SAPPHIRINE-KORNERUPINE ROCKS FROM THE REYNOLDS RANGE, CENTRAL AUSTRALIA - CONSTRAINTS ON THE UPLIFE HISTORY OF A PROTEROZOIC LOW-PRESSURE TERRAIN

Sapphirine-kornerupine-bearing rocks from the Reynolds Range, Northern Territory, Australia preserve spectacular metamorphic reaction textures that provide valuable insights into the regional metamorphic uplift history. The rocks occur in pods that are several meters in diameter within high-temperature, low-pressure (750 to 800-degrees-C and approximately 4 to 5 kbar) granulite facies exposures of the early Proterozoic Lander Rock beds, a laterally extensive sequence of folded pelitic and quartzose metasediments. The pods are not associated with large volumes of partial melts and are likely to have formed by metasomatism near the peak of M2 metamorphism. The rocks in the pods consist of high-temperature Mg- and Al-rich minerals such as boron-free kornerupine, and are coarse-grained (0.5 to > 15 cm), non-foliated, and locally nearly monomineralic. The growth of the coarse minerals in the pods largely post-dated the high-grade regional metamorphic D2 fabric and completely reconstructed the precursor rocks. The retrograde metamorphic reaction textures show that the early retrogression from the M2 granulite facies conditions was characterized not by isobaric cooling as previously proposed, but by nearly isothermal decompression. These data imply that the Reynolds Range did not follow a simple anticlockwise P-T-t path. Because rocks such as these preserve information from a only restricted portion of the metamorphic history and can preserve evidence of decompression reactions more clearly than many more ordinary lithologies, they can be especially important for discerning metamorphic P-T-t paths.