Abstract:
Archaean gneisses in the Vestfold Block, Antarctica are cut by abundant tholeiite and rare alkaline dykes. At least five generations of dykes have been recognized on the basis of intrusive relationships, petrography and geochemistry. Rb-Sr isotopic data indicate that intrusion of the tholeiites occurred over a period of c. 1000 Ma, during three clearly defined events (viz. c. 2400, 1800, and 1400 Ma). Dykes in the southwestern part of the Vestfold Block were recrystallized during a late Proterozoic (c. 1000-1100 Ma) high-grade event. Mineral-whole rock Rb-Sr isotopic data show that the terrain was also variably affected by an even younger thermal event c. 500 Ma ago which is correlated with the Pan-African Orogeny. Isotopic, major element and trace element data suggest that the tholeiite dyke suites were derived by varying degrees of partial melting of compositionally distinct, in some cases rather heterogeneous subcontinental mantle source regions, combined with variations in the extent and nature of crystal fractionation. Extensive crustal contamination does not appear to have played a significant role in determining compositional trends within the different suites, although minor contamination cannot be discounted. The oldest dykes, a diverse group of high-Mg tholeiites, and the youngest, c. 1400 Ma tholeiites, appear to have been derived from isotopically similar source regions, which were nevertheless characterized by quite different abundances of highly incompatible elements (Pb, Rb, Ba, Th, K, La, and Ce). 1800 Ma tholeiites were derived from a more radiogenic mantle source region, characterized by long-term enrichment in incompatible elements. Such enrichment is interpreted to be a consequence of metasomatism of their mantle source region. 2400 Ma high-Mg dykes in Enderby Land have virtually identical chemical and isotopic compositions to those in the Vestfold Block, indicating generation from a similar source region; however, 1200 Ma tholeiites were apparently derived from a more radiogenic source region like that of the c. 1800 Ma Vestfold Block dykes. Hence, both Sr isotopic and geochemical data indicate that the ancient subcontinental lithospheric mantle beneath Gondwana was extremely heterogeneous.