THE PETROGENESIS OF THE KIRWAN BASALTS OF DRONNING MAUD LAND, ANTARCTICA

The 420 m thick sequence of Kirwan basalt crops out along the southernmost 50 km of the Kirwanveggen Escarpment (74°S, 6°W). There is little variation in major element chemistry of these basalts (SiO2 49·3-51·6 wt.%; MgO 5·1-6·6 wt.%), but the concentrations of certain incompatible elements (e.g., Zr) vary by factors of approximately two or more. Most interelement plots show rather poor correlation (r<0·78), but rocks from opposite ends of the data array can be related by 30% fractionation of plagioclase, clinopyroxene, olivine, and magnetite in the proportions 51:35:11:3. Plagioclase is much more abundant in phenocryst assemblages (85%) and it appears that selective transport of plagioclase to the surface occurred.The range in incompatible element concentrations cannot be explained by crystal fractionation and is most probably a result of the parent liquids of these basalts being derived by slightly different degrees of partial melting of a common source, or alternatively of open-system (RTF) magma processes. The strontium isotope data for the freshest rocks (R0=0·7049-0·7065) may be explained by ∼7% contamination by crustal material with an R0 of 0·709 and bulk Sr of∼800 ppm, but there is little supporting evidence from other trace element variations for this hypothesis. Oxygen isotope determinations on whole-rock-plagioclase pairs show that alteration has resulted in a 0·5%o shift in (δ18O. Alteration also appears to have resulted in a greater spread of data, particularly for the LIL elements and Sr isotopes. The Sr and Nd isotopic composition of the suite is close to bulk Earth at 172 Ma and this, together with REE and other trace element data, shows these basalts to be similar in composition to the more primitive basalts among the Karoo basalt lavas. It is suggested that the Kirwan basalts were derived from a source which was similar to that of the southern Lebombo variant of the Sabie River Basalt Formation of the Karoo Volcanic Province. This part of the Karoo was closest to the Kirwanveggen before the break-up of Gondwanaland. © 1990 Oxford University Press.