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

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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 , organic matter approaching bituminous grade, phengitic white mica ( Å), relatively Fe-rich chlorite, and sphene. The low-pressure Knoxville shales have no measurable illite crystallinity because of mixed layering, no detectable 2M white mica, densities less than , organic matter of lignite grade or lower and recognizable microfossils or plant fragments in some samples, non-phengitic white mica (b < 9.030 Å), relatively Fe-poor chlorite, and locally anatase. Comparison of these measures of metamorphism of the shale matrix with those of the high-pressure metashale and the low-pressure Knoxville shale, as well as consideration of phase equilibria of lawsonite + quartz and albite, indicates that the shale matrix of the central melange belt was subjected to conditions of 100 < T < 250°C and 3 < P < 10 kb. The high phengite content of white mica in many matrix samples suggests that much of the central melange belt has been buried to depths of 15 to 20 km. Thus, the major metamorphic difference between the shale matrix and the exotic blocks such as eclogite is not pressure but rather that the blocks were subjected to considerably higher temperatures than the matrix ever was. The measures of metamorphism used to differentiate between the melange matrix and Knoxville shales can be used as a semiquantitative measure of the depth of burial for shales of unknown metamorphic histories. In this way, remnants of the nonsubducted slope cover can be distinguished from bedded units which were deeply subducted in the Franciscan.