Mineral Parageneses in Franciscan Metamorphic Rocks, Panoche Pass, California

Abstract

Approximately 33 square miles of Franciscan terrane near Panoche Pass in the Diablo Range of the central California Coast Ranges were mapped at an initial scale of 1:12,000. Serpentinite intrusions present in the area are not mantled by metasomatic glaucophane schist aureoles, but do appear to contain tectonic inclusions of chloritized eclogite, “glaucophane eclogite,” amphibolite, glaucophane schist, and greenstone. Petrographic and X-ray investigations of country rocks reveal systematic mineral parageneses in metabasalts, metacherts, and metagraywackes. Conversion of greenstones to blueschists involved reaction of albite + actinolite + chlorite to form crossite + lawsonite; greenstones contain calcite, mafic glaucophane schists calcite or aragonite. Least metamorphosed metacherts contain very minor albite + calcite; with progressive recrystallization these phases are replaced by acgirme, riebeckite, and aragonite. Metagraywacke paragenesis is from albite metagraywackes containing accessory lawsonite + calcite or aragonite, to jadeitic pyroxene + law-sonite metagraywackes carrying accessory glaucophane + aragonite. The “jadeitic pyroxene + lawsonite isograd” is recognized on the basis of metagraywacke assemblages. The aragonite isograd cannot be located precisely because of rapid postmetamorphic inversion to calcite. More extreme conditions of recrystallization appear to have been required to produce aragonite and the jadeitic pyroxene + lawsonite (+ quartz) compatibility than to convert greenstones to blueschists. Mineral parageneses and inferred phase relationships in the system CaCO 3 and in the pseudoternary system Ab-An-H 2 O suggest that metamorphism at Panoche Pass involved temperatures of about 200–300°C, lithostatic pressures approaching 7–8 kb, and high chemical potential of H 2 O. Presence of a separate aqueous phase during the metamorphism is indicated. Based on observed mineral assemblages, experimental phase equilibrium data, tectonic and spatial occurrences of blueschists, available geophysical evidence, fossil occurrences, and radiometric dates, the following tentative geologic model is proposed. Franciscan rocks principally of the Diablo Range are hypothesized to have been deposited in an oceanic trench bordering the continental margin in later Jurassic and possibly earliest Cretaceous time. Rapid deposition and subsidence could have accounted for thermal disequilibrium and consequent relatively high pressure, low temperature recrystallization in axial portions of the trench. Thickness of the Franciscan rocks in the Diablo Range is thought to have approached 30 km near the trough axis. Diastrophism nearly contemporaneous with deposition locally deformed the structural trough, resulted in periodic intrusion of serpentinized mantle material and in rapid uplift, and allowed preservation of the blueschist facies mineral assemblages. Elsewhere, portions of the trench apparently were not destroyed and continued to receive sediments well into Cretaceous time.