Geochemical and Sr-Nd isotopic mapping of source provinces for the Mawson charnockites, east Antarctica

Abstract

Large charnockite plutons (∼970 Ma) were intruded into high-grade metamorphic rocks of the Mawson Coast area, east Antarctica, immediately after peak granulite metamorphism in the region. Detailed Sr-Nd isotopic studies suggest that charnockites in the central zone have significantly higher initial 87Sr/86Sr ratios (0.7205-0.7334), lower initial ∈Nd values (-8.8 to -11.1) and older Nd depleted mantle model ages (TDMNd) (2.03-2.16 Ga) than those from the western and eastern zones (0.7076-0.7263; -4.0 to -8.4; 1.68-1.96 Ga, respectively). The isotopic compositions of the country rocks also mimic such regional variations. Apart from the isotopic variations, the charnockites can also be divided into two major geochemically distinctive categories, i.e. high-and low-Ti groups, with the high-Ti group being characterized by higher TiO2, P2O5, K2O, FeO*, Na2O+K2O, Zr, Nb, Y, La, Ce, Rb, Ba, Rb/Sr and Ga-Al and lower MgO, CaO, Al2O3, Na2O, Mg#, Cr and Ni than the low-Ti group. However, the two groups show no obvious difference in terms of Sr-Nd isotopic signatures. Compared with Palaeozoic I-type granites from the Lachlan Foldbelt, both groups of charnockites are generally higher in TiO2, P2O5, K2O, Zr, Nb, Y, La, Ce and Ba, and lower in MgO, CaO, Sr/Ba and Mg#, with the low-Ti charnockites being somewhat transitional between the high-Ti charnockites and I-type granites. On the spiderdiagram, both charnockites and I-type granites show similar degrees of negative Nb anomaly, implying derivation from similar Nb-depleted sources. We consider the Mawson charnockites, like those in the northern Prince Charles Mountains (PCM), were derived by high-temperature partial melting of dry, granulitic crustal sources, with charnockites in the central zone being related to a significantly older source province than those in the western and eastern zones. The charnockite magmatism occurred probably in response to Meso-Neoproterozoic continental collision related to construction of the Rodinia supercontinent. The igneous charnockites in the Mawson-PCM areas and the provincial distribution of their TDMNd ages and initial 87Sr/86Sr ratios are potentially important markers for precise reconstruction of the Rodinia and Gondwana supercontinents. This study demonstrates that the isotopic systematics, especially the TDMNd ages in granitic rocks, may provide an important means for the delineation of distinctive crustal terranes and their tectonic configuration, especially in structurally complex regions. Show more