HISTORY OF A MULTIPLY METAMORPHOSED QUARTZ DIORITE EXOTIC BLOCK AND ITS MELANGE MATRIX IN THE FRANCISCAN COMPLEX AT CAZADERO, CA, DESCRIBING THREE CYCLES OF BLOCK HEATING AND COOLING WITH BRECCIATION, EXHUMATION, AND REBURIAL, AND DESCRIBING THE MELANGE MATRIX

The block protolith was an M-type biotite-hornblende quartz diorite in an oceanic island arc (Saha et al, 2005) dated by zircon U/Pb at 165+1 Ma. (Erickson et al 2004; http://library.sonoma.edu/regional/subject/subgeo.html). The block location is UTM 10 S 0488568E 4262693N 1927 CONUS datum.

The pluton with its arc was subducted and fragmented at <160 Ma and at least one segment incompletely metamorphosed (M1) to a blue ferrorichterite-albite-brown chlorite fels at uncertain T and P. This pluton segment was then moved to a much cooler and lower-pressure environment, probably <3kb and <250oC, and there hydrofractured by fluids escaping the system, forming 1-10 mm thick microbreccia veins that next recrystallized completely.

The segment then was moved (resubducted?) to a new environment at ~ 250+50oC and 5+2 kb where a second incomplete metamorphism (M2) occurred, forming common pumpelleyite+yellow chlorite veins and patches.

A ~100m block of this granitoid metamorphite was then moved to the surface and incorporated as one of >249 >3m exotic blocks into the developing King Ridge Road (KRR) olistostrome mélange (Erickson 1995) by ~147 Ma (Tithonian). Blocks were emplaced onto the sea floor and then buried by independently moving sand.

The sandstone matrix of the KRR is massive unsheared/unfoliated litharenite or felsarenite, with rare local plane beds. No turbidites are present in it, no debris flows, no fossils, and very little shale or conglomerate. It has no paleocurrent indicators. In rough order the sand grains are metamorphic lithics, volcanic lithics, quartz, plagioclase, Kspar, biotite, and muscovite, a disaggregated granodiorite and its wall rocks. This matrix sand had a continental arc source, different from the oceanic source of the exotic blocks contained in it.

The entire KRR was next buried several km and moderately folded, and a third metamorphism (M3) at ~250+50oC and 2+1kb produced laumontite veins in it. Faulting, folding, and metamorphism ended by ~100 Ma. Apatite fission track ages of ~37 Ma for the block and its matrix (Erickson et al, 2004) show that they were at ~100oC then and have since been exhumed by isostatic uplift and erosion.