Palaeoproterozoic subduction-related and passive margin basalts in the Halls Creek Orogen, northwest Australia

Palaeoproterozoic basaltic rocks in the Halls Creek Orogen form part of two stratigraphic sequences on either side of a major structure, the Angelo-Halls Creek-Osmond Fault System. The two sequences have contrasting geological histories and probably formed in different tectonostratigraphic terranes. To the east, basalts of the Biscay Formation, which are part of the Halls Creek Group, were erupted at ca 1880 Ma and deformed and metamorphosed first at low grade between ca 1845 and ca 1820 Ma. To the west, basalts of the Tickalara Metamorphics were deposited after 1865 Ma, and were metamorphosed at medium to high grade and intruded by tonalite and leucogranite sheets at 1850 to 1845 Ma. Two groups of metabasalts are identified in the Biscay Formation. Group 1 samples have compositions similar to enriched (E-)MORB. Group 2 samples have lower TiO₂, P₂O₅, Cr, Y, Nb and Zr contents, and trace-element ratios (e.g. Ti/V and Zr/Nb), similar to low-TiO₂ continental flood basalts. Metabasalts from the Tickalara Metamorphics consist of depleted and enriched types. The depleted samples have high field strength element (HFSE) and rare-earth element (REE) abudances similar to oceanic island arc/backarc basin tholeiites. The enriched samples have compositions similar to E-MORB, and are similar to group 1 samples from the Biscay Formation. Basalts of the Biscay Formation were erupted on a passive continental margin, whereas those of the Tickalara Metamorphics formed in an oceanic island arc/backarc basin or ensialic marginal basin, the two terranes being brought together by ca 1820 Ma. This is consistent with the evolution of the Halls Creek Orogen during the Palaeoproterozoic by plate-tectonic processes similar to those operating in the Phanerozoic.