Franciscan blueschist-facies metaconglomerate, Diablo Range, California

Abstract

Certain Franciscan conglomerate units are shown to be metaconglomerate and to exhibit selective blueschist-facies metamorphism of various constituents. Petrographic and x-ray studies were made on pebbles and matrix of 18 Franciscan conglomerate units forming blocks in melanges and lenses in stratiform bodies of graywacke in the vicinities of Mount Hamilton and Pacheco Pass. The conglomerate ranges from little-deformed varieties (textural grade 1) to tectonite with pronounced secondary planar structures (textural grade 2). Significant minerals in conglomerate of textural grade 1 are quartz + albite ± lawsonite ± minor glaucophane + CaCO 3 , and those of textural grade 2 are lawsonite + glaucophane + quartz ± jadeitic pyroxene ± albite ± aragonite. Thus the most intense metamorphism is empirically associated with internal deformation; however, the converse is not invariably true. Lawsonite in granitic clasts favors feldspar, but its abundance varies within single clasts, from clast to clast, and between adjacent conglomerate units; it commonly forms crystallographically oriented prisms in albite and is locally concentrated near the margins of albite crystals. Glaucophane, which is scarce in conglomerate of textural grade 1, is locally concentrated as rinds in some clasts of fine-grained basalt or it forms rare fibrous prisms in granitic clasts. In rocks of textural grade 2, it commonly forms beards on green amphibole in the matrix and in granitic clasts and completely replaces some mafic clasts. Jadeitic pyroxene replaces albite in conglomerate of textural grade 2 and is concentrated in the outer parts of certain clasts. Carbonate, commonly aragonite, shows crosscutting relations with lawsonite, glaucophane, or jadeitic pyroxene. Paragenetic relations between lawsonite and glaucophane are equivocal, but probably both minerals formed earlier than jadeitic pyroxene. Albite is apparently the only feldspar in the metaconglomerate units. The chemistry of granitic clasts is reflected in their metamorphosed mineral assemblages. They are deficient in potassium and enriched in sodium relative to average granite of similar silica content. The mineral assemblages of the metaconglomerate are consistent with those reported from nearby metagraywacke units, although there is generally no structural continuity with the metagraywacke. Petrographic features indicate postdepositional blueschist-facies metamorphism of the conglomerate units and demonstrate the presence of a fluid phase during metamorphism.