The metamorphic footprint of western Laurentia preserved in subducted rocks from southern Australia

Abstract

Polymetamorphic metapelitic rocks in central-west Tasmania, southern Australia, contain high-pressure mineral assemblages that formed during Cambrian-aged subduction and relict garnet with published Lu–Hf ages of c. 1285–1240 Ma. These garnet ages, along with published detrital zircon data from throughout western Tasmania and western North America, have been used to propose the presence of Mesoproterozoic Laurentian crust in western Tasmania. In this study, we combine zircon petrochronology with compositional information from the inclusion assemblages in relict garnet to extract Mesoproterozoic pressure–temperature data from subduction-overprinted rocks, which effectively constitute an interpreted remnant of Laurentian crust now residing in central-west Tasmania. The new data suggest Mesoproterozoic metamorphism involved two stages. The first event is recorded by c. 1480–1235 Ma zircon that formed in a garnet-absent, plagioclase-present, high-thermal gradient environment at pressures no greater than ~5–5.5 kbar. The second event recorded by c. 1285–1240 Ma relict garnet was characterized by the development of a moderate-pressure kyanite–plagioclase–biotite-bearing mineral assemblage, which formed at ~8.5 kbar and ~590–680°C. These pressure–temperature constraints are attributed to extension within a deep basin system associated with the cryptic East Kootenay Orogeny in North America, which coincides with the final stages of c. 1450–1370 Ma upper Belt-Purcell Basin sedimentation. Taking into account new detrital zircon U–Pb–Hf isotopic data from central-west Tasmania in this study and existing zircon provenance data from throughout western Tasmania and the Belt-Purcell Basin, our results strengthen the hypothesis of a Laurentian footprint that potentially encompasses much of western Tasmania and relates to both Nuna and Rodinian tectonism.