Sm-Nd evidence for high-grade Ordovician metamorphism in the Arunta Block,central Australia

Sm-Nd ages from the Harts Range in the south-eastern Arunta Inlier in centr al Australia indicate that regional metamorphism up to granulite facies occ urred in the Early Ordovician (c. 475 Ma). This represents a radical depart ure from previous tectonic models for the region and identifies a previousl y unrecognized intraplate event in central Australia. Peak metamorphic asse mblages (800 degrees C and 10.5 kbar) formed at around 476 +/- 14 Ma and un derwent approximately 4 kbar of near-isothermal decompression at 475 +/- 4 Ma. A coarse-grained unfoliated garnet-clinopyroxene-bearing marble inferre d to have recrystallized late in the decompressional evolution, gives an ag e of 469 +/- 7 Ma. Two lines of evidence suggest the Early Ordovician tecto nism occurred in an extensional setting. First, the timing of the high-grad e lower crustal deformation coincides with a period of marine sedimentation in the Amadeus and Georgina basins that was associated with a seaway that developed across central Australia. Second, isothermal decompression of low er crustal rocks was associated with the formation of a regional, sub-horiz ontal mid-crustal foliation. In the Entia Gneiss Complex, which forms the s tructurally lowest part of the Harts Range, upper-amphibolite facies metamo rphism (c. 700 degrees C, 8-9 kbar) occurred at 479 +/- 15 Ma. There is no evidence that P-T conditions in the Entia Gneiss Complex were as high as in the overlying units. This implies that the extensional system was reworked during a later compressional event. Sm-Nd data from the mid-amphibolite fa cies (c. 650 degrees C and 6 kbar) detachment zone that separates the Irind ina Supracrustal Assemblage and Entia Gneiss Complex give an age of 449 +/- 10 Ma. This age corresponds to the timing of a change in the pattern and s tyle of sedimentation in the Amadeus and Georgina basins, and indicates tha t the change in basin dynamics was associated with mid-crustal deformation. It also suggests that compressional deformation culminating in the Devonia n to Carboniferous (400-300 Ma) Alice Springs Orogeny may have begun as ear ly as c. 450 Ma. At present, the extent of Early Ordovician tectonism in ce ntral Australia is unknown. However, granulite facies metamorphism and asso ciated intense deformation imply an event of regional extent. An implicatio n of this work is that high-grade lower crustal metamorphism and intense de formation occurred during the development of a broad, shallow, slowly subsi ding intraplate basin.