U–Pb evidence of ∼1.7 Ga crustal tectonism during the Nimrod Orogeny in the Transantarctic Mountains, Antarctica: implications for Proterozoic plate reconstructions

Abstract

The Pacific margin of East Antarctica records a long tectonic history of crustal growth and breakup, culminating in the early Paleozoic Ross Orogeny associated with Gondwanaland amalgamation. Periods of older tectonism have been proposed (e.g. Precambrian Nimrod and Beardmore Orogenies), but the veracity of these events is difficult to document because of poor petrologic preservation, geochronologic uncertainty due to isotopic resetting, and debated geological field relationships. Of these, the Nimrod Orogeny was originally proposed as a period of Neoproterozoic metamorphism and deformation within crystalline basement rocks of the Nimrod Group, based on ∼1000 Ma K–Ar mineral ages. Later structural and thermochronologic study attributed major deformation features in the Nimrod Group to Ross-age basement reactivation. Yet, new SHRIMP ion microprobe U–Pb zircon age data for gneissic and metaigneous rocks of the Nimrod Group indicate a period of deep-crustal metamorphism and magmatism between ∼1730–1720 Ma. Igneous zircons from gneissic Archean protoliths show metamorphic overgrowths of ∼1730–1720 Ma, and an eclogitic block preserved within the gneisses contains zircons yielding an average metamorphic crystallization age of ∼1720 Ma. Deformed granodiorite that intrudes the gneisses and associated metasedimentary rocks yields a concordant zircon crystallization age of ∼1730 Ma. Despite scant petrologic evidence for these metamorphic and igneous events, the zircon ages from these diverse rock types indicate major crustal thickening, possibly due to collision, in the late Paleoproterozoic. We therefore recommend revival of the term Nimrod Orogeny to describe Paleoproterozoic tectonic events in rocks of the East Antarctic shield. Similarities in the ages of igneous and metamorphic events in the Nimrod Group and geologic units elsewhere in present-day East Antarctica, southern Australia and southwestern North America suggest they may have played a role in early supercontinent assembly. In particular, similarity with the Laurentian Mojave province is consistent with Proterozoic plate reconstructions joining ancestral East Antarctica with western Laurentia.