ISOTOPIC AND CHEMICAL CONSTRAINTS ON THE CRUSTAL EVOLUTION AND SOURCESIGNATURE OF FERRAR MAGMAS, NORTH VICTORIA-LAND, ANTARCTICA

Isotopic (Nd and Sr) and chemical compositions of the 177 Ma Kirkpatri ck Basalt and Ferrar Dolerite from north Victoria Land, Antarctica, ar e examined in order to address the role of crustal assimilation and th e characteristics of their mantle source. Results for the Scarab Peak chemical type (SPCT) that constitutes the flow unit capping the lava s equence [Mg-number, Mg/(Mg + Fe+2) = 24, MgO = 2.4%, SiO2 = 57.1%, ini tial Sr-87/Sr-86 = 0.7087-0.7097, epsilon(Nd) -4.3) confirm previous r eports that attribute variations in the concentrations of the more mob ile elements and calculated initial Sr-87/Sr-86 to mid-Creataceous alt eration and elevated delta(18)O to low-temperature interaction with me teoric water. The underlying lavas and the sills that are of the Mt. F azio chemical type (MFCT) display a much wider range of both chemical and isotopic compositions (Mg-number = 40-65, MgO = 3.7-7.5%; SiO2 = 5 2.6-58.3%, initial Sr-87/Sr-86 = 0.7087-0.7117, epsilon(Nd) = -5.6 to -4.8). The effects of rock alteration on apparent initial Sr-87/Sr-86 are demonstrated by large differences between the initial ratio of min eral separates or leached fractions and whole rocks. Cretaceous altera tion produced Rb and Sr-redistribution within the lava sequence that r esults in erroneous calculated initial Sr-87/Sr-86 ratios. These effec ts are responsible for the large initial Sr-87/Sr-86 variations previo usly proposed which, combined with the large range in whole-rock delta (18)O, were purported to show very large degrees of crustal assimilati on. The variations in epsilon(Nd) are restricted and indicate much sma ller 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 wit h assimilation of material similar to early Paleozoic basement. The lo wer Sr-87/Sr-86 and higher epsilon(Nd) Of the SPCT suggest that they w ere derived by extensive fractionation of a more primitive, less conta minated, precursor of the MFCT. The most isotopically primitive Ferrar rocks from the region still have a high initial Sr-87/Sr-86 and low i nitial Nd-143/Nd-144; 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 eith er case Ross-type material may have been involved in the development o f the enriched isotopic signature.