The geological evolution of the Reynolds Range, central Australia: evidence for three distinct structural-metamorphic cycles

Abstract

The Reynolds Range is affected by three distinct structural-metamorphic cycles, DI- MI, DII- MII and DIII- MIII, which represent the local expressions of a series of orogenic events that affected large sections of the Arunta Block, central Australia. The tectonic cycles recognized in the Reynolds Range correlate extremely well with similar events in the nearby Anmatjira Range. DI- MI took place around 1820 Ma and only affected the NW Reynolds Range. DII- MII occurred between 1760 and 1650 Ma and involved NE-SW shortening with identical timing relationships between the deformations and metamorphism along the entire Reynolds Range. DII commenced with the formation of a fabric, SII 1, preserved as inclusion trails in porphyroblasts. SII 1 formed early during a progressive event, DII 1−2, which involved the formation and tightening of upright NW-SE-trending isoclinal folds. A penetrative subvertical NW-SE-trending fabric, SII 2, developed axial planar to these folds. The general symmetry of DII 2 microstructures suggests that FII 2 folds were formed during progressive coaxial deformation under plane strain conditions. DII 3 structures are associated with conjugate sets of crenulation bands. Within the macroscopic crenulation bands, large sections of earlier folds and fabrics are rotated to a near recumbent position. The dominant set of crenulation bands is upright and trends ENE-WSW while the conjugate to this set trends NW-SE. No penetrative fabric formed during DII 3. DII 4 is associated with a system of amphibolite-grade NE-dipping thrusts interconnected by numerous smaller shear zones. The sense of movement is NE-over-SW. During DII essentially penetrative coaxial deformation ( DII 2) evolved into essentially localized non-coaxial deformation ( DII 3- DII 4). DII deformation occurred concomitant with a low-pressure granulite-facies metamorphic event ( MII, 1730 Ma). Peak metamorphic sillimanite + cordierite + orthopyroxene + ilmenite + spinel + garnet + K-feldspar + quartz assemblages define SII 2. Oriented sillimanite + biotite assemblages that formed early during DII 3 and unoriented andalusite + chlorite + muscovite assemblages that formed late during DII 3 indicate that cooling of MII was initially isobaric. During DII 4 thrusting staurolite + garnet + biotite + quartz and biotite + kyanite + quartz assemblages, defining the mylonitic fabric, overprint the late DII 3 andalusite assemblages. DII 4 shear movement is therefore accompanied by a slight rise in pressure. DII 4 shear zones gradually increase in grade eastward along the Reynolds Range. This trend coincides with the variation in peak metamorphic grade attained during MII, indicating that the rocks had not yet fully cooled when DII 4 thrusting commenced. DIII- MIII is late Devonian to Carboniferous in age and caused minor differential movement (<100 m) and retrogression along DII 4 shear zones.