EARLY PROTEROZOIC METAMORPHISM AT THE GRANITES GOLD MINE, NORTHERN-TERRITORY - IMPLICATIONS FOR THE TIMING OF FLUID PRODUCTION IN HIGH-TEMPERATURE, LOW-PRESSURE TERRANES

Gold mineralization at The Granites, Northern Territory, Australia, oc curs in a sequence of amphibolite facies pelitic and semipelitic schis ts forming part of the Early Proterozoic Granites-Tanami block. Peak m etamorphism at The Granites at approximately 600-degrees-C and approxi mately 3.5 kbars occurred during the development of near-vertical foli ations and associated stretching fabrics and, in keeping with many Aus tralian Proterozoic metamorphic belts, was followed by isobaric coolin g, reflecting the extremely transient nature of the thermal perturbati on. Cummingtonite and hornblende schists occur in alteration assemblag es surrounding mineralized calcite veins in the pelitic and semipeliti c schists, as well as in the footwall and host gold lodes, and are ass ociated with high-temperature metasomatic addition of Ca and depletion of K. The mobilization of the significant volumes of fluid necessary for metasomatism and associated mineralization during convergent defor mation is modeled as a consequence of the lithospheric-scale deformati on geometry appropriate for the generation of high-temperature, low-pr essure terranes (e.g., Loosveld and Etheridge, 1990; Sandiford and Pow ell, 1991). We show that if crustal thickening is accompanied by appre ciable mantle lithospheric thinning, in excess of 50 percent devolatil ization of the low crust may occur during prograde metamorphism and ac tive crustal thickening.