Revised geochronology of magmatism in the western Capricorn Orogen at 1805-1785 Ma: diachroneity of the Pilbara-Yilgarn collision

The June Hill Volconics occupy a central stratigraphic position in the tectonic evolution of the Capricorn Orogeny that occurred along the southwestern margin of the Pilbara Craton in Palaeoproterozoic time. The volcanic rocks have been interpreted alternatively as due to continental rifting, foreland-basin development, or backarc extension. Previous multigrain zircon U-Pb results from these rocks, as well as from an unnamed rhyolitic porphyry that is interstratified in turbidites of the overlying Ashburton Formation, are influenced by the combined effects of Pb loss and xenocrystic inheritance. We report new SHRIMP zircon ages of 1799 8 and 1786 11 Ma, respectively from the June Hill Volcanics and the unnamed porphyry, both of which have been deformed during the Capricorn Orogeny. Close similarity between these ages and results from various late-stage Capricorn granitoids (including our new SHRIMP result of 1795 8 Ma for the Minnie Creek granite) imply rapid development of the western Ashburton Trough, accumulating and deforming similar to 10 km of turbiditic sediments in about 10 million years, Comparison of our new June Hill age with previous SHRIMP results from the central Ashburton Trough implies that foreland-basin development of the Capricorn Orogeny was significantly diachronous,younging westward during oblique Pilbara-Yilgarn convergence.