Brown, Dillon1, Hand, Martin1, Morrissey, Laura2,1
1Department of Earth Sciences, University of Adelaide, Adelaide, SA, Australia. 2Future Industries Institute, University of South Australia, Mawson Lakes, SA, Australia
Ultrahigh- and high-pressure terranes are identified based on the occurrence of mafic eclogite-facies mineral assemblages, which effectively record burial and subduction metamorphic conditions. In such terranes however, mafic mineral assemblages are volumetrically minor compared to the continental rocks that host them, which typically preserve anhydrous quartzofeldspathic amphibolite-facies mineral assemblages. Continued debate centres around two hypotheses accounting for the petrology of the continental rocks: (1) the protoliths to the continental rocks did not respond to burial and subduction, and (2) the continental rocks developed high-pressure mineral assemblages which were subsequently overprinted during exhumation. However, less is known about continental rocks that preserve hydrous amphibolite-facies assemblages and schistose fabrics, which are also documented in high-pressure rock systems. In western Tasmania, south-east Australia, eclogite-facies mafic boudins with previously constrained subduction metamorphic conditions of 18–21.5 kbar and 650–700 °C are hosted by weakly foliated to mylonitic metapelitic continental rocks preserving hydrous amphibolite-facies assemblages dominated by siliceous muscovite, quartz, and garnet. Weakly foliated metapelites preserve relict kyanite and two possible textural generations of garnet, moderately foliated metapelites record evidence of partial melting, and mylonitic metapelites contain sillimanite within the rock fabric. Monazite LA–ICP–MS U–Pb geochronology documents two instances of monazite growth in the metapelites: a possible Mesoproterozoic growth event at c. 1385 Ma and a younger, yet poorly constrained growth event in the Cambrian. Rutile LA–ICP–MS U–Pb geochronology more precisely constrains Cambrian-aged metamorphism in the metapelites between 520–505 Ma. Monazite-garnet petrochronology reveals that Mesoproterozoic-aged monazite formed in a system with little or no influence of garnet whereas Cambrian-aged monazite formed in the presence of garnet during subduction. Metamorphic mineral equilibrium modelling of Cambrian subduction indicates that the weakly foliated metapelites best approximate peak metamorphism, recording metamorphic conditions of 13–17 kbar and 600–720 °C. Migmatitic metapelites record lower pressure conditions of 8–13 kbar and 660–740 °C and mylonitic metapelites equilibrated at 3.5–7 kbar and 590–680 °C. We infer that the amphibolite-facies metapelites record different stages of Cambrian-aged exhumation and, unlike their mafic counterparts, do not record burial or subduction. We attribute the inferred eradication of peak subduction mineral assemblages in the metapelites to the influence of fluid and localised deformation.
Dillon is a PhD student from the University of Adelaide working under the themes of metamorphic petrology, geochemistry, and petrochronology. His research focuses on understanding the geodynamic character and tectonism associated with the Cambro-Ordovician East Gondwana margin.