Petrogenesis of the Gifford Creek Carbonatite Complex, Western Australia

Abstract

The 1370 Ma Gifford Creek Carbonatite Complex (GCCC) comprises a diverse suite of alkaline dyke and sill complexes that cover an area of ~ 250 km² in the Gascoyne Province, Western Australia. Most carbonatite types are interpreted to be related products of fractional crystallisation, with calcite carbonatites representing cumulate rocks and dolomite carbonatites representing crystallised products of the derivative liquids. Genetic relationships between these carbonatites and other alkaline igneous units are less clear. The ankerite–siderite carbonatites and magnetite-biotite dykes are likely of related magmatic origin as both have distinctly high LREE and low HFSE contents. The ankerite–siderite carbonatites have mantle-like δ¹³C isotope values of − 6.1 to − 7.1‰ and similar geochemistry to other known magmatic ferrocarbonatites. Silica-rich alkaline veins found near the centre of the complex have trace element signatures that are antithetic to the magnetite–biotite dykes, so these veins are interpreted to represent products of alkali- and F-rich magmatic-hydrothermal fluids exsolved from the magnetite–biotite dykes during their emplacement. Carbon, O, Sr, and Nd isotope data are consistent with an enriched mantle source for the origin of the GCCC, with mantle enrichment likely caused by plate convergence processes associated with the c. 2.0 Ga Glenburgh Orogeny. There is no evidence to link mantle plume activity with formation of the GCCC; rather, alkaline magmatism is interpreted to result from low degree melting of the metasomatised mantle during reactivation of the crustal suture zone at 1370 Ma. The carbonatitic magmas utilised the Lyons River Fault to traverse the crust to be emplaced as the GCCC. Post magmatic alteration has variably modified the O and Sr isotope compositions of carbonates from these rocks. We therefore appeal for careful evaluation of isotopic data from ancient carbonatites, as isotopic resetting may be more common than currently recognised.