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

Charnockite plutons were intruded into Meso-Neoproterozoic (similar to 1000 Ma) high-grade metamorphic zone in the northern Prince Charles M ountains (PCM), East Antarctica, immediately after peak granulite meta morphism in the region. Detailed geochemical and Sr-Nd isotopic studie s were carried out on these plutons and related rocks, which enables i mportant constraints to be placed on the regional tectonic setting as well as the origin of igneous charnockites. The PCM charnockites are g eochemically distinctive, characterised by having much higher TiO2, P2 O5, K2O, K2O/Na2O, 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 SiO2, sharply contrast with those of the I-type granites. Isotopicall y, the PCM charnockites are relatively uniform and evolved, characteri sed by limited ranges of initial Sr-87/Sr-86 ratios (0.7063 to 0.7100) , initial epsilon(Nd) values (mainly -4.0 to -5.9), and Nd depleted ma ntle model ages (1.60 to 1.98 Ga), implying derivation from pre-existi ng crustal sources. The geochemical and isotopic features are most con sistent with crystal fractionation of dry hot magmas, with pyroxene, K -feldspar, plagioclase, apatite, zircon, ilmenite and magnetite as ear ly-crystallizing phases. Although the involvement of a mantle-derived magma via AFC process cannot be ruled out, we consider that the charno ckitic magmas were mainly derived from pre-existing subduction-related crustal sources, geochemically and isotopically similar to those of t he I-type granites. The partial melting probably occurred under dry gr anulitic conditions at elevated temperatures (950-1050 degrees C), wit h orthopyroxene, plagioclase and magnetite being residual phases. Unde r such conditions, elements including K, Rb, Ba, La, Ce, Nd, Zr, Nb an d Y will be strongly incompatible and partition into the melt. The rel atively low U and Th values in the charnockites are probably due to U- , Th-depletion in the sources, which may have been caused by dehydrati on and U-, Th-removal during amphibolite-to-granulite transition of th e sources. We consider that the PCM charnockites and related regional metamorphism resulted from Meso- to Neoproterozoic continental collisi on between Archaean and Palaeoproterozoic cratonic blocks in East Anta rctica. The Meso-Neoproterozoic collision was probably a global event, possibly related to construction of the Rodinia Supercontinent. Durin g this collisional period or earlier orogenic events in the region (e. g. are-continent collision in the Palaeo-Mesoproterozoic), calcareous sediments formed at plate margins or back-are basins would have been t ectonically transported to depth. Release of CO2-rich fluids upon tect onic burial may have been responsible for amphibolite-to-granulite tra nsition without causing dehydration melting to generate I-type granite s. Instead, subsequent uplift of the dehydrated, but fertile, granulit e crust during a period of crustal rebound may have facilitated decomp ression melting to produce high-temperature, water-deficient, charnock itic melts. Syn- to post-collision lithosphere delamination, asthenosp here upwelling and magma underplating may have also aided in heating t he lower crust above its water-deficient solidus temperature.