Revealing the complex development of the Arunta region, central Australia

J.C. ClaouŽ-Long and A. Cross

The rocks exposed in the Arunta region of central Australia are renowned as a natural laboratory for studying the effects of complex polyphase metamorphic processes. Aspects of them have received study by the RSES Geochronology Group in previous years but the sum of available modern dating has been sparse and difficult to fit into an event framework.

Last year we reported the beginning of an ambitious partnership between geochronologists at Geoscience Australia and geologists at the Northern Territory Geological Survey who are remapping the fundamental geology of central Australia and using SHRIMP U-Pb geochronology intensively to identify and correlate major rock packages over very wide areas. The pattern of data now emerging shows why there has been great difficulty in comprehending this terrane until now. Zircons in rocks of the Arunta region record a repeated episodic progress of crustal events over a period of more than 2000 Ma, with more than ten distinct magmatic and metamorphic events overprinting one another. There can be few regions of the earthÕs crust which have experienced such prolonged and repeated reworking and which pose such a challenge to isotope geochronology.

The earliest preserved crust is late Archaean in age, and other basement rocks date from the ca. 1860 Ma ÔBarramundiÕ event for which there is evidence across northern Australia. Detrital zircon ages indicate that sediments were deposited on this basement after 1840 Ma and these were intruded by plutonic systems at ca. 1810 Ma and again at ca. 1780 Ma, following which zircons over widespread areas record metamorphism at ca. 1730 Ma while a more restricted region experienced earlier reworking at ca. 1750 Ma. A previously unrecognised event at ca. 1685 Ma is recorded by magmatic and metamorphic rocks in different areas, and by the magmatic age of a mafic dyke implying extensional tectonics at that time. A restricted region of the southern Arunta preserves previously unknown magmatic and metamorphic systems formed at ca. 1640 Ma in the newly recognised ÔLiebig eventÕ Ð which corresponds to a previously enigmatic deflection in the Australian Proterozoic Palaeomagnetic Polar Wander Path. The whole region then experienced widespread magmatism and metamorphic recrystallisation at ca. 1590 Ma in what is locally termed the ÔChewingsÕ event Ð which corresponds with a similar major event recorded in the Gawler and other Australian cratons. Another magmatic episode is locally expressed at ca. 1130 Ma Ð corresponding to the ÔGrenvillianÕ of north America. Finally, there is evidence that the already-complex Proterozoic basement was again reworked during the Phanerozoic, with both the Ordovician ÔLarapintaÕ event impacting in the eastern Arunta, and the later Alice Springs orogeny juxtaposing fault-bound basement units from different crustal levels later in the Palaeozoic.

Work is now proceeding to correlate this detailed event framework with the deposition systems operating in Proterozoic sedimentary basins, and to achieve wider correlations with other Proterozoic terrains in Australia and north America. The expectation is that these now-separate fragments of Proterozoic crust will be shown to have developed as linked crustal systems.