PETROLOGY, ISOTOPE GEOCHEMISTRY, AND GEOCHRONOLOGY OF A MULTIPLY METAMORPHOSED GRANITOID EXOTIC BLOCK IN A FRANCISCAN OLISTOSTROME MELANGE, CAZADERO, CALIFORNIA
The pluton was subducted at <160Ma and partially metamorphosed to a ferrorichterite-albite-clinochlore fels at unknown T and 4+-1 kb. It cooled (<200°C) and was fragmented by many thin breccia veins, which then recrystallized. Metamorphism occurred a second time, forming abundant pumpelleyite and chlorite at ~ 250+-50°C and 5+-2 kb.
A ~100m block of this metamorphite was then transported to the surface and incorporated into the Kings Ridge Road olistostrome mélange (Erickson, 1995) by ~147+-3 Ma (Tithonian). The melange was next weakly subducted, and a third metamorphism at ~250+-50°C and 2+-1 kb produced abundant laumontite veins in its sandstone matrix. Metamorphism ended by 135+-3 Ma (Valanginian); local faulting ended by 103 Ma.
Block apatite yielded a poor quality fission track age of 36+-11 Ma. Block sandstone matrix yielded high-quality apatite fission track ages of 36+-2 and 38+-2 Ma with slow-cooling track length distributions. These data suggest that the mélange and block were exhumed to the surface over a protracted period, cooling below ~100oC in early Tertiary time.
Block location is UTM 10 S 0488568E 4262693N NAD 27 datum.