REMELTING THE GONDWANAN MANTLE

The Southwest Indian Ridge is ultraslow spreading ridge, with a full rate of ~14 mm/yr. It extends ~7,700 km from the Bouvet to the Rodriguez Triple Junction (Fig. 1b). It formed at ~150 Ma following the breakup of Gondwana at 184 Ma. From 80 Ma the ridge propagated rapidly 2500 km to the northeast from the Discovery II Transform to its present position at the Rodriguez Triple Junction. The ridge has two principle sections: the western SWIR extending from the Bouvet TriJ to the Andrew Bain FZ, and the Marion Rise extending from the latter to close to the Rodriguez TJ. The Marion Rise consists of a rugged 2000-km long platform, which plunges ~2 km 1400 km to the NE to 63°E. The Andrew Bain FZ marks a sharp break in isotopic, major and trace element basalt and residual mantle chemistry, as exemplified by the storied DUPAL isotopic anomaly to the east, as opposed to the South Atlantic isotopic province to the west. A more gradual break occurs east of the Gallieni FZ at 52°E. The platform basalt and peridotite are the most refractory on the SWIR, yet in the presence of abundant peridotite, the crust is thin and discontinuous. Thus, the platform is supported by a buoyant mantle depleted in an earlier melting event. We attribute this to the result of recycling the different mantle provinces from beneath Gondwana; reflecting the pre-breakup lateral transition from Archaean Craton to the East and West, and the Proterozoic East African Orogenic Belt sandwiched in between. The broad Proterozoic EAOB represents a orogenic cycle from ~950 to 450 Ma, encompassing collapsed island arcs, and smaller cratonic fragments, sandwiched between two Archaean cratons that collided with it to form Gondwana ~600 Ma. The Andrew Bain and Gallieni FZ’s coincide with, and likely nucleate from the western and eastern boundaries of the orogenic belt. These latter represent tectonic escape-related shear zones formed during the collision of East and West Gondwana. This juxtaposition of Archaean Cratons, and mixed Proterozoic crust, mantle, and cratonic microplates can provide an explanation for the very different SWIR isotopic and major element domains whose boundaries more or less coincide with the elevated platform between the Gallieni and Andrew Bain FZ’s, and the region of refractory basalt and peridotite