Cambrian eclogite-facies metamorphism in the central Transantarctic Mountains, East Antarctica: Extending the record of early Palaeozoic high-pressure metamorphism along the eastern Gondwanan margin

Abstract

The Nimrod Complex in the Miller and Geologists ranges of the central Transantarctic Mountains comprises Mesoarchean and Palaeoproterozoic crust that was incorporated into the Ross Orogen during metamorphism and deformation along the eastern Gondwanan convergent margin in the late Neoproterozoic and early Palaeozoic. In the Miller Range, boudinaged mafic domains are hosted within upper-amphibolite- to granulite-facies mylonitic rocks that were deformed during the Ross Orogeny. The mafic rocks preserve evidence of a precursor eclogite-facies mineral assemblage containing garnet, omphacite, amphibole, quartz and rutile. Mineral equilibria forward modelling and Zr-in-rutile thermometry indicate that the eclogite assemblage records peak pressure-temperature conditions of 16–18 kbar and 675–760 °C. A secondary metamorphic assemblage of clinopyroxene-plagioclase symplectite, plagioclase coronae rimming garnet, fine-grained amphibole, and interstitial ilmenite orthopyroxene overprint the high-pressure assemblage. Compositions and modal proportions of retrograde garnet, orthopyroxene and amphibole suggest near isothermal exhumation to conditions of 8–10 kbar and 660–840 °C, followed by a long-lived mid-crustal residence. In-situ LA–ICP–MS UPb dating of zircon and rutile yields concordant ages of 536 ± 14 Ma and 536 ± 20 Ma, respectively. The Cambrian-aged zircons show an absent to weakly negative Eu anomaly signature and flat heavy rare earth element (HREE) distributions that are typical of zircons crystallised in the absence of plagioclase and presence of garnet, and which is diagnostic of eclogite-facies conditions. The Cambrian-aged zircon population contrasts with older, highly discordant Palaeoproterozoic zircons that have negative Eu anomalies and are HREE enriched. These zircons are interpreted to be protolithic, and their ages are consistent with existing geochronology from host rocks in the region. The Cambrian-aged zircon and rutile are inferred to represent eclogite-facies metamorphism associated with Ross orogenesis. The early Palaeozoic eastern Gondwanan margin contains a diverse array of eclogite precursors that document contrasting geodynamic styles. Eclogites in northern Victoria Land and central Tasmania record comparatively warm thermal gradients, short-lived high-pressure conditions, and rapid exhumation within their continental crust hosts, whereas oceanic eclogites in the southern New England Fold Belt record refrigerated, long-lived convergent-margin metamorphism. The Miller Range eclogite-facies rocks in this study appear to have been derived from shallow subduction of ancient continental crust, after which they stalled at thermally elevated mid-crustal depths prior to their final exhumation. The discovery of Cambrian-aged eclogite-facies rocks in the Miller Range expands the spatial footprint of high-pressure metamorphism during the Ross Orogeny along the early Palaeozoic eastern Gondwanan margin.