Evolution of the Southwest Indian Ocean and its relative geochemical evidences from magmatism

The Southwest Indian Ridge(SWIR) is an important component of global mid-oceanic ridge system, and it has been a focus for geo-scientists of the world because of its very slow average spreading rate about 14 mm/y.In this paper, we systematically reviewed,from tectonic and magmatic points of view, the formation and evolution history of SWIR and the distribution of magmatic rocks and the origin of mantle heterogeneity.The formation of SWIR began with the break-up of Gondwana and the middle part of SWIR (26°E to 42°E) was formed in the earliest stages. The present-day tectonic patterns of SWIR (e.g., multi-order ridge segments and oblique spreading) are the results of multi-stage ridge jumps and extensions. The mantle hotspots play a key role in the break-up of Gondwana and have a significant impact on geomorphology and magmatism of SWIR. Both the Bouvet and Marion hotspots have left their geochemical traces in the magmatic rocks in the western and central portions of SWIR.In addition, the basalts from the western part of SWIR show isotopic affinities with the Atlantic-Pacific-type basalts. In the middle part of SWIR(39°E to 41°E), the basalts display significant DUPAL anomalies, which are closely related to the initial formation and break-up of Gondwana.The delamination of subduction-modified lithospheric mantle in the Mesoproterozoic to Nesoproterozoic orogenic belts led to the mantle source heterogeneity in the middle part of SWIR.In the east part of SWIR(about 46°E to 52°E), the chemical anomalies are probably caused by the contamination of continental crust. To the east of the Melville transform fault, the ridge was formed in the latest stages, and the mantle source of basalts experienced metasomatism due to enriched mantle components, showing isotopic affinities with the mantle source of the adjacent Central Indian Ridge (CIR) and Southeast Indian Ridge (SEIR).