Paleoproterozoic orogenesis in the southeastern Gawler Craton, South Australia

Integrated structural, metamorphic and geochronological data demonstrate the existence of a contractional orogen preserved in the ca 1850Ma Donington Suite batholith along the eastern margin of the Gawler Craton, South Australia. The earliest structures are a pervasive gneissic foliation developed in the Donington Suite and interleaved metasedimentary rocks. This has been overprinted by isoclinal and non-cylindrical folding, and zones of pervasive non-coaxial shear with north-directed transport, suggesting that deformation was the result of orogenic contraction. SHRIMP U-Pb zircon data indicate that a syn-contractional granitic dyke was emplaced at 1846 +/- 4Ma. Overprinting the contractional structures are a series of discrete, migmatitic high-strain zones that show a normal geometry with a component of oblique dextral shear. U-Pb zircon data from a weakly foliated microgranite in one such shear zone give an emplacement age of 1843 +/- 5Ma. Rare aluminous metasedimentary rocks in the belt preserve a granulite-grade assemblage of garnet+biotite+plagioclase+K-feldspar+silicate melt that formed at similar to 600MPa and similar to 750 degrees C. Peak metamorphic garnets are partially replaced by biotite+sillimanite+ cordierite assemblages suggesting post-thermal peak cooling and decompression, and are indicative of a clockwise P-T evolution. Chemical U-Th-Pb electron microprobe ages from monazites in retrograde biotite yield a minimum estimate for the timing of retrogression of ca 1830Ma, indicating that decompression may be linked to the development of the broadly extensional shear zones and that the clockwise P-T path occurred during a single tectonothermal cycle. We define this ca 1850 Ma phase of crustal evolution in the eastern Gawler Craton as the Cornian Orogeny.