Geochemical and Geochronological Signatures in Australian Proterozoic Igneous Rocks

Abstract

Summary Most igneous activity in Australian Proterozoic domains is related to major orogenic events, or else to crustal extensional events in which there is an earlier volcanogenic rift phase overlain by a non-volcanic thermal subsidence phase. The igneous suites are either predominantly mafic, felsic or bimodal, and there are no major suites of intermediate igneous rocks. With time, the amount of igneous material decreases volumetrically in the Proterozoic sedimentary sequences and there are also distinct compositional changes. Depleted tholeiites are more common in the early Proterozoic sequences, whereas enriched tholeiites and alkaline rocks are dominant after 1800 Ma. Felsic igneous rocks are rare in the earliest Proterozoic sequences, but dominate in the period 1880-1400 Ma. They are also compositionally time controlled, and certain major magmatic felsic suites at c . 1860, 1790, 1740, 1670, 1500 Ma have distinct chemical parameters which are common to most domains. All the major suites of felsic igneous rocks show enrichment in K, Rb, Th, U, La, and Ce relative to Phanerozoic analogues. The felsic rocks are predominantly I-types, derived from Proterozoic infracrustal rather than Archaean precursors; rare S-types also appear to have Proterozoic sources. Our preferred model has the felsic igneous rocks derived from underplated sources which are progressively accreted to the base of the crust, as a result of a series of extensional episodes. There are no direct compositional equivalents of modern subduction-related igneous sequences suggesting that subduction as it is known today was not a dominant process in the production of the igneous suites. There are also no recognizable ophiolites, ocean-floor crust, or other evidence of a Wilson cycle.