FACIES AND GEOCHEMICAL ANALYSIS OF THE TRIASSIC VOLCANIC ROCKS IN TWO PENDANTS, EASTERN CALIFORNIA WITH IMPLICATIONS FOR ARC PALEOGEOGRAPHY, VOLCANOLOGY, AND SUBDUCTION INITIATION

The continental passive margin along the west coast of Laurentia transitioned from transpressional faulting to a subduction zone and continental magmatic arc in Pennsylvanian to Permian time, but the exact timing of initial arc magmatism, eruptive styles and sequences, and depositional setting remain poorly understood. Several pendants in the Sierra Nevada in California preserve the transition from passive margin to an Early Triassic magmatic arc. Triassic lenses of limestone and the presence of pelecypod fossils in the Ritter Range pendant suggest a subaqueous depositional setting, but evidence in volcanic rocks has not been gathered. Mapping and facies analysis, whole-rock and zircon trace element geochemical data, and three new zircon U-Pb ages from pyroclastic rocks in the Ritter Range (RRp) and Mount Morrison (MMp) pendants provide insight into the timing and style of the initial stages of Triassic arc development and paleography at ~37°N.

Units in the RRp succession were deposited by pyroclastic density currents and dome eruptions. An unusual unit containing elongate pumice (5-75 cm long) supported by fine-grained pyroclastic material likely represents a deposit of pumice spalled from a subaqueous dome carapace that floated through the water column and sank into unconsolidated pyroclastic material. These observations combined with previous work showing caldera-margin facies in the MMp indicates a large silicic volcanic complex throughout the MMp and RRp.

Previous zircon studies of pyroclastic rocks in the MMp, RRp, and Saddlebag Lake pendant yielded crystallization ages of 217 - 221 +/- 2 Ma, which marks some of the earliest arc magmatism in this area. New zircon U-Pb samples from two pyroclastic units in the RRp and one in the MMp yielded ages within error of each other centered at ~220 +/- 2 Ma, suggesting these units are coeval and part of an evolving magmatic system. Preliminary whole-rock and zircon geochemical data suggest units in these pendants are cogenetic, and geochemical signatures indicate a thick continental crust. The earliest volcanic activity was likely subaqueous and erupted from large-volume silicic systems now preserved in several pendants in the eastern Sierra Nevada.