HIGH-P AMPHIBOLITE BLOCKS FROM MELANGE, NACIMIENTO BELT, COASTAL CALIFORNIA: A FIRST REPORT

We have examined two high-grade metamorphic blocks in shale matrix mélange in the Nacimiento belt near York Mountain, San Luis Obispo County, California. One of these is an amphibolite block about 70 meters long. Most of the block is metabasite with hornblende (Al2O3 = 14.91-15.37%, TiO2 = 0.72-0.89), epidote, and rutile rimmed by titanite. These rocks are locally overprinted by an assemblage including pumpellyite and actinolite that appears in veins, and appears to replace what may have been plagioclase in some samples. The block also contains a subordinate amount of metachert, which is quartz rich and contains garnet and phengite, in addition to hornblende. A 5-meter-long block we examined appears to be a fine-grained blueschist at first glance. The mineral assemblage includes glaucophane, pumpellyite, lawsonite, phengite, chlorite, and titanite with rare cores of hornblende (up to 10.05% Al2O3) and cores of rutile within titanite. The presence of rutile in metabasite assemblages of both blocks suggests metamorphic pressures in excess of about 1.2 GPa for amphibolite metamorphism. Amphibole compositions suggest metamorphic temperatures in the vicinity of 700°C for the amphibolite block whereas the calcic amphibole in the blueschist is too poorly preserved to use its composition for semi-quantitative thermobarometry. Both blocks record metamorphism similar to the high-grade blocks that are common in the Franciscan Complex east of Salinia, however this is the first report of such rocks in the Nacimiento belt, a testimony to the rarity of such blocks. Although the Nacimiento belt resembles the Franciscan in many ways, the former lacks epidote-blueschist facies coherent units (such as the South Fork Mountain schist) and high-grade blocks (with apparently rare exceptions). The contrast may reflect a difference between accretion-metamorphic processes and/or exhumation/preservation processes.