Morrissey, Laura1, Halpin, Jacqueline2, Hand, Martin3, Payne, Justin4
1 Future Industries Institute, University of South Australia, Adelaide, Australia; 2 Institute for Marine and Antarctic Science, University of Tasmania, Hobart, Australia; 3 Department of Earth Sciences, University of Adelaide, Adelaide, Australia; 4 UniSA STEM, University of South Australia, Adelaide, Australia
The Mesoproterozoic Rayner Complex, Antarctica, and the Eastern Ghats Province, India, belong to a single terrane that records some of the highest known temperatures for regional metamorphism on Earth. This formerly contiguous terrane separates Archean cratons of India and Antarctica, and the ultra-high temperature metamorphism is thought to record the collision between cratonic India and Antarctica. The timing of ultra-high temperature metamorphism at c. 1000–900 Ma corresponds to the timing of Rodina amalgamation, making the Rayner–Eastern Ghats terrane one of the largest known Rodina-aged belts and a key constraint in Rodinia reconstructions. However, the position of the Rayner–Eastern Ghats terrane in the Rodinia supercontinent is debated, with the most recent reconstructions suggesting the Rayner–Eastern Ghats terrane may have been isolated from main Rodina supercontinent. One of the challenges of placing the Rayner Complex into a supercontinent construct relates to the paucity of information about the protoliths and geodynamic setting of this region prior to ultra-high temperature metamorphism.
Mac.Robertson Land in East Antarctica provides a transect through the Mesoproterozoic Rayner Complex into the Archean–Paleoproterozoic crust of cratonic East Antarctica. Outcrops extend from the Mawson Coast in the north, through the northern Prince Charles Mountains (nPCM) and Fisher Terrane, to the Archean southern Prince Charles Mountains (sPCM). New Lu–Hf isotopic data from magmatic protoliths in the Fisher Terrane shows that this region contains the some of the oldest and most isotopically juvenile protoliths in the Rayner Complex. Magmatic rocks in the Fisher Terrane are c. 1300 Ma calc-alkaline rocks that were reworked at c. 1020–980 Ma. The c. 1300 Ma rocks have εHf(t) of +6 to +10, and the younger granites have εHf(t) of +2 to +5.5. In contrast, limited data from 990–950 Ma granites and charnockites from the nPCM suggests these are more evolved, with εHf(t) of -5 to 0. Previously published data from c. 1100–980 Ma magmatic protoliths along the Mawson Coast have even more evolved εHf(t) values of -10 to + 2.
The isotopically juvenile nature of the calc-alkaline rocks in the Fisher Terrane suggest that it was an arc built on highly extended, juvenile crust with little input from evolved sources. The shift to more evolved εHf values from south to north may confirm models that place the Rayner Complex in a highly extended back arc setting off the margin of cratonic India. This setting involved nearly continuous magmatism from c. 1490–950 Ma, with peak metamorphic temperatures reached between c. 970–900 Ma. Importantly, there is no evidence of magmatism after c. 950 Ma and the terrane is characterised by extremely slow cooling. Tectonic models that place the Rayner Complex in an isolated setting during Rodinia amalgamation must account for the fact that there is no evidence for continued consumption of oceanic crust after 950 Ma.
Laura Morrissey completed a PhD in metamorphic petrology and tectonics at the University of Adelaide in 2016. She is now a research fellow at the University of South Australia.