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

W. E. Glassley, J. Stimac

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±kspar±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 ban s 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 + plug (<5.8 kb) and garnet + sillimanite -> 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, re-equilibration occurred at crustal depths less than ^sim18 km. The original granulite facies assemblage appears to have formed at pressures in excess of 8 kb, but precise definition of the conditions of formation are precluded by difficulty in identifying the equilibrium high pressure garnet composition. Associated with the gneissic xenoliths are mafic xenolit s 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. The unusually shallow occurrence of granulite facies gneisses requires, in addition, that a significant quantity of fluid be released as the dehydration reactions are occurring. The total volume of water released, and the heat required for these dehydration reactions, are sensitive functions of the original rock mineralogy. We present estimates of the heat budget and water flow volumes for the 1 million year lifetime of the geothermal field based on (1) seismic evidence regarding the magma chamber geometry, (2) water stabl isotope chemistry, (3) enthalpy and stoichiometry of the dehydration reactions, and (4) the distribution of thermal wells in the region.

AAPG Search and Discovery Article #90097©1990 Fifth Circum-Pacific Energy and Mineral Resources Conference, Honolulu, Hawaii, July 29-August 3, 1990