A tectonic block (approx 6 m in diameter) in the NE Diablo Range shows evidence of three episodes of retrograde blueschist-facies metamorphism + or - deformation under progressively declining P-T conditions. Retrograde metamorphism began with the tectonic incorporation of fragments of the original metamorphic terrain into serpentinite, which provided Mg-bearing fluids for the reactions of both the first and second retrograde events. There is extensive CaCO3 veining which matches that of the associated metasediments. In the third retrograde event, aragonite crystallized, accompanied by lawsonite, chlorite, jadeitic pyroxene and crossite. Such tectonic blocks may have originally formed in a pre-Franciscan subduction zone setting; fragments of blueschist and eclogite from this metamorphic terrain were tectonically incorporated in a serpentinite diapir and developed alteration zones against the enclosing ultramafic rock. When the serpentinite reached the Earth's surface, such blocks were transferred to the Franciscan as detritus and were then re-subducted and metamorphosed along with their associated sedimentary sequences.-R.A.H.
Additional publication details
Metamorphic history of a high-grade blueschist exotic block from the Franciscan complex, California.