PROTEROZOIC LOW-PRESSURE HIGH-TEMPERATURE METAMORPHISM AND AN ANTICLOCKWISE P-T-T PATH FOR THE HAZELDENE AREA, MOUNT ISA INLIER, QUEENSLAND, AUSTRALIA

In the Hazeldene area, situated in the Mount Isa Inlier, Queensland, the metamorphic grade changes from chlorite zone, through biotite and cordierite zones, to sillimanite/K-feldspar zone. Microstructural studies of rocks near the sillimanite isograd demonstrate that cordierite grew early during the development of a steep foliation (S2), was replaced by biotite, andalusite and sillimanite at the metamorphic peak late in S2, and in turn by kyanite + chlorite adjacent to localized small post-D2 shear zones. Although the anticlockwise P-T-t path is well defined, the precise P-T conditions are uncertain because of problems with experimental and thermodynamic data. The best estimate for the metamorphic peak for rocks close to the sillimanite isograd is around 600-degrees-C at 4 kbar. The metamorphism has been dated at 1544 Ma, and was synchronous with a major crustal shortening event. Because proposed extensional events occurred more than 60 Ma earlier, their contribution to the peak metamorphic thermal perturbation would have been insignificant. The syn-metamorphic Mica Creek Pegmatites, the abundance of high heat-producing elements in the nearby pre-D2 Sybella Granite, and advective heat by fluids which caused considerable metasomatism in the Hazeldene area, may have each contributed to the thermal budget. However, the metamorphic thermal gradient may be 80-degrees-C km-1 or higher, strongly suggesting a local magmatic control. As none are known in the area, such synmetamorphic plutons would have to lie beneath the exposed high-grade rocks.