Hand, MartinMorrissey, LauraKelsey, DavidMarch, Samantha Nicole2025-07-212025-07-212025https://hdl.handle.net/2440/146174The formation of metamorphic mineral assemblages reflects an interplay between bulk composition, pressure, temperature, and kinetics. Contextualising these assemblages and their associated pressure–temperature (P–T) constraints in the broader evolution of a rock system therefore requires careful evaluation, particularly in polymetamorphic terranes. Early high-temperature metamorphism in such terranes can create a largely dehydrated rock system, where localised retrograde hydration can lead to a spatially variable metamorphic response during overprinting. If unrecognised, differences in mineral assemblage responsivity may lead to misinterpretations of reaction textures and geochronological data, potentially resulting in erroneous tectonic models. This thesis examines mineral responsivity and its implications for how tectonic events are recorded in two polytectonic regions with contrasting thermobarometric character. The Western Gneiss Region (WGR) in Norway is a high-pressure (HP) to ultrahigh-pressure (UHP) terrane metamorphosed during the c. 420 Ma subduction of continental crust, which had previously undergone high-grade metamorphism during the Mesoproterozoic to early Neoproterozoic. The southern North Australian Craton (NAC) in central Australia records late Paleoproterozoic (c. 1640 Ma) magmatism and localised high-grade metamorphism, overprinted by high geothermal gradient Mesoproterozoic (c. 1150 Ma) metamorphism. Despite being subducted to depths >30 kbar during the c. 420 Ma Caledonian Orogeny, only a small volume of rocks in the WGR preserve HP–UHP conditions. The majority of the terrane consists of amphibolite-facies felsic gneiss, suggesting either widespread retrogression during exhumation from mantle depths, or a general lack of mineral responsivity during subduction. In the Lavik area of the southern WGR, volumetrically minor mafic eclogite (<2 vol%) are enclosed by amphibolite-facies quartzofeldspathic gneiss. Peak conditions in mafic eclogite are 23–25 kbar and 575–605 °C. Although mafic eclogite is hydrous and comparatively low temperature, there is little evidence for mineral transformation during prograde burial. Instead, garnet inclusion assemblages and mineral compositions are indicative of significant reaction overstep at eclogite-facies conditions, implying prolonged metastability during early burial. Mafic eclogite occurs within localised high-strain domains, where deformation may have acted as a catalyst for eventual mineral responsivity. Consistent with this, adjacent low-strain mafic rocks contain well-preserved pre-Caledonian assemblages, suggesting mafic rocks did not uniformly respond to subduction. The enclosing amphibolite-facies quartzofeldspathic gneiss contains a garnet-bearing assemblage with peak conditions of 11–14 kbar and 670–740 °C, associated with up–P metamorphism. The elevated thermal gradient recorded by quartzofeldspathic gneiss (~60 °C kbar-1) relative to mafic eclogite (~20 °C kbar-1), combined with structural relationships, indicate the assemblage in the host gneiss formed after the eclogite was exhumed. This phase of metamorphism is interpreted to record regional transtension and compensatory inflow of material during crustal-scale necking. Crucially, there is no evidence that the volumetrically dominant quartzofeldspathic gneiss responded during Caledonian HP subduction. In the absence of the subduction record partially preserved by volumetrically minor high-strain mafic rocks, there would be no indication in the Lavik region that continental subduction had occurred. Along the southern margin of the NAC, Paleoproterozoic-aged U–Pb zircon and monazite geochronology from high-temperature (HT) to ultrahigh-temperature (UHT) Mg–Al-rich assemblages has historically been used to suggest (U)HT collision between the Aileron and Warumpi Provinces during the c. 1640 Ma Liebig Orogeny. Contrary to this long-held hypothesis, Lu–Hf garnet geochronology couples HT–UHT mineral assemblages with the c. 1150 Ma Musgrave Orogeny, illuminating the polymetamorphic history of this terrane and the inhibited response of accessory minerals during metamorphic overprinting. Early metamorphism during the 4–5 kbar and 770–800 °C Liebig Orogeny created refractory Mg–Alrich bulk compositions, which were then variably overprinted during the 8–10 kbar and 850–915 °C Musgrave Orogeny. Accessory mineral response during the Musgrave Orogeny was limited due to the refractory nature of these