Morphotectonic and petrological variations along the southern Central Indian Ridge

Abstract

In order to ascertain the effect of geomorphic and tectonic domains on the formation, enrichment, and ascension of the ridge axis melt, structural and petrological data from a nearly 300-km-long axial stretch along the slow-to-intermediate-spreading (40–60 mm/year) southern Central Indian Ridge (SCIR) were