Granulite Facies Xenoliths in Clear Lake Volcanic Rocks and the Distribution of Heat Around Geothermal Energy Sources: ABSTRACT

Abstract

Within the Clear Lake (CA) volcanic field, a broad range of felsic and mafic xenoliths occur at several of the volcanic centers that erupted through Franciscan Complex rocks. A small proportion of these xenoliths are composed of granulite facies mineral assemblages and appear to be of crustal origin. Most of these xenoliths contain fine-grained gneissic textures. The xenoliths preserve a complex recrystallization history in which garnet-opx-plagioclase-qtz{plus minus}kspar{plus minus}sillimanite assemblages are partially replaced either by multiple generations of plagioclase-opx-hercynite coronas around garnets or cordierite overgrowths on hercynite-spinel. In some cases, hydration of the xenoliths is recorded by the development of biotite, which often occurs as nearly concentric bands in the interior of the xenoliths. Textural and compositional evidence suggests that recrystallization records the following reaction sequence, at the indicated pressures: garnet + quartz -> opx + plag ( hercynite + quartz (3.8 to 2.2 kb), followed by garnet + sillimanite + quartz -> cordierite (<5.9 kb). The geobarometric results indicate that, for all recrystallization events, reequilibration occurred at crustal depths less than {approximately}18 km. The original granulite facies assemblage appears to have formed at pressures in excess of 8 kb, but precise definition of themore » conditions of formation are precluded by difficulty in identifying the equilibrium high pressure garnet composition. Associated with the gneissic xenoliths are mafic xenoliths that are interpreted to represent fragments from deep crustal magma bodies. The presence of shallow to intermediate depth granulites in this region demonstrates that a significant quantity of the heat released during cooling and recrystallization of the magma bodies is used in the recrystallization of the rocks enclosing the magma chambers.« less