The Petrogenesis of the Kirwan Basalts of Dronning Maud Land, Antarctica

The 420 m thick sequence of Kirwan basalt crops out along thesouthernmost 50 km of the Kirwanveggen Escarpment (74?S, 6?W).There is little variation in major element chemistry of thesebasalts (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 interelementplots show rather poor correlation (r<0?78), but rocks fromopposite ends of the data array can be related by 30% fractionationof plagioclase, clinopyroxene, olivine, and magnetite in theproportions 51:35:11:3. Plagioclase is much more abundant inphenocryst assemblages (85%) and it appears that selective transportof plagioclase to the surface occurred. The range in incompatible element concentrations cannot be explainedby crystal fractionation and is most probably a result of theparent liquids of these basalts being derived by slightly differentdegrees of partial melting of a common source, or alternativelyof open-system (RTF) magma processes. The strontium isotopedata for the freshest rocks (R₀=0?7049–0?7065) may beexplained by 7% contamination by crustal material with an R₀of 0?709 and bulk Sr of800 ppm, but there is little supportingevidence from other trace element variations for this hypothesis.Oxygen isotope determinations on whole-rock-plagioclase pairsshow that alteration has resulted in a 0?5%o shift in (¹⁸O.Alteration also appears to have resulted in a greater spreadof data, particularly for the LIL elements and Sr isotopes.The Sr and Nd isotopic composition of the suite is close tobulk Earth at 172 Ma and this, together with REE and other traceelement data, shows these basalts to be similar in compositionto the more primitive basalts among the Karoo basalt lavas.It is suggested that the Kirwan basalts were derived from asource which was similar to that of the southern Lebombo variantof the Sabie River Basalt Formation of the Karoo Volcanic Province.This part of the Karoo was closest to the Kirwanveggen beforethe break-up of Gondwanaland.