PETROGENESIS OF THE BUNBURY BASALT, WESTERN-AUSTRALIA - INTERACTION BETWEEN THE KERGUELEN PLUME AND GONDWANA LITHOSPHERE

Subsequent to the initial rifting of Greater India from Australia/Anta rctica at similar to 132 Ma, widespread early Cretaceous volcanism occ urred on the continental margins. Two examples are the similar to 117 Ma Rajmahal Traps in northeast India and the similar to 123-130 Ma Bun bury 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 char acteristics the Bunbury Basalt is divided into two lava types: Casuari na and Gosselin. Relative to the 130 Ma Casuarina lavas, the 123 Ma Go sselin lavas have higher MgO content, lower abundances of high field s trength elements (P, Ti, Zr, Nh and Ta), higher Sr-87/Sr-86 (similar t o 0.7080), and Pb-207/Pb-204 (>15.61) and lower Nd-143/Nd-144. These i sotopic features, combined with relative depletion in Nb and Ta, are c haracteristic of continental crust. The low MgO and Ni contents (5.3-6 .9% and <100 ppm, respectively) of all Bunbury lavas indicate that the y experienced considerable crustal processing. Although the isotopical ly extreme Gosselin lavas have higher MgO contents than Casuarina lava s, their compositions reflect fractional crystallization at lower pres sures than Casuarina lavas; we infer that the Gosselin magmas evolved by fractional crystallization and crustal assimilation within the uppe r crust. The present-day Sr and Nd isotopic ratios in Casuarina lavas are similar to those in younger lavas from the Ninetyeast Ridge and Ke rguelen Archipelago which define the Kerguelen hotspot track. These is otopic similarities are consistent with a genetic relationship to the Kerguelen plume, but relatively high Pb-207/Pb-204 ratios (15.57-15.62 ) indicate that Casuarina lavas contain a small amount of the crustal component 207 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 (similar to 110- 115 Ma) for the large igneous province (Kerguelen Plateau) associated with the plume; moreover, Western Australia was apparently similar to 1000 km from the plume during this time. Therefore, the Bunbury Basalt may be unrelated to the plume; in this case. the geochemical similari ties of Casuarina and younger plume-related lavas an fortuitous.