THE P-T-DEFORMATION PATH FOR A MIDPROTEROZOIC, LOW-PRESSURE TERRANE - THE REYNOLDS RANGE, CENTRAL AUSTRALIA

The Proterozoic low-pressure, high-temperature (LPHT) terrane of the Reynolds Range occurs in a 130-km-long, NW-trending belt in the central part of the Arunta Block, central Australia. The Reynolds Range has been affected by two mid-Proterozoic tectonic cycles, DI and DII, associated with two metamorphic events, MI and MII. DI-MI effects are restricted to the older of two sedimentary successions, the Lander Rock beds, which are separated from the younger Reynolds Range Group by an angular unconformity. The dominant structural-metamorphic features formed during DII-MII affected both sedimentary successions and the various granites that intruded them, and reworked most DI-MI effects. The DII deformation history can be subdivided into one prograde, two peak, and one retrograde stage. Average P-T calculations in the southeastern half of the range indicate a peak-metamorphic pressure of 4.1 +/- 0.3 kbar. Because the calculated values are derived from the same stratigraphic level corresponding to the base of the Reynolds Range Group, which is exposed throughout the area, it is likely that pressures were similar in the entire range. In fact, however, the peak-metamorphic temperature shows a dramatic increase from greenschist facies (c. 400-degrees-C) in the northwest to granulite facies (740 +/- 60-degrees-C) in the southeast, indicating that MII was associated with anomalously high heat flows. The P-T path is anticlockwise, with isobaric cooling from the metamorphic peak indicated by corona textures. However, the evidence of a prograde increase in pressure is indirect and based on the compressional nature of the structures. Peak-metamorphic mineral assemblages and retrograde mineral assemblages in amphibolite facies shear zones show the same metamorphic zonation, suggesting they formed in response to the same thermal event. If this is true, the implication is that a thermal perturbation external to the crust was maintained for a considerable period of time (110 Ma, based on zircon dating). As it is not clear whether Proterozoic, asthenosphere-active, thermal perturbations operated for this long, the alternative interpretation must be considered, namely that the peak-metamorphic events are separate from the shear zone event associated with reheating of the area.