Abstract
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+ or -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 degrees C. U-Pb zircon geochronology on one sample yields ages of nearly equal 216 Ma which along with the existing K-Ar hornblende age of 200+ or -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.