PETROLOGY, ISOTOPE GEOCHEMISTRY, AND GEOCHRONOLOGY OF A MULTIPLY METAMORPHOSED GRANITOID EXOTIC BLOCK IN A FRANCISCAN OLISTOSTROME MELANGE, CAZADERO, CALIFORNIA

The block protolith was an M-type biotite-hornblende quartz diorite pluton. Igneous crystallization and cooling is dated by U/Pb on zircon at 165+-1 Ma, by Ar40/Ar39 on relict magnesiohornblende at 162+-2 Ma, and by U/Pb on apatite at 160+-1 Ma. Amphibole Pb206/204=18.66 and Pb207/Pb204=15.62, which plots with arcs. Apatite Sr87/Sr86=0.703537+-20; apatite Sm147/Nd144=0.130 and Nd143/Nd144=0.512904+-7; epsilon Nd is +6.6, and epsilon Sr is –13.8. On an epsilon diagram the pluton plots with primitive oceanic arcs like the Marianas.

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.