Origin and Geodynamic significance of opx granitoids from Bunger Hills Area, East Antarctic

Abstract

<p>The information on magmatic orthopyroxene (opx) granitoids (charnockites) is essential for geodynamic models of continent collision, amalgamation, and postorogenic events. There is also the petrological issue - the role of the crust and mantle material in the formation of opx granitoids in orogenic and post-orogenic settings. Our research objects are three multiphase coeval plutons of the opx granitoids in the Banger Oasis, East Antarctica. Only two phases can be distinguished: the first phase comprising of silica and potassium rich middle-coarse grained qz opx monzodiorites, monzonites and granites with porphyric perthite alkali feldspar, whereas the second phase is more basic and comprises of alkali feldspar less fine-middle grained qz gabbroes and qz-opx diorites and gabbro-diorites with gabbroic and ophitic microstructure.  These plutons intrude granulite facies metamorphic bedrock. The age of those plutons is around 1170 ± 10 Ma. The first phase has the age around 1170-1190 Ma, whilst the second phase has the age of 1150 1170 Ma. The interval of peak metamorphism is constrained by 1250-1170 Ma. We combine together thermodynamic modelling and geochemistry (isotope geochemistry in Sm-Nd, Rb-Sr and Pb-Pb systems, Hf and O stable isotopes data on zircons) to create a petrological model for opx granitoids formation. As a result of such modelling we are able to prove that pluthons were crystallized at grnulitic facies and dry conditions. Within the first phase of the crystal fractionation the process is traced. These phases have also different isotope characteristics. For example, the ε(Nd)<sub>0</sub> is around -7 to -12 for the second more basic phase and – 16 to -22 for the first opx granitoids phase. It is assumed that the structures of the Banger oasis are the result of the reworking of the Yilgarn craton in the Early Proterozoic. Afterwards there was a collision and amalgamation of the supercontinent Rodinia, and at the final stage, the intrusion of opx granitoids occurred. We consider a presence of mantle material in the formation of charnockite melts. It is possible that the differentiation of tholeiitic magma and the mixing with the crustal component took place.  That might also go along with delamination of the thickened continental crust after the completion of collisional orogeny. Mixing with mantle material could occur at lowest levels of the crust and afterwards the mixed melt moves consequently to the higher levels and differentiates in chambers. Last phases are more enriched in mantle component according to modelling and geochemistry.</p>