MAGMATIC COMPOSITIONS AND ALTERATION OF RHYOLITIC ASH-FLOW TUFFS IN THE EASTERN SIERRAN ARC OF CALIFORNIA

The basement rocks of the Sierra Nevada, California, are thought to have formed by the collision of two magmatic arcs, a western oceanic arc and an eastern 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 petrology of Triassic volcanic rocks in SLP, and to better understand the hydrothermal alteration that occurred during or after deposition. The basal SLP volcanic section includes a structurally repeated sequence of three ash-flow tuffs that range in age from 232 to 219 Ma, with interlayered deformed conglomerates and mafic volcanic and volcaniclastic rocks. Compared to the (para)autochthonous section, the extent of deformation and alteration appears to increase toward the south and west in structurally higher thrust sheets, nearer to adjacent Late Cretaceous plutons. The tuffs are classified as low-silica rhyolites, with SiO2 contents between 70 and 75%. Fractionated REE patterns with La/Yb ~10 and Eu/Eu* ~0.8 distinguish SLP tuffs from typical group 1, high-silica rhyolites. The La/Yb ratio increases and the Eu anomaly deepens as SiO2 increases, signifying middle and heavy REE abundances decreased with fractionation and indicating that plagioclase and hornblende were fractionated from the melt that erupted to form the tuffs. Alkali abundances decrease as SiO2 increases, suggesting that the tuffs were hydrothermally altered by Na loss and carbonate addition during or soon after deposition. Nd isotopic ratios show little variation (eNd = -1.9±0.3) and 87Sr/86Sr ratios are weakly negatively correlated with the extent of this alteration, suggesting that hydrothermal alteration of the SLP tuffs was not primarily the result of interaction with marine fluids.