EARLY ARC VOLCANIC ACTIVITY RECORDED BY PYROCLASTIC FLOWS, TRIASSIC OF THE CENTRAL SIERRA NEVADA OF CALIFORNIA

Geodynamic models for subduction initiation and early marine arc evolution are testable using the stratigraphy and geochemistry of tephra records. Pyroclastic rocks in the Saddlebag Lake, Ritter Range and Mount Morrison pendants in the east-central Sierra Nevada preserve a Late Triassic record of marine arc initiation and silicic magmatic arc evolution. The silicic pyroclastic materials in these pendants are dacitic to rhyolitic ignimbrite sheets, with SiO₂ contents between 70 and 75%. Bulk rock alkali contents are high and variable, reflecting primarily the effects of hydrothermal alteration during deposition and lithification. Whole rock REE abundances of La, Eu, and Yb are similar to modern continental arcs, however, the patterns of REEs are different from modern oceanic arcs. In the modern Izu-Bonin oceanic arc, Tamura et al (2009) stated that Rhyolite 1 (R1) is depleted in LREE, R2 is flat to enriched in LREE, R3 is enriched in LREEs, and LREE enrichment is associated with deeper negative Eu anomalies. In contrast, the east-central Sierra Nevada dacites and rhyolites (DR) are LREE-enriched with small negative Eu anomalies: DR1 is depleted in mid-HREE and DR2 is less depleted in mid-HREE. Zircons in DR1 and DR2 ignimbrites are HREE-enriched, consistent with crystallization from LREE-enriched silicic melts broadly similar to those bulk rock compositions. Our results are consistent with an origin for silicic magmas in the early Sierra marine arc by fractional crystallization of granodiorite or by partial melting of mafic lower crust, but whole rock isotope ratios are consistent with a partial melting origin, as inferred by Tamura et al. for the modern Izu-Bonin arc.