PALAEOPROTEROZOIC PLATE COLLISION IN NORTHERN AUSTRALIA: THE HALLS CREEK OROGEN

In contrast to accretionary and collisional models for Palaeoproterozoic orogenic belts in North America and Europe, models for similar belts in Australia have assumed that tectonic and magmatic activity occurred within a craton that was assembled during the late Archaean. Remapping of the Halls Creek Orogen in the Kimberley region of Western Australia, combined with geochronological, geochemical and geophysical data, has revealed a complexity that intracratonic models cannot explain.

Geophysical data show differences in the buried basement on either side of the orogen. Detrital zircon populations suggest that the eastern basement is Archaean, whereas the western basement consists of a series of accreted late Archaean to early Palaeoproterozoic terranes.

Three tectonostratigraphic terranes are exposed in the orogen, each with histories that reflect disparate tectonic settings between 1910 and 1820 Ma. Turbiditic metasedimentary rocks that had previously been correlated across the orogen, were deposited, and deformed and metamorphosed at different times in each terrane. The eastern terrane was a passive continental margin between 1910 and 1840 Ma. Within the central terrane an oceanic island arc or ensialic marginal basin developed at 1865 Ma. The western terrane formed between 1870 and 1850 Ma as a rift within older terranes accreted prior to 1900 Ma.

Granites and gabbros intruded the western terrane between 1865 and 1850 Ma. The central terrane was intruded by tonalite sheets at 1850 Ma. Further granites and gabbros intruded mainly the central terrane between 1835 and 1805 Ma. Each magmatic event has contrasting chemical and Nd isotope compositions and was derived from separate sources in different tectonic settings.

The Palaeoproterozoic tectonic evolution of the Halls Creek Orogen is best explained by terrane accretion between 2500 and 1850 Ma, followed by collision between the Kimberley Craton and the rest of the North Australian Craton at 1820 Ma.