Petrogenesis of the Bunbury Basalt, Western Australia: interaction between the Kerguelen plume and Gondwana lithosphere?

Abstract

Subsequent to the initial rifting of Greater India from Australia/Antarctica at ∼ 132 Ma, widespread early Cretaceous volcanism occured on the continental margins. Two examples are the ∼ 117 Ma Rajmahal Traps in northeast India and the ∼ 123–130 Ma Bunbury Basalt in Western Australia, which erupted in the Perth Basin, a fault-bounded trough separating Archaean and Proterozoic crust. On the basis of spatial distribution, age, petrography, and geochemical characteristics the Bunbury Basalt is divided into two lava types: Casuarina and Gosselin. Relative to the 130 Ma Casuarina lavas, the 123 Ma Gosselin lavas have higher MgO content, lower abundances of high field strength elements (P, Ti, Zr, Nb and Ta), higher 87Sr/ 86Sr (∼ 0.7080), and 207Pb/ 204Pb (> 15.61) and lower 143Nd/ 14Nd . These isotopic features, combined with relative depletion in Nb and Ta, are characteristic of continental crust. The low MgO and Ni contents (5.3–6.9% and < 100 ppm, respectively) of all Bunbury lavas indicate that they experienced considerable crustal processing. Although the isotopically extreme Gosselin lavas have higher MgO contents than Casuarina lavas, their compositions reflect fractional crystallization at lower pressures than Casuarina lavas; we infer that the Gosselin magmas evolved by fractional crystallization and crustal assimilation within the upper crust. The present-day Sr and Nd isotopic ratios in Casuarina lavas are similar to those in younger lavas from the Ninetyeast Ridge and Kerguelen Archipelago which define the Kerguelen hotspot track. These isotopic similarities are consistent with a genetic relationship to the Kerguelen plume, but relatively high 207Pb/ 204Pb ratios (15.57 – 15.62) indicate that Casuarina lavas contain a small amount of the crustal component present in Gosselin lavas. Although it is possible that the Bunbury Basalt is a response to long-term incubation of the plume beneath eastern Gondwana, the eruption ages of Bunbury lavas (123 and 130 Ma) significantly exceed the oldest measured ages (∼ 110–115 Ma) for the large igneous province (Kerguelen Plateau) associated with the plume; moreover, Western Australia was apparently ∼ 1000 km from the plume during this time. Therefore, the Bunbury Basalt may be unrelated to the plume; in this case, the geochemical similarities of Casuarina and younger plume-related lavas are fortuitous.