Evaluation of prograde regional metamorphic conditions : their implications for the heat source and water activity during metamorphism in the Williyama Complex, Broken Hill, Australia

Abstract

The Willyama Complex around Broken Hill shows a NW to SE increase in metamorphic grade from andalusite-muscovite to sillimanite-muscovite, sillimanite-K feldspar and two pyroxene grade. Mineral assemblages in the dominant pelitic and lesser mafic gneisses (andalusite → sillimanite, cummingtonite-plagioclase, cordierite) indicate a low-intermediate pressure metamorphic path ; and by using solid-solid, silicate-based barometers (cordierite-garnet, orthopyroxene-garnet-plagioclase, garnet-plagioclase-sillimanite) pressures of 5-6 kbar are inferred for a large part of the two pyroxene zone including Broken Hill itself. The overall increase in pressure to the SE is accompanied by a small rise in temperature (T from 650 to 800 °C) and a gradual decrease in water activity (from 1.0 to ~ 0.5) above the second sillimanite isograd. A gradual transition from amphibolite to granulite facies conditions is indicated. The decrease in water activity appears to be related to the onset of partial melting ; i.e. as in situ partial melting proceeds in response to temperature rise, water activity declines. The latter is compatible with increased dilution of an aqueous fluid by another component, perhaps carbon dioxide, or with fluid-absent conditions. The low-intermediate pressure prograde P-T path and nearly constant pressure retrograde path cannot be explained by a heat source related to tectonic thickening alone and instead an external heat source (perhaps a deep crustal mafic magma) may be required. Such a heat source may have wider applicability to other low-intermediate pressure terrains.