Polymetamorphic evolution of the Vestfold Block in East Antarctica and implications for the amalgamation of terranes

Abstract

The Vestfold Block, a typical polymetamorphic Archean terrane in East Antarctica, is a key area to understand amalgamations of Rodinia and East Gondwana continents. However, multiphase overprinting makes it difficult to determine the timing and nature of each tectonothermal event. In this study, we present P–T estimates, zircon, monazite U(–Th)–Pb and biotite/K–feldspar Rb–Sr isochron ages of paragneisses from the SE Vestfold Block. One paragneiss sample, which is assigned to the Chelnok Paragneiss, has experienced a protracted metamorphism from the Neoarchean to the early Paleoproterozoic. Phase equilibria modeling constrained the peak P–T conditions to 7.2–9.6 kbar and 850–880 ℃, and the post–peak metamorphism to 4.2–5.6 kbar and 720–790 ℃, respectively. On the other hand, a paragneiss sample close to the ice sheet documented a high–grade metamorphic event at 918 ± 23 Ma, with peak P–T conditions of 6.0–8.0 kbar and 860–880 ℃. Biotite/K–feldspar Rb–Sr dating for these two samples yields isochron ages of 474 ± 12 and 442 ± 7 Ma, respectively, representing the cooling ages of the Pan–African reworking. Collectively, an integrated application of diverse chronometers, combined with published data, indicates that the Vestfold Block may have experienced at least three major thermal events with variable intensities and extents. Initially, the supracrustal rocks in this region pervasively underwent a protracted high–grade thermal event from the Neoarchean to the early Paleoproterozoic, which formed the backbone of the block. Thereafter, the southern Vestfold Block experienced a Grenvillian granulite facies metamorphism, indicating that the Vestfold Block has been locally involved in the Rayner orogeny (i.e. the late Mesoproterozoic/early Neoproterozoic collision between the Indian craton and East Antarctica). Ultimately, the whole Vestfold Block may have been reworked under relatively low temperatures during the Pan–African Prydz tectonic event.