Late Pan-African granitoids from the Grove Mountains, East Antarctica: Age, origin and tectonic implications

Abstract

The Grove Mountains of East Antarctica are an inland continuation of the Pan-African Prydz Belt. The area is made up of high-grade metamorphic complex and numerous intrusive granitoid bodies including foliated charnockite, sheeted granite and charnockitic and granitic dykes. U–Pb zircon dating reveals that the charnockite, charnockitic dyke, granite and granitic dyke were emplaced in succession during the period from 550 to 500 Ma. Trace element abundances indicate that all the granitoids are of A-type affinity, characterized by enrichment in REE, Y, Ba, Sr, Ga and HFS elements (Zr, Nb, Th). Sr–Nd isotopic analyses yielded high initial 87Sr/ 86Sr ratios (0.7095–0.7156) and low ɛ Nd( T) values (−9.2 to −13.4). Depleted mantle-based Nd model ages range from 2.0 to 2.3 Ga. These geochemical and isotopic signatures point to their ultimate derivation from a long-term enriched subcontinental lithospheric mantle. The charnockites of the Grove Mountains have low SiO 2 contents and high abundances of K and Ti; they are likely produced by partial melting of an alkaline basaltic protolith at elevated temperatures (980–1050 °C) at deep crust. The protolith could represent underplated magma derived from metasomatized Paleoproterozoic mantle during the late stage of the orogeny. The associated charnockitic dykes have somewhat higher SiO 2 and more distinct negative Eu, Nb, Sr, P and Ti anomalies in spidergrams, suggesting that the dykes have undergone a greater degree of fractional crystallization. Granites were emplaced later than charnockites by 20–40 Ma. However, these two rocks have rather similar trace element and Sr–Nd isotopic characteristics, indicating that they share the same enriched basaltic source. For the granitic rocks alone, two types with different REE patterns and emplacement ages could be identified, suggesting two-stage partial melting for their derivation. Relative to charnockites, the dominant intrusive granites have slightly higher ɛ Nd( T) values, which could be compared with most metamorphic rocks from the Grove Mountains, hence a crustal contamination is implied in their petrogenesis. Granitic dykes are less silicic than granites; they were probably generated by a higher degree of melting of the same source. Geothermobarometry of the granitoids (charnockites and granites) and associated metamorphic rocks demonstrates that the Grove Mountains underwent an earlier near-isothermal decompression of an overthickened crust, followed by a near-isobaric cooling accompanying deep emplacement (≥15 km) of granitoids during the time interval of 550–500 Ma. Implicitly, the Prydz Belt must represent a Pan-African orogen formed by collision between the Indo-Antarctic and Australo-Antarctic continental blocks.