Paleoproterozoic orogenesis in the southeastern Gawler Craton, South Australia

Abstract

Integrated structural, metamorphic and geochronological data demonstrate the existence of a contractional orogen preserved in the ca 1850 Ma 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 ± 4 Ma. 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 ± 5 Ma. Rare aluminous metasedimentary rocks in the belt preserve a granulite-grade assemblage of garnet + biotite + plagioclase + K-feldspar + silicate melt that formed at ∼600 MPa and ∼750°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 1830 Ma, 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.