U-Pb ages from the Harts Range, central Australia: evidence for Early Ordovician extension and constraints on Carboniferous metamorphism

Abstract

Sensitive high-resolution ion microprobe (SHRIMP) U–Pb analyses of monazite and zircon from the Harts Range indicate the southeastern Arunta Inlier has been affected by two major Palaeozoic tectonothermal events. During the early Ordovician (467 ± 8 Ma), regional deformation produced a sub-horizontal upper amphibolite-facies (> 700°C and 7 kbar) shear fabric that overprinted c. 800°C migmatitic assemblages. In the intracratonic Amadeus Basin south of the Harts Range region, subsidence rates increased in the early Ordovician with the accumulation of easterly, and southerly, derived mature sediments. This is contrary to what would be expected if deformation in the Harts Range region occurred in a compressional setting, and suggests that the early Ordovician deformation may have occurred in an extensional intraplate setting. The terrain remained at depth until the mid-Carboniferous when up to 20 km of exhumation occurred during the latter stages of the intracratonic Devonian to Carboniferous (400–300 Ma) Alice Springs Orogeny. Monazite U–Pb data indicates the structurally lowest parts of the terrain were at c. 6 bar and ≥c. 600°C at 343 ± 8 Ma, and subsequently underwent high-temperature decompression, resulting in overprinting of the kyanite-grade assemblages with sillimanite-bearing assemblages during south-directed thrusting. During exhumation, pegmatite emplaced in the southeastern Harts Range at 330 ± 6 Ma (zircon U–Pb) was strongly deformed at temperatures around 560°C and depths around 15 km. The U–Pb data from the Harts Range indicate that the record of intraplate deformation in central Australia was more complex than previously thought.