Pan-African Tectonism in the Western Maud Belt: P–T–t Path for High-grade Gneisses in the H.U. Sverdrupfjella, East Antarctica

Abstract

Extensive high-grade polydeformed metamorphic provinces surrounding Archaean cratonic nuclei in the East Antarctic Shield record two tectono-thermal episodes in late Mesoproterozoic and late Neoproterozoic–Cambrian times. In Western Dronning Maud Land, the high-grade Mesoproterozoic Maud Belt is juxtaposed against the Archaean Grunehogna Province and has traditionally been interpreted as a Grenvillian mobile belt that was thermally overprinted during the Early Palaeozoic. Integration of new U–Pb sensitive high-resolution ion microprobe and conventional single zircon and monazite age data, and Ar–Ar data on hornblende and biotite, with thermobarometric calculations on rocks from the H.U. Sverdrupfjella, northern Maud Belt, resulted in a more complex P–T–t evolution than previously assumed. A c. 540Ma monazite, hosted by an upper ampibolite-facies mineral assemblage defining a regionally dominant top-to-NW shear fabric, provides strong evidence for the penetrative deformation in the area being of PanAfrican age and not of Grenvillian age as previously reported. Relics of an eclogite-facies garnet–omphacite assemblage within strainprotected mafic boudins indicate that the peak metamorphic conditions recorded by most rocks in the area (T ¼ 687–758 � C, P ¼ 9� 4–11� 3kbar) were attained subsequent to decompression from P >12� 9kbar. By analogy with limited U–Pb single zircon age data and on circumstantial textural grounds, this earlier eclogitefacies metamorphism is ascribed to subduction and accretion around 565Ma. Post-peak metamorphic K-metasomatism under amphibolite-facies conditions is ascribed to the intrusion of postorogenic granite at c. 480Ma. The recognition of extensive PanAfrican tectonism in the Maud Belt casts doubts on previous Rodinia reconstructions, in which this belt takes a pivotal position between East Antarctica, the Kalahari Craton and Laurentia. Evidence of late Mesoproterozoic high-grade metamorphism during the formation of the Maud Belt exists in the form of c. 1035Ma zircon overgrowths that are probably related to relics of granulite-facies metamorphism recorded from other parts of the Maud Belt. The polymetamorphic rocks are largely derived from a c. 1140Ma volcanic arc and 1072 � 10Ma granite.