Petrography and Rb-Sr mineral age of mafic dyke rock from Niban Iwa, Lützow-Holm Complex, East Antarctica

Abstract

A WNW-ESE-trending mafic dyke intruding across major structures in high-grade metamorphic rocks was found at Niban Iwa (translated as ‘Number Two Rock’) in the Proterozoic Lützow-Holm Complex of East Antarctica. It is holocrystalline and aphyric, and comprises biotite, hornblende, plagioclase, orthoclase, quartz, apatite, and titanite. Chemically the dyke rock is alkali basalt with high K₂O/Na₂O and total Fe contents, and low Cr and Ni contents, indicating that it was formed by the differentiation of olivine from a primary alkali basaltic magma derived from the subcontinental mantle. The Rb-Sr mineral isochron age was obtained of 487 ± 15 Ma with Sr_IR = 0.70486 ± 0.00007. Considering that the metamorphic age of the gneisses at Niban Iwa was estimated to be 532 Ma, the dyke probably intruded after metamorphism as part of the post-orogenic igneous activity following the collision of East and West Gondwana.