Timing and exhumation of eclogite facies shear zones, Musgrave Block, central Australia

Timing constraints on shear zones can provide an insight into the kinematic and exhumation evolution of metamorphic belts. In the Musgrave Block, central Australia, granulite facies gneisses have been affected, to varying degrees, by mylonitic deformation, some of which attained eclogite facies. The Davenport Shear Zone is a dominant strike-slip system that formed at eclogite facies conditions (T approximate to 650 degrees C and P approximate to 12.0 kbar). Sm-Nd mineral isochrons obtained from equilibrated high-pressure assemblages, as well as 40Ar-39Ar data, show that the eclogite and greenschist facies high-strain overprints were coeval, at c. 550 Ma. Mylonitic processes do not appear to have reset the U-Pb system in zircon, but may have partially disturbed it. The thermal gradient in the Musgrave Block crust at c. 550 Ma was c. 16 degrees C km(-1) and at c. 535 Ma was c. 18 degrees C km(-1), based on P-T estimates of eclogite and greenschist facies shear zones, respectively. These estimates are similar to present-day geothermal gradients in many stable continental shield areas, suggesting that the region did not undergo a significant transient perturbation of the geotherm. Therefore, in the Musgrave Block, cooling subsequent to eclogite facies metamorphism appears to have been controlled by exhumation, rather than by the removal of a heat source. Estimated exhumation rates in the range 0.2 to greater than or equal to 1.5 mm year(-1) are comparable with other orogenic belts, rather than cratonic areas elsewhere.