Tectonic evolution of Madagascar over three billion years of Earth's history

Abstract

Earth’s last major supercontinent, Gondwana, formed during the Neoproterozoic by convergence along the East African Orogen. Madagascar contains at least two Gondwana-forming suture zones, and hosts several suites of Neoproterozoic rocks that have been correlated with other Rodinian and Gondwanan terranes. Madagascar also hosts several Paleoproterozoic sedimentary sequences, which provide constraints on the Earth’s plate configuration from the Paleoproterozoic through to the early Cambrian. These factors make Madagascar an important place to study supercontinent assembly and dispersal throughout Earth’s history. In this thesis we integrate a range of isotopic, geochemical and structural datasets to decipher the timing and nature of tectonic events in these regions. A primary focus of this research is to constrain the location and timing of the controversial Betsimisaraka Suture in Madagascar and India. We have used detrital zircon U–Pb and Lu–Hf isotopes to constrain the location of this suture. We propose that the Karwar Block of western peninsular India is an extension of the western Dharwar Craton and not part of the Antananarivo Domain of Madagascar as has been suggested in some models. We also suggest that India was paleogeographically isolated from central Madagascar during the Paleoproterozoic. From northern Madagascar, we use magmatic zircon Hf and O isotopes to demonstrate that the northern component of the Bemarivo Domain is distinct from the southern part of the Bemarivo Domain. We propose that the northern terrane formed in a juvenile arc system that included the Seychelles, the Malani Igneous Suite of northwest India, Oman, and the Yangtze Belt of south China, which at the time were all outboard from continental India and south China. The final assembly of northern Madagascar and amalgamation of the northern and southern terranes occurred along the Antsaba subduction zone, with collision occurring at c. 540 Ma. In central Madagascar we have integrated remote sensing, field data and medium-temperature thermochronology (Rb–Sr mica and U–Pb apatite) to unravel complex deformation in the Ikalamavony and Itremo domains. We suggest deformation in west-central Madagascar formed in response to the c. 650 Ma collision of Azania with Africa along the Vohibory Suture in southwestern Madagascar. In eastern Madagascar, deformation is syn- to post-550 Ma, which formed in response to the final closure of the Mozambique Ocean along the Betsimisaraka Suture. A series of Paleoproterozoic sedimentary sequences overlie many of the domains in Madagascar. New U–Pb and Lu-Hf isotopes from a range of these sequences are indistinguishable from each other, and we therefore refer to these sequences as the Greater Itremo Group. We correlate these with equivalent sedimentary sequences in the Muva Supergroup in the Tanzania Craton and define an epicontinental Itremo-Muva Basin. We show that the Greater Itremo Group is distinct from Indian samples, and therefore India and Madagascar were not contiguous during the Paleoproterozoic. The integration of various isotopic datasets from Madagascar and its paleo-neighbours have provided new insights into the evolution of supercontinent assembly and dispersal during the Neoproterozoic. Although there remain some areas that require further study, this multi-disciplinary research supports a model where central Madagascar and India amalgamated during the Neoproterozoic along the Betsimisaraka Suture.