Isotopic behavior and age interpretations of U-Pb, Sm-Nd, and 40Ar/39Ar systems in polymetamorphic granulite terranes: A case study from the Prydz Belt and adjacent Vestfold Block, East Antarctica

Abstract

A combined zircon U-Pb, garnet-based mineral−whole-rock Sm-Nd, and hornblende/biotite 40Ar/39Ar geochronological study was conducted on polymetamorphic granulite-facies rocks from the Prydz Belt and adjacent Vestfold Block in East Antarctica. Zircon U-Pb data and zircon-garnet rare earth element distributions indicate that granulite-facies mineral assemblages of the Meknattane Nunataks−Hamm Peak area and the Reinbolt Hills in the Prydz Belt were formed at ca. 1000−900 Ma. Some Grenville-aged (i.e., late Mesoproterozoic to early Neoproterozoic) zircons underwent partial U-Pb resetting, without new growth, during Pan-African-aged (i.e., late Neoproterozoic to Cambrian) metamorphism. Mineral−whole-rock Sm-Nd dating yielded isochron ages of 865−672 Ma for the Meknattane Nunataks−Hamm Peak area, 504−483 Ma for the Reinbolt Hills, and 670−589 Ma for the Vestfold Hills. These results suggest that Pan-African-aged reworking was inhomogeneous, leading to either partial or complete resetting of the garnet Sm-Nd isotopic system in different areas. Apparently, Sm-Nd ages might have geological meaning only when the isotopic system of garnet (including other contemporaneous minerals) has been completely reset. In this regard, garnet-based Sm-Nd geochronology is less robust when applied to polymetamorphic granulite terranes. Hornblende 40Ar/39Ar dating produced an older age of 526 Ma for the Vestfold Hills, whereas biotite 40Ar/39Ar ages showed slight differences among the studied areas, with ages of 514−497 Ma for the Meknattane Nunataks−Hamm Peak area, 482−467 Ma for the Reinbolt Hills, and 520−509 Ma for the Vestfold Hills. These age patterns imply that 40Ar/39Ar dating of minerals can constrain the cooling time of the latest metamorphic event only, and cooling times to the Ar closure temperature of biotite differ among different parts of the Prydz Belt. Integration of the new results with available Sm-Nd and 40Ar/39Ar data indicates that the Prydz Belt underwent rapid tectonic uplift and cooling from lower to mid- and upper-crustal depths during postorogenic evolution. The Vestfold Block might have been located at the margin of the high-temperature Prydz Belt and underwent relatively low-temperature reworking and earlier cooling during the Pan-African-aged orogeny.