Cordilleran Section - 97th Annual Meeting, and Pacific Section, American Association of Petroleum Geologists (April 9-11, 2001)

Paper No. 0
Presentation Time: 1:00 PM-5:00 PM


PEARCE, L. Sara, Geology Department, Sonoma State Univ, 1801 East Cotati Avenue, Rohnert Park, CA 94928 and ERICKSON, Rolfe C., Geology, Sonoma State Univ, 1801 E. Cotati Avenue, Rohnert Park, CA 94928,

The block is an albite - Kspar - aragonite - chlorite - jadeitic pyroxene - lawsonite - glaucophane fels breccia. Aragonite is only present in sparse veins. The breccia is composed of fragments of coarse-grained glaucophane - lawsonite fels in a recrystallized fine-grained glaucophane - lawsonite fels matrix. A late dike, now metamorphosed to a fine-grained chlorite - lawsonite fels, cuts the block.

Aragonite, glaucophane and albite were stable at maximum prograde conditions, circa 300oC and 7 +/- 1 kb, based on Figure 6 of Evans (1990). The alumina in glaucophane geobarometer gives a minimum pressure of 7.8 kb and the chlorite geothermometer used on chlorite in the dike gives a temperature of 275oC. Jadeitic clinopyroxene was forming from glaucophane and lawsonite in the coarse-grained fragments. Veins of retrograde pumpelleyite are present. The gas phase had very low XCO2. Later, in an olistostrome, the block went to <3 kb pressure and <300oC, forming laumontite veins in the matrix.

The protolith was a basaltic andesite (52.4% SiO2)of probable continental arc origin. A REE profile matches NMORB while a Pearce spidergram has arc-typical low Ta - Nb anomaly and high K, Rb, and Ba peaks, modified by low Ce, Sm,and Yb values.

Brecciation and recrystallization of the primary fels ocurred at ~7 kb pressure in a single event, by hydrofracturing caused by water escaping the recrystallizing subducting plate.

Our model is that (1) a subducting oceanic (Farallon?) plate released water into a wedge of depleted mantle forming the protolith basaltic andesite magma, which erupted in a continental arc, probably the ancestral Sierras; (2) a portion of a flow was carried into the trench and subducted to ~7.5 kb pressure; (3) the protolith was metamorphosed to glaucophane - lawsonite fels, brecciated by hydrofracturing, and recrystallized; (4) the block of metabreccia was transported to the surface, perhaps by a diapir; (5) the block was deposited in an olistostrome; (6) the olistostrome was subducted to < 3 kb pressure and <300oC, forming laumontite veins.