Library

Notes: Abstract The initial 87Sr/86Sr ratios of twelve basalt flows of Jurassic age on Storm Peak in the Queen Alexandra Range are anomalously high and range from 0.7094–0.7133. The average value is 0.7112±0.0013 (1σ). The concentrations of rubidium and strontium have arithmetic means of 60.6±19.4 ppm and 128.8±11.9 ppm, respectively. The corresponding average Rb/Sr ratio is 0.47 which is also anomalously high for rocks of basaltic composition. In addition, these rocks have high concentrations of SiO2 (56.50%) and K2O (1.29%) and are depleted in Al2O3 (12.92%), MgO (3.44%) and CaO (7.91%) compared to average continental tholeiites. They are nevertheless classified as basalts on the basis of the composition of microphenocrysts. The initial 87Sr/86Sr ratios and all of the chemical parameters of the flows exhibit systematic stratigraphic variations. These are interpreted as indicating the occurrence of four eruptive cycles. In a typical cycle the initial 87Sr/86Sr ratios of successive flows and their concentrations of SiO2, FeO (total iron), Na2O, K2O, P2O5, Rb and Sr decrease in ascending stratigraphic sequence while the concentrations of TiO2, Al2O3, MgO, CaO and MnO increase upward. The initial 87Sr/86Sr ratios of the flows show a strong positive correlation with the strontium concentration. Similar correlations are observed between the initial 87Sr/86Sr ratios and all of the major oxide components. These relationships are incompatible with the hypothesis that these flows are the products of crystal fractionation of a-34 magma at depth under closed-system conditions. It is suggested that the flows resulted from the hybridization of a normal tholeiite basalt magma by assimilation of varying amounts of granitic rocks in the Precambrian basement which underlies the entire Transantarctic Mountain chain. Mixtures of two components having different 87Sr/86Sr ratios and differing strontium concentrations are related to each other by hyperbolic mixing equation. Such an equation was fitted by least squares regression of data points to a straight line in coordinates of initial 87Sr/86Sr and the reciprocals of the concentrations of strontium. This equation and plots of strontium versus other oxides were then used to estimate the chemical composition of the parent basalt magma and of the granitic contaminant by substituting reasonable estimates of their 87Sr/86Sr ratios. The chemical composition of the parent basalt (87Sr/86Sr=0.706) is generally compatible with that of average continental tholeiite, but is distinctive by having a low concentration of strontium (117 ppm). The chemical composition of the contaminant (87Sr/86Sr=0.720) is enriched in strontium (173 ppm), SiO2, FeO (total iron) and the alkalies but is depleted in Al2O3, MgO and CaO. The data for strontium indicate that the lava flows on Storm Peak contain between 20 and 40% of this granitic contaminant. The contamination of basalt magma is not a local event but is characteristic of the Jurassic basalt flows and diabase sills throughout the Transantarctic Mountains and in Tasmania.