High-pressure granulite-facies metamorphism in central Dronning Maud Land (East Antarctica): Implications for Gondwana assembly

Abstract

Central Dronning Maud Land (DML; East Antarctica) is located in a key region of the Gondwana supercontinent. The Conradgebirge area (central DML) consists of orthogneisses, derived from both volcanic and plutonic protoliths, and minor metasedimentary rocks, intruded by Cambrian syn- to post-metamorphic plutons and dykes.Mafic-ultramafic boudins in the metavolcanic and metaplutonic gneisses from Conradgebirge consist of amphibolites and high-grade garnet-bearing pyroxene- and amphibole-rich granofels. They occur either as discontinuous levels or as pods boudinaged within highly-strained and strongly-migmatized gneisses. Bulk-rock major and trace-element compositions, together with geochemical discriminant diagrams (e.g., Th/Yb versus Ta/Yb and V versus Ti), suggest derivation from enriched mantle source for the mafic rocks boudinaged in metaplutonic gneisses, whereas a calc-alkaline signature is common for the mafic boudins in metavolcanic rocks. The microstructural study and P–T modelling of an ultramafic metagabbroic rock reveal a prograde metamorphic evolution from amphibolite-facies (ca. 0.5GPa; 500°C) up to high-P granulite-facies conditions (ca. 1.5–1.7GPa; 960–970°C). Partial melting is testified by “nanogranitoid” inclusions enclosed in garnet. An almost isothermal decompression down to ca. 0.4GPa and 750–850°C produced well-developed An+Opx-bearing symplectites around garnet. A final isobaric cooling at nearly 0.4GPa is testified by Grt coronas around high-T symplectites.The above reconstruction traces a clockwise loading-heating P-T evolution with a peak metamorphism at high-P granulite-facies conditions suggesting crustal thickening at nearly 570Ma, followed by a tectonically assisted rapid exhumation, and then, by an isobaric cooling. 40Ar-39Ar dating of amphibole and biotite at ~505–480Ma testify mineral re-equilibration at upper crustal level (T<650°C) during the isobaric cooling.This tectono-metamorphic scenario seems representative of the evolution resulting from the Neoproterozoic/Early Palaeozoic (600–500Ma) collision between parts of East- and West-Gondwana blocks that led to the final assembly of Gondwana.