GEOCHEMISTRY, GEOCHRONOLOGY, AND THERMOBAROMETRY OF HIGH-PRESSURE AMPHIBOLITES FROM THE CENTRAL ARC AND MELANGE BELT NEAR COLFAX, CALIFORNIA

The western slopes of the Sierra Nevada consist of five north-south trending accretionary belts that range in age from Ordovician to Jurassic. One of these belts, the Central Arc and Mélange Belt, preserves fragmented ophiolitic rocks within a siliciclastic matrix. Locally, the ophiolitic rocks are amphibolitic with some having high-pressure and high-temperature mineralogies. One amphibolite exposure near the town of Colfax, which was described in a previous abstract, occurs as blocks within the unmetamorphosed sedimentary Colfax and Clipper Gap Units. These amphibolites contain hornblende retrograded to actinolite, plagioclase, epidote, and garnet. Ti phases consist of rutile and ilmenite overgrown by titanite. Here we present geochemistry, geochronology, and quantitative thermobarometry to characterize the timing and conditions of formation of these amphibolites.

Rock samples were collected from the Colfax amphibolite bodies and were sent to a commercial lab for whole-rock and trace-element geochemistry. For one sample, zircons were separated using standard heavy mineral separation techniques, then dated using LA-ICP-MS at the Arizona LaserChron Center. Mineral chemistry of the rocks was determined by electron microprobe and used to calculate pressure and temperature conditions via aluminum-in-hornblende barometry, hornblende-plagioclase thermometry, and zircon-in-rutile thermometry.

Trace-element geochemistry shows that Colfax amphibolites are N-MORB in composition based on V/Ti and Th/Nb versus Nb/Yb discriminant diagrams. However, we note that Nb is depleted relative to Th on a MORB-normalized spider diagram, which may indicate an SSZ contribution. This combination of MORB and SSZ signatures appears to be common in Central Arc and Mélange Belt amphibolites. Geochronology and thermobarometry results are in progress and will also be presented.