Charnockite composition in relation to the tectonic evolution of East Antarctica

Abstract

Charnockitic suites in central Dronning Maud Land (DML), Mac.Robertson Land (MRL), and the Bunger Hills area are compositionally varied and probably include both mantle and lower-crustal components. In this paper we present new geological and geochemical data on the DML charnockitic rocks, and compare their geochemistry with that of charnockitic rocks from several other Antarctic high-grade terranes, particularly MRL and the Bunger Hills. These areas have different geological histories and one of the main aims of this study is to investigate possible links between charnockite composition and the tectonic history of their host terranes. Antarctic charnockitic rocks form two distinct compositional groups. 510 Ma DML charnockites are relatively alkalic and ferroan, with high K 2O, Zr, Ga, Fe / Mg, and Ga / Al, and very low MgO, characteristic of A-type (alkaline, commonly anorogenic) granitoids. The more mafic DML rocks, at least, were derived by fractionation of a relatively alkaline high-P–Ti ferrogabbro parent magma. Most other early Palaeozoic charnockitic rocks in Antarctica are of similar composition. In contrast, MRL (c. 980 Ma) and Bunger Hills (c. 1170 Ma) charnockites are mainly calc-alkalic or calcic and magnesian, and the associated mafic components are tholeiitic. MRL and Bunger Hills charnockites are late-orogenic, whereas DML charnockites are post-orogenic, and appear to have been emplaced after post-collision extension and decompression. These two mineralogically and geochemically distinct charnockite groups may thus reflect a compositional trend in an evolving orogen, either accretional or collisional, respectively.