PETROLOGY OF A BRECCIATED GLAUCOPHANE-LAWSONITE META-ARC BASALT BLOCK, FRANCISCAN COMPLEX, SONOMA COUNTY,CA

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 300^oC 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 275^oC. 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 X_CO2. Later, in an olistostrome, the block went to <3 kb pressure and <300^oC, 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 <300^oC, forming laumontite veins.