MARINE VOLCANICLASTIC RECORD OF SUBDUCTION INCEPTION AND EARLY SIERRAN ARC EVOLUTION, RITTER RANGE PENDANT, CALIFORNIA

Marine volcaniclastic rocks in pendants of the eastern Sierra Nevada preserve a critical record of subduction inception and early evolution of the Sierra Nevada magmatic arc. While these sections have been steeply tilted, shortened and are variably hydrothermally altered, comparatively low-strain zones preserve textural information and high field strength element records of magmatic evolution. We present whole rock and zircon trace element geochemical data and sixteen new zircon SIMS Pb/U ages from the Ritter Range pendant, providing insight into arc inception and evolution at ~37.7°N as well as suggesting stratigraphic correlations between volcanic rock sequences in pendants in the eastern Sierra Nevada. These new results suggest that: (1) the first-order magma production rate as recorded by marine early Mesozoic volcanism was episodic, with distinct pulses of ignimbrite deposition at ca. 220 to 207 Ma and 174 to 167 Ma; (2) similarities in age and trace element geochemistry suggest correlation of the Triassic sections in the eastern Ritter Range pendant with the western Saddlebag Lake pendant to the north (~Koip sequence of Kistler) and the western Mount Morrison pendant to the south; (3) ignimbrites range from dacite to rhyolite in bulk composition, and are petrographically similar to modern “monotonous intermediate” dacite or phenocryst-poor low-silica rhyolite; (4) ignimbrites can be correlated to granodioritic to granitic components of the Late Triassic Scheelite Intrusive Suite and Middle Jurassic Palisade Crest Intrusive Suite, suggesting testable hypotheses for linking this volcanic record with pluton construction in the underlying Triassic – Jurassic batholith. Ignimbrites and felsic granodiorites in the early Mesozoic east Sierran arc thus may provide complementary records of long-lived and episodic marine upper-crustal arc magma systems.