Metamorphic and crustal evolution of Australian-Antarctic Proterozoic margins

Abstract

Regional, long-lived (>>50 Myr), high temperature–ultrahigh temperature (HT–UHT) metamorphism challenges conventional notions on the thermal state of the crust as it requires the sustaining of high energy thermal gradients (>>75 °C/kbar). Regional, long-lived, high thermal gradient metamorphism occurring at continental margins further questions the relationship between continental amalgamation, the mechanisms that promote and maintain elevated heat flow, and thus the tectonic setting(s) of which this metamorphic expression is the hallmark. The Musgrave–Albany–Fraser–Wilkes Orogen (MAFWO) is outstanding in its record of regionally expansive, prolonged, and thermally extreme conditions during the Mesoproterozoic (D1/M1: 1345–1260 Ma; D2/M2: 1220–1130 Ma). Occurring along the margins of the Australian and East Antarctic Archean–Paleoproterozoic cratons, this system is geographically and geodynamically central to models for the Proterozoic amalgamation of the Australian–Antarctic continent in the Rodinia system. Despite its significance, precise details of the Mesoproterozoic metamorphic evolution of some constituent terranes of the MAFWO are unclear. A metamorphic framework for the orogen in its entirety is also non-existent. To address this deficiency, this thesis firstly presents an integrated metamorphic, geochronological and geochemical (together “petrochronology”), and isotopic study that characterises the metamorphic and crustal evolution of two key and understudied tectonic regions at opposing ends of the MAFWO. High thermal gradients at mid-crustal levels (6.0–6.5 kbar, 900 °C) prevailed in the east Musgrave Inlier, central Australia (east MAFWO), between ca. 1220–1140 Ma. Despite the protracted record of age data, the excursion to peak UHT metamorphic conditions was transient, and was followed by a slight increase in pressure. It is argued that high thermal gradient conditions occurred in response to magmatic loading and heat advection of coevally-emplaced granite. These new constraints provide insight into local-scale heat sources operating within a regional, mantle-driven thermal system. The Bunger Hills and Highjump Archipelago, East Antarctica (west MAFWO), are characterised by similarly high thermal gradient conditions to the east MAFWO (5.5–7.1 kbar, 800–960 °C and 6–9 kbar, 850–950 °C, respectively). Metamorphic age data also suggest an extremely long duration of high temperatures (>150 Myr) but peak metamorphism itself is constrained to a comparatively shorter time period (ca. 1220–1180 Ma). In contrast to the east Musgrave Inlier, peak metamorphism was followed by a pressure decrease that is interpreted to reflect the extension of thickened crust. New isotopic datasets from the Bunger Hills have allowed for re-examination of the pre-existing tectonic setting and thus potential tectonic control(s) on metamorphism. The Bunger Hills is now understood to represent a detached fragment of the Archean Yilgarn Craton that underwent (para)autochthonous crustal reworking during the Paleo–Mesoproterozoic. The tectonic evolution of the Bunger Hills is therefore strongly allied with the west Albany–Fraser Orogen, southwest Australia (also west MAFWO). In light of these new constraints, the current state of knowledge of regional, long-lived, high thermal gradient metamorphism allied with Proterozoic Australian–East Antarctic continental assembly is reappraised. Mantle-heating is concluded as the overarching thermal driver but the specific mechanism diverges between the two-stage Mesoproterozoic evolution. D1/M1 was magmatically juvenile, spatiallyconfined, controlled by the pre-D1 tectonic geometry and is reconcilable with a model of extensional accretionary orogenesis. D2/M2 was comparatively prolonged (high-T >80 Myr), consistently hot (~150 °C/kbar), and involved voluminous felsic–charnockitic magmatism with a significant mantle source contribution. These features are congruent with mantle lithosphere removal, and specifically lithosphere delamination. Precise details of the tectonic setting within D1/M1 and D2/M2 and the geodynamic trigger for D2/M2 remain unresolved, but the pre-metamorphic geological architecture is a key contributing factor. The ubiquitous metamorphic record of D2/M2, occurring across the entire MAFWO, suggests that the converging cratons had assembled prior, with the nature and timing of final amalgamation central to the generation of unusually high heat flow and the longevity of thermally anomalous conditions.