THE PETROGENESIS OF MESOZOIC GONDWANA LOW-TI FLOOD BASALTS

New major, trace element and isotopic data for Jurassic basalts from SE Australia indicate that they are strikingly similar to the Jurassic tholeiitic rocks of Tasmania and the Transantarctic Mountains. These rocks are all characterised by low TiO2, P2O5, Na2O, Fe2O3, Ti/Zr, Ti/Y and epsilon-Nd, and high SiO2, Rb/Ba, Rb/Sr, Sr-87/Sr-86 and Pb-207/Pb-204, relative to oceanic basalts. They therefore comprise a major province, termed the Ferrar magmatic province, which extended for 3000-4000 km across the Gondwana supercontinent. A review of the other Mesozoic low-Ti CFB's suggests that the Ferrar rocks are an extreme example of these magma types. It is striking that both the major and trace element compositions are different from oceanic basalts, which suggests that these features are linked, and it is argued that they were derived from distinctive source regions in the sub-continental mantle. Such source regions were variably depleted in major and minor elements, and then relatively enriched in highly incompatible elements and Sr and Pb isotopes, which is best explained by the introduction of a small amount of subducted sediment. The tectonic setting of the Ferrar magmatism is poorly constrained, but at present there is no clear geochemical evidence for the involvement of asthenospheric plume material in the petrogenesis of these low-Ti CFB's.