PETROLOGY OF A META-ANORTHOSITIC GABBRO BLOCK, FRANCISCAN COMPLEX, SONOMA COUNTY, CALIFORNIA
OUTLAW, Robert K., Department of Geology, Sonoma State Univ, 1801 East Cotati Ave, Rohnert Park, CA 94928, sphene@earthlink.net, ERICKSON, Rolfe C., Geology, Sonoma State Univ, 1801 East Cotati Avenue, Rohnert Park, CA 94928, and DEIBERT, Jack, Department of Geology and Geography, Austin Peay State Univ, Clarksville, TN 37044,

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 71 kb. The chlorite geothermometer gives a metamorphic temperature above 273C. 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 (LaN/LuN=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.

Cordilleran Section - 97th Annual Meeting, and Pacific Section, American Association of Petroleum Geologists (April 9-11, 2001)
Session No. 18--Booth# 5
Undergraduate Research (Sponsored by the Council on Undergraduate Research) (Posters)
Sheraton Universal: Grand Ballroom
1:00 PM-5:00 PM, Monday, April 9, 2001