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Rocky Mountain (56th Annual) and Cordilleran (100th Annual) Joint Meeting (May 3–5, 2004)

Paper No. 4
Presentation Time: 8:00 AM-5:00 PM

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


ERICKSON, Rolfe C., Geology, Sonoma State Univ, 1801 E. Cotati Avenue, Rohnert Park, CA 94928, MATTINSON, Jim, Department of Geological Sciences, Univ of California at Santa Barbara, Santa Barbara, CA 93106, DUMITRU, Trevor A., Geological and Environmental Sciences, Stanford Univ, Bldg 320, Lomita Mall, Stanford, CA 94305-2115 and SHARP, Warren, Berkeley Geochronology Center, 2455 Ridge Rd, Berkeley, CA 94709, rolfe.erickson@sonoma.edu

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.

Session No. 26--Booth# 48

Volcanology, Igneous and Metamorphic Petrology (Posters)
Tuesday, 4 May 2004: 8:00 AM-5:00 PM
Geological Society of America Abstracts with Programs. Vol. 36, No. 4, p.39
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