PETROLOGY OF A META-ANORTHOSITIC GABBRO BLOCK, FRANCISCAN COMPLEX, SONOMA COUNTY, CALIFORNIA

The sample is a phengite(<1%) - epidote(<1%) - chlorite(<1%) -titanite(2%)- stilpnomelane(8%) - magnesioriebeckite(16%) - albite(71%) - schist, found as a cobble in Salmon Creek, CA, UTM ~ 4249500N 505400E. Jadeite, lawsonite and quartz are absent. The sample possesses a weak foliation of the albite, crosscut by radiating acicular magnesioriebeckite and stilpnomelane. Phengite geobarometry, the absence of jadeite+quartz and the presence of stilpnomelane (Miyano and Klein,1989) gives a maximum prograde metamorphic pressure of 7±1 kb. The chlorite geothermometer gives a metamorphic temperature above 273°C. Interpretation of PT conditions was aided by Fig. 6F of Evans (1990). Very low CaO (0.89%) and Sr (15 ppm) suggests the loss of these components by subduction dewatering. A Pearce spidergram strongly suggests an oceanic island basalt protolith. Strong negative P and Ti anomalies indicate strong apatite and titanite fractionation. The REE profile exhibits strong LREE enrichment (La_N/Lu_N=4.5) and a moderate negative Eu anomaly (Eu/Eu^*=0.72). The sample is likely from a differentiated gabbroic pluton from a zone similar to the Upper Border Group (UBG) of the Skaergaard pluton. Evidence suggests a model of: 1) A seamount on the Farallon Plate moved off its hotspot source followed by differentiation and solidification of residual magma chamber; 2) Seamount at least partially subducted and fragmented and the plutonic (anorthositic gabbro) clast foliated; 3) Shearing stops, followed by rapid formation of stilpnomelane and magnesioriebeckite; 4) Clast borne to surface quickly, perhaps by a diapir; 5) Clast released at the surface and incorporated into a Franciscan olistostrome.