Geochemical and SrNd isotopic study of charnockites and related rocks in the northern Prince Charles Mountains, East Antarctica: implications for charnockite petrogenesis and proterozoic crustal evolution

Abstract

Charnockite plutons were intruded into Meso-Neoproterozoic (∼1000 Ma) high-grade metamorphic zone in the northern Prince Charles Mountains (PCM), East Antarctica, immediately after peak granulite metamorphism in the region. Detailed geochemical and SrNd isotopic studies were carried out on these plutons and related rocks, which enables important constraints to be placed on the regional tectonic setting as well as the origin of igneous charnockites. The PCM charnockites are geochemically distinctive, characterised by having much higher TiO 2, P 2O 5, K 2O, K 2 O Na 2 O , Zr, Nb, Y, Pb, La, Ce, and Ba, and lower MgO, CaO, Mg#, Th, U, Sr Ba , and Rb Ba , than, for example, I-type granites from the Lachlan Foldbelt. The decrease in Zr, Nb, Y and Ce with increasing SiO 2, sharply contrast with those of the I-type granites. Isotopically, the PCM charnockites are relatively uniform and evolved, characterised by limited ranges of initial 87 Sr 86 Sr ratios (0.7063 to 0.7100), initial ϵ Nd values (mainly −4.0 to −5.9), and Nd depleted mantle model ages (1.60 to 1.98 Ga), implying derivation from pre-existing crustal sources. The geochemical and isotopic features are most consistent with crystal fractionation of dry hot magmas, with pyroxene, K-feldspar, plagioclase, apatite, zircon, ilmenite and magnetite as early-crystallizing phases. Although the involvement of a mantle-derived magma via AFC process cannot be ruled out, we consider that the charnockitic magmas were mainly derived from pre-existing subduction-related crustal sources, geochemically and isotopically similar to those of the I-type granites. The partial melting probably occurred under dry granulitic conditions at elevated temperatures (950–1050°C), with orthopyroxene, plagioclase and magnetite being residual phases. Under such conditions, elements including K, Rb, Ba, La, Ce, Nd, Zr, Nb and Y will be strongly incompatible and partition into the melt. The relatively low U and Th values in the charnockites are probably due to U-, Th-depletion in the sources, which may have been caused by dehydration and U-, Th-removal during amphibolite-to-granulite transition of the sources. We consider that the PCM charnockites and related regional metamorphism resulted from Meso- to Neoproterozoic continental collision between Archaean and Palaeoproterozoic cratonic blocks in East Antarctica. The Meso-Neoproterozoic collision was probably a global event, possibly related to construction of the Rodinia Supercontinent. During this collisional period or earlier orogenic events in the region (e.g. arc-continent collision in the Palaeo-Mesoproterozoic), calcareous sediments formed at plate margins or back-arc basins would have been tectonically transported to depth. Release of CO 2-rich fluids upon tectonic burial may have been responsible for amphibolite-to-granulite transition without causing dehydration melting to generate I-type granites. Instead, subsequent uplift of the dehydrated, but fertile, granulite crust during a period of crustal rebound may have facilitated decompression melting to produce high-temperature, water-deficient, charnockitic melts. Syn- to post-collision lithosphere delamination, asthenosphere upwelling and magma underplating may have also aided in heating the lower crust above its water-deficient solidus temperature.