Periodic Paleoproterozoic calc-alkaline magmatism at the south eastern margin of the Yilgarn Craton; implications for Nuna configuration
Résumé
The age and composition of magmas provide fundamental information to chart the tectonic setting of crustal development through time and hence refine paleogeographic reconstructions. The Biranup Zone of the Albany-Fraser Orogen in southwestern Australia preserves a protracted record of magmatism associated with the formation and subsequent break-up of the Paleoproterozoic supercontinent Nuna. Yet, the configuration of Proterozoic Australia within Nuna is not well constrained. New U-Pb zircon geochronology on four samples of mafic intrusions in the north eastern Biranup Zone yield U-Pb crystallisation ages of 1809 ± 17 Ma, 1798 ± 12 Ma, 1796 ± 12 Ma, and 1755 ± 12 Ma coeval with known pulses of felsic magmatism elsewhere in the orogen. The Lu-Hf isotope composition of zircon crystals from these mafic intrusions, and the metasedimentary rocks they intrude, reveal juvenile magmatic input into the Archean Yilgarn Craton with this magmatism commencing as early as c. 1.90 Ga. Following this juvenile magmatism, the margin of the craton was affected by at least three pulses of calcic to calc-alkaline magmatism between 1.81 and 1.65 Ga. Secular changes in zircon Hf isotope composition are comparable in both duration and periodicity to 50–100 Ma changes in magma composition and zircon chemistry observed in modern volcanic arcs of the American Cordillera and in Cambrian to Carboniferous accretionary complexes of eastern Australia. Isotopic patterns and whole-rock chemistry are apparently consistent with the 1.81–1.70 Ga Paleoproterozoic igneous rocks of the Albany-Fraser Orogen forming in a magmatic arc above a Pacific-type subduction zone which extended along the south eastern margin of the Yilgarn Craton. This interpretation is consistent with paleomagnetic data which places the Yilgarn Craton on the periphery of the supercontinent Nuna at 1.90–1.60 Ga.