Isotopic and chemical constraints on the crustal evolution and source signature of Ferrar magmas, north Victoria Land, Antarctica

Abstract

Isotopic (Nd and Sr) and chemical compositions of the 177 Ma Kirkpatrick Basalt and Ferrar Dolerite from north Victoria Land, Antarctica, are examined in order to address the role of crustal assimilation and the characteristics of their mantle source. Results for the Scarab Peak chemical type (SPCT) that constitutes the flow unit capping the lava sequence [Mg-number, Mg/(Mg+Fe+2)=24, MgO=2.4%, SiO2=57.1%, initial87Sr/86Sr=0.7087–0.7097, (eNd=−4.3) conform previous reports that attribute variations in the concentrations of the more mobile elements and calculated initial87Sr/86Sr to mid-Cretaceous alteration and elevated δ18O to low-temperature interaction with meteoric water. The underlying lavas and the sills that are of the Mt. Fazio chemical type (MFCT) display a much wider range of both chemical and isotopic compositions (Mg-number=40–65, MgO=3.7 7.5%; SiO2=52.6–58.3%, initial87Sr/86Sr=0.7087–0.7117, eNd=−5.6 to −4.8). The effects of rock alteration on apparent initial87Sr/86Sr are demonstrated by large differences between the initial ratio of mineral separates or leached fractions and whole rocks. Cretaceous alteration produced Rb and Sr redistribution within the lava sequence that results in erroneous calculated initial87Sr/86Sr ratios. These effects are responsible for the large initial87Sr/86Sr variations previousl7 proposed which, combined with the large range in whole-rock δ18O, were purported to show very large degrees of crustal assimilation. The variations in eNd are restricted and indicate much smaller degrees of assimilation. The least altered of the MFCT rocks show good chemical and isotopic correlations that can be integrated into a model involving fractionation of pyroxene and plagioclase coupled with assimilation of material similar to early Paleozoic basement. The lower87Sr/86Sr and higher eNd of the SPCT suggest that they were derived by extensive fractionation of a more primitive, less contaminated, precursor of the MFCT. The most isotopically primitive Ferrar rocks from the region still have a high initial87Sr/86Sr and low initial143Nd/144Nd; this may reflect either earlier assimilation or an enriched source. The chemical and isotopic similarities, as well as the close geographic correspondence of the Ferrar Group to granitoids produced during the early Paleozoic Ross Orogeny suggest that in either case Ross-type material may have been involved in the development of the enriched isotopic signature.