Petrology of early Palaeozoic lamprophyres from Schirmacher Oasis, east Antarctica

Lamprophyre dykes are conspicuous within the Proterozoic high-grade metamorphic rocks of Schirmacher Oasis. These dykes have been classified chemically into alkaline and calc-alkaline types on the basis of K2O/Na2O ratio and CIPW norm. The close association, age and geochemical similarities of both the lamprophyres and melasyenite in the area indicate their consanguinity and derivation from same source region. These lamprophyres have a high concentration of incompatible elements like Ba, Rb, Sr and Zr. These rocks also have a high Mg# and depict a negative Ta-Nb-Ti anomaly. The chondrite normalised REE patterns show fractionated trends with Large Variation in LREE and small variation in HREE concentration in both the rock groups. The calc-alkaline lamprophyre is more enriched in LREE than the alkaline lamprophyre. It is proposed that metasomatised phlogopite-bearing enriched mantle source has undergone partial melting to generate lamprophyric magma. This magma is generated in a continental within-plate tectonic setting. The collision of East and West Gondwana during Pan-African orogeny, resultant overthickening of continental lithosphere and generation of widespread magmatic activity in the region due to partial melting has culminated in the intrusion of lamprophyres around 450 Ma ago.