Zhou, Dr Jiu-long1, Li, Prof. Xian-Hua1,2, Li,Prof. Zheng-Xiang3
1State Key Laboratory Of Lithospheric Evolution, Institute Of Geology And Geophysics, Chinese Academy Of Sciences, Beijing, China, 2College of Earth Sciences, University of Chinese Academy of Sciences, Beijing, China, 3Earth Dynamics Research Group, ARC Centre of Excellence for Core to Crust Fluid Systems (CCFS) and The Institute for Geoscience Research (TIGeR), School of Earth and Planetary Sciences, Curtin University, Perth, Australia
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We present new and existing geological and geochemical observations from Madagascar and Seychelles, both are believed to have faced the Mirovoi superocean during Rodinia time, in order to shed new light on the evolution of Rodinia’s western margin. (1) 870-750 Ma plutonic rocks in central Madagascar (i.e., the Imorona-Itsindro Suite) have a general bi-modal nature. The felsic components have a ferroan A-type composition, were likely derived from ancient continental crust, and show a positive zircon εHf(t) excursion during a 790 Ma high-flux magmatic pulse. The gabbroic components host layered Fe-Ti-V oxides and chromitites, and were likely derived from an enriched lithospheric mantle with no clear asthenospheric input. (2) Clastic sedimentary rocks in central Madagascar coeval with the Imorona-Itsindro Suite contain detrital zircons of dominantly Archean to Paleoproterozoic ages. (3) Neoproterozoic granitoids in Seychelles were emplaced during two discrete magmatic pulses: an older and weaker one at 810 Ma and a younger and more intense one at 760–745 Ma. Primary low-δ18O signals are absent from the 810 Ma granitoids but characterize the 750 Ma granitoids. The 750 Ma Mahé-type (Nd-isotopically juvenile) granitoids appear to display a stronger 18O-depletion than the 750 Ma Praslin-type (Nd-isotopically enriched) granitoids. Some of the 750 Ma 18O-depleted rocks even document intra-zircon δ18O decreases from core to rim. Thus, the Seychelles granitoids likely acquired their low-δ18O signatures through the dynamic magma process of crustal cannibalization, instead of through source inheritance from a pre-existing basement. (4) Neoproterozoic magmatism in the Bemarivo Terrane (northern Madagascar) also has a two-episodes age-distribution: an older 760–745 Ma one represented by the Antsirabe Nord Suite, and a younger 740–700 Ma one represented by the Manambato Suite and the Daraina-Milanoa Group. The Antsirabe Nord granitoids are ferroan A-type in chemistry, have negative ɛHf(t) values and low-δ18O signatures, and were likely generated by remelting of an ancient basement source. In contrast, the Manambato granitoids and Daraina-Milanoa rhyolites are chemically calc-alkaline and Hf-isotopically juvenile, with progressively disappearing low-δ18O signatures. Generation of these rocks involves a significant input of asthenospheric materials.
The pre-745 Ma Neoproterozoic igneous rocks in central Madagascar, the Bemarivo Terrane, and Seychelles bear strong resemblance to each other in geochemistry and petrogenesis, and likely have been produced within continental rift(s). However, the post-745 Ma igneous rocks, only found in the Bemarivo Terrane, likely signified the onset of arc magmatism. We thus conclude that Rodinia’s western margin experienced a transition from a protracted passive margin setting at 870–745 Ma to an arc setting at 740–700 Ma at this segment of the eastern edge of the Mirovoi superocean. The 740 Ma subduction initiation age here is significantly younger than previously proposed.
Jiu-Long Zhou obtained his Ph.D. degree from the China University of Geosciences (Beijing) in 2016, and then received postdoctoral training at the Institute of Geology and Geophysics, Chinese Academy of Sciences. His research interests focus on utilizing geochemical methods to understand supercontinent cycles and associated igneous processes.