Recurrent transitional group charnockites in the east Amery Ice Shelf coast (East Antarctica): Petrogenesis and implications on tectonic evolution

Abstract We present new geochemical and isotopic Sm–Nd data on two transitional group charnockite generations which were emplaced within the same crustal block in the east Amery Ice Shelf coast in East Antarctica at ca. 980 Ma (Reinbolt charnockite) and ca. 500 Ma (Jennings charnockite). Thus it is the only known area in Antarctica where recurrent charnockite emplacements occurred providing a basis to trace the secular changes in charnockite composition and to put additional constraints on their nature and tectonic settings. Our data show that in spite of a prolonged time gap between the two charnockite generations their chemical and Sm–Nd isotopic compositions resemble very closely. Both generations comprise low-silica and relatively high-Fe* rocks with high TiO2, and other HFSEs. We suppose that both generations originated by partial melting of low crustal to sub-crustal mafic sources at high pressures and temperatures. However, the Cambrian Jennings charnockite has consistently higher TiO2, P2O5 and LaN/YbN arguing for their derivation from a garnet-bearing source under higher temperature conditions. The Sm–Nd data show that all charnockites irrespective of their age have very similar isotopic compositions suggestive of derivation from the same or very similar source(s), which precludes significant Sm–Nd fractionation before or during the Cambrian episode. We also present new U–Pb zircon (SHRIMP) age of ca. 980 Ma obtained for apparently undeformed charnockite from Gillock Island, which argues for essentially thermal rather than deformational nature of the subsequent (the Cambrian) episode. We conclude that at ca. 550–500 Ma this part of East Antarctica experienced thermal reworking from hypothetical mafic underplating at the base of the crust.