Crustal development in the Agnew region, Western Australia, as shown by Rb/Sr isotopic and geochemical studies

Rb/Sr geochronology on a folded greenstone-granitoid complex in the Agnew area, Western Australia, yields four distinct ages of igneous activity that conform with stratigraphic and intrusive relationships. They are (1) Differentiated gabbro-granophyre from a stratigraphically old greenstone sequence (2) Voluminous tonalite, the Lawlers Tonalite (3) A less voluminous leucogranite, and a large complex pegmatite cutting the Perseverance nickel orebody (4) Aplitic leucotonalite. Strontium IRs and major and trace-element chemistry suggest that the tonalite-granodiorite and the later leucotonalite were obtained by partial melting of tholeitic material with garnet as a controlling mineral phase. The leucogranite more likely resulted from remelting of tonalite-granodiorite under similar conditions with garnet again a controlling mineral phase. These four magmatic events constitute a massive addition to the crust from the mantle lasting about 250 Ma. About 90% of the silicic material was emplaced 80 Ma after activity began (tonalite-granodiorite stage) and at least 99% had been emplaced after 140 Ma (leucogranite stage). (Auth.)