rocks, where the suppression of melting also restricted accessory mineral (re)crystallisation. Similar thermal conditions, coupled with the limited responsiveness of accessory minerals during the subsequent Musgrave Orogeny create the appearance of a single-cycle (U)HT metamorphic event. Supportive of the polymetamorphic record proposed from Mg–Al-rich rocks, neighbouring rocks that did not develop refractory bulk compositions during M1 metamorphism underwent migmatisation, HT metamorphism, and crucially, accessory mineral (re)crystallisation during the Musgrave Orogeny. In addition to accessory mineral responsivity, the propensity of rocks to melting also affects thermal constraints. Migmatitic metapelitic and metamafic granulites in the Warumpi Province record peak temperatures of ~820 °C, whereas spatially associated refractory Mg– Al-rich metapelitic granulites record higher peak temperatures between 865 °C and >920 °C. This temperature disparity reflects the energetic demands of crustal anatexis, where fertile rocks may intersect the solidus and commence melting at lower–T than refractory rocks. The conversion of heat energy to temperature rise is buffered during partial melting, resulting in lower modelled temperatures. In contrast, Mg–Al-rich rocks with substantially higher solidi will experience relatively unimpeded temperature rise. In order for migmatitic rocks to record the same peak temperatures as refractory rocks, the system would require an additional 22–37% enthalpy input. Mg–Al-rich rocks are suggested to have formed and been preconditioned during early hydrothermal alteration and metamorphism. Thermal buffering could plausibly explain local-scale temperature variations in many granulite-facies terranes. The revised understanding for accessory mineral systematics in rocks that once defined the c. 1640 Ma Liebig Orogeny necessitates a reassessment of both this major event and the subsequent c. 1150 Ma Musgrave Orogeny in the southern NAC. Magnetotelluric imaging of a conductivity interface between the Warumpi Province and the Aileron Province to the north has underpinned suggestions of collision during the Liebig Orogeny, where the Warumpi Province would be an exotic continental ribbon that collided with the southern margin of the NAC. However, mineral equilibria forward modelling of representative Liebig-aged subdomains in polymetamorphic rocks from the Warumpi Province reveal peak metamorphic conditions of 4–5 kbar and 770–800 °C, inconsistent with significant crustal thickening. Instead, these P–T constraints indicate a high thermal gradient regime. Correspondingly, outcrop relationships exhibit little evidence of significant deformation during the Liebig Orogeny. Analysis of the pre-existing record of detrital zircon geochronology and isotopic signatures shows similarities between the Aileron and Warumpi Provinces, further supporting a shared history in the southern NAC. Rather than a collisional regime, these observations are consistent with an extensional setting during the Liebig Orogeny. In this model, the Warumpi and Aileron Provinces formed a contiguous unit, with southward-retreating subduction inducing extension and magmatically-driven low–P and high–T metamorphism with limited associated deformation. Given the reinterpretation of (U)HT metamorphic conditions as being associated with the Musgrave Orogeny rather than the Liebig Orogeny, along with the increasing recognition of documented Mesoproterozoic metamorphism, a reassessment of the tectonic significance of the Musgrave Orogeny in the southern NAC is required. Lu–Hf garnet geochronology and mineral equilibria forward modelling of samples spanning the Aileron Province, Warumpi Province, and Casey Inlier reveal a regionally extensive high–T response to the Musgrave Orogeny throughout the southern NAC. Lu–Hf garnet dates range from 1160–1130 Ma, corresponding to the timing of metamorphism in the Musgrave Province, south of the Amadeus Basin. High thermal gradient metamorphism throughout these geological units (80–195 °C kbar-1) suggest a shared history in which the Musgrave Province is the southernmost extension of the NAC. The intrusion of the Pitjantjatjara Supersuite in the Musgrave Province is interpreted to have caused high–T contact metamorphism, where subsequent ductile outflow of hot material induced similar high thermal gradient metamorphism in the northward Casey Inlier, Warumpi Province, and Aileron Province.enMetamorphismU-Pb geochronologyLu-Hf geochronologygeochemistryP-T modellingNorth Australian CratonWestern Gneiss RegionThesis