HIGH-PRECISION GEOCHRONOLOGICAL TEST OF BIG TANK: MAGMATIC EVOLUTION OF UPWARDLY ZONED WOOLEY CREEK BATHOLITH WITH IMPLICATIONS FOR ZIRCON RESIDENCE TIMES

Post-emplacement, regional tilt has exposed a nearly complete cross-section of the ca. 160 Ma, 600 km2, calcic, upwardly zoned Wooley Creek batholith and related Slinkard pluton, Klamath Mtns., California. Diorite and gabbro in the base grade into quartz diorite, tonalite and granodiorite toward the top. Andesitic to dacitic dikes in the roof are interpreted as feeders to the associated volcanic system. Quantitative thermobarometry from aureole rocks document 5.5 kbar difference from top to bottom and emplacement depths of ~30 km at the base to 15 km at the top. A broad enclave-rich zone of magma mingling within the central quartz diorite zone may represent the level of magma recharge. High-precision, U-Pb, single crystal, chemical abrasion zircon dates (CATIMS) will be combined with detailed mapping, petrology, geochemistry, O and Nd isotopic characterization and fabric analysis to determine the magmatic evolution of the batholith. Age trends will differentiate between 1) Big Tank: cooling from a single intrusion with ages younging toward the center, 2) multiple, discrete, rapidly crystallizing pulses, and 3) a series of recharge and rejuvenation events. Preliminary SHRIMP data have established a minimum of 3 myr for duration of zircon crystallization in the system. Anticipated CATIMS precisions are ± 0.06 to 0.02 myr. In addition to determining the magmatic evolution, the results will quantify the pulses and durations of extrusive eruptions, and impact zircon residence time considerations in volcanic systems.