Abstract
The Central Belt of the northern Sierra Nevada in California consists of ophiolitic fragments representing Triassic to Jurassic arc rocks. Some of these rocks are high-pressure and high-temperature garnet and rutile mafic amphibolite that indicate the existence of a Triassic-Jurassic age subduction zone. 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 localized at the base of the ultramafic body has led to proposals that this is a metamorphic sole formed at the initiation of subduction. In order to understand the timing, origin, and pressure-temperature history of these rocks I use a combination of outcrop-scale field mapping, petrology, geochemistry, geochronology, and quantitative mineral thermobarometry to investigate the deformation history of the high-grade amphibolite blocks. Field mapping indicates that the amphibolite occurs as lenses at multiple levels within the structurally lowest part of the ultramafic body. Thin-section study indicates that the unit identified in the field as a garnet amphibolite is a strongly foliated garnet-epidote-rutile amphibolite. Whole-rock geochemical analysis indicates an enriched, MORB-like protolith with a supra-subduction zone signature. Garnet-plagioclase-quartz barometry and garnet-hornblende thermometry indicates metamorphism at a pressure and temperature of 9.0-9.8 kbar at 470℃. U-Pb zircon geochronology on one sample yields protolith formation ages of ca. 216 Ma.
The protolith formation age of 216 Ma, 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. This, along with the MORB-like protolith with a supra-subduction zone signature and the position of the high-grade amphibolite at the base of the ophiolite, is consistent with amphibolite formation during initiation of subduction.