GEOCHRONOLOGY AND THERMOBAROMETRY OF THE GARNET AMPHIBOLITE AT THE BASE OF THE TUOLUMNE OPHIOLITE, SIERRA NEVADA CENTRAL BELT, CALIFORNIA

The Central Belt of the Sierra Nevada consists of ophiolitic fragments, some of which are high-pressure and high-temperature amphibolite, along with Triassic to Jurassic arc rocks. One of the more notable amphibolite exposures occurs in the Tuolumne Ophiolite, west of the town of Chinese Camp, where previously-published work has described chert and coarse-grained garnet-rutile-epidote mafic amphibolite lenses with a K-Ar age of 200±10 Ma and Ar-Ar ages of 200-178 Ma on hornblende. The position of the high-grade rocks concentrated at the base of the ultramafic sequence has led to proposals that this is a metamorphic sole. In order to understand the timing, origin, and P-T history of these rocks we implement a combination of outcrop-scale field mapping, geochronology, and quantitative mineral thermobarometry.

Rock samples were collected from the garnet amphibolite and sent for U-Pb zircon geochronology with the LA-ICP-MS at the Arizona LaserChron Center. Thin sections samples were analyzed for mineral chemistry with an electron microprobe and the resulting data used to determine pressure and temperature conditions through garnet-plagioclase-hornblende-quartz barometry and garnet-hornblende thermometry.

Field mapping indicates that the amphibolite occurs as lenses at multiple levels within the structurally lowest part of the ultramafic body. Preliminary thermobarometry indicates metamorphism at a pressure and temperature of 9.0-9.8 kbar at 470^oC. U-Pb zircon geochronology on one sample yields ages of ~216 Ma which along with the existing K-Ar hornblende age of 200±10 Ma indicates a short time span between formation of the mafic protolith and high-pressure and high-temperature metamorphism, consistent with formation during initiation of subduction near a mid-ocean ridge.