Metamorphic History of a High-Grade Blueschist Exotic Block from the Franciscan Complex, California

Abstract

A high-grade blueschist tectonic block from the Franciscan Complex of the northeast Diablo Range shows evidence of three episodes of retrograde blueschist facies metamorphism ± deformation developed under progressively declining P-T conditions. The first retrograde metamorphism involved formation of an outer rind of actinolite + chlorite + rutile ± phengite, and the growth of coarse-grained chlorite + pumpellyite within the block. During the second event the rind and outer edge of the blueschist were folded, sheared and fractured, and primary glaucophane within the blueschist was replaced by albite, medium-grained chlorite, and glaucophane-crossite. The third retrograde metamorphism was marked by the pseudomorphic replacement of rind actinolite by aragonite and quartz. Aragonite also crystallized extensively within th block, accompanied by lawsonite, chlorite, jadeitic pyroxene, and crossite; this last mineral assemblage is identical to that of the surrounding Franciscan metasedimentary rocks. Features characteristic of the first and second retrograde metamorphic events are readily observed in other high-grade tectonic blocks of the Franciscan Complex and the correlative Otter Point Formation of Oregon. In contrast, evidence of a third retrograde metamorphism, matching that of presently associated Franciscan and Otter Point rocks, has been found in some but not all blocks examined so far. The high P/T conditions of prograde metamorphism and the available metamorphic age determinations suggest that the tectonic blocks 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 they developed alteration rinds through interaction with the enclosing ultramafic rock. The available data suggest that the examined exotic block and at least some others were transferred to the Franciscan as detritus from a body of serpentinite that reached the earth's surface. Such blocks were then resubducted and metamorphosed along with their presently associated sedimentary sequences.