THE ECLOGITE TO BLUESCHIST TRANSITION IN FRANCISCAN METABASITE

The Franciscan subduction complex contains mafic blocks that record counter-clockwise pressure-temperature paths with blueschist facies overprints on earlier eclogite. The eclogite to blueschist transition is commonly attributed to decreasing pressure-temperature conditions during exhumation, however, published data suggest that Franciscan eclogite and blueschist blocks have different bulk compositions. The extent that the eclogite to blueschist transition within the Franciscan complex reflects different pressure-temperature conditions or bulk compositions remains an open question.

We examined a single block that preserves an eclogite facies core, blueschist facies margin, and actinolite outer rind. The eclogite assemblage consists of omphacite, rutile, and garnet that contains inclusions of omphacite, epidote, phengite, rutile, sphene, and quartz. The blueschist assemblage consists of glaucophane, chlorite, pumpellyite, and albite. Mineral modes vary systematically from eclogite to blueschist.

Mutli-equilibrium thermobarometry shows a counter-clockwise pressure-temperature path similar to published paths for Franciscan eclogite. Garnet inclusions record burial to peak eclogite conditions from <10 up to ~22 kbar between 550-625 ˚C. The initial blueschist overprint occurred at pressures above albite stability and records conditions down to ~8 kbar and 375 ˚C.

Bulk rock, major element analyses display systematic compositional variations from eclogite to blueschist. Eclogite has higher CaO/(CaO + MgO + FeO_Total + MnO + Na₂O) and lower Al₂O₃/(CaO + Na₂O + K₂O) than blueschist. Published Franciscan bulk compositions show similar trends between discrete eclogite and blueschist blocks and suggest that retrogression of eclogite to blueschist was accompanied by open system processes.

Franciscan mafic eclogite was altered to blueschist by combined changes in pressure, temperature, and bulk composition. Blueschist facies assemblages reflect lower temperatures, though may still record relatively high pressures. Changes in bulk composition occurred by metasomatic interaction with serpentinite. The data are consistent with models of exhumation and metasomatism within a serpentinite-rich subduction channel.