Comparative Study of Melange Matrix and Metashales from the Franciscan Subduction Complex With the Basal Great Valley Sequence, California

Abstract

The central belt of the Franciscan subduction complex of California is largely a zone of pelitic-matrix melange bearing exotic blocks of blueschist and eclogite. In outcrop, the matrix appears to be a black shale with an anastomosing fracture cleavage. In this study, the petrology of the melange matrix was compared to that of the high-pressure/low-temperature metashales from the Diablo Range and the low-pressure/low-temperature metashales from the base of the Great Valley sequence. The shale matrix and high-pressure metashales (7 < P < 10 kb) contain the assemblage quartz + albite + phengitic white mica + chlorite + sphene + trace amounts of lawsonite, pumpellyite, or calcite. By contrast, low-pressure Knoxville shales (2 < P < 3 kb) contain quartz + albite + mixed-layer clay minerals + calcite. Most matrix samples from the exotic block-bearing melange and high-pressure metashale have illite crystallinities of 4 to 10 mm, up to 100% 2M white mica, densities exceeding $$2.6 gms/cm^{3}$$, organic matter app...