified as group 1 rhyolitic ignimbrite sheets, with SiO₂ contents between 70 and 75%. Alkali abundances decrease as SiO₂ increases, suggesting that the tuffs were hydrothermally altered during or after deposition. REE and other immobile element abundances distinguish SLP tuffs from typical group 1 ignimbrites. The REE do not have the distinctive seagull pattern found in group 1 ignimbrites. The La/Yb ratio increases and the Eu anomaly deepens as SiO₂ increases, signifying that the slope of the REE pattern increased with fractionation. This indicates that plagioclase and hornblende were being fractionated from the melt that erupted to form the tuffs. This mineral assemblage is observed in Triassic plutons east of SLP, indicating a process between SLP rhyolite tuffs and early Sierra Nevada granodiorite – granite plutons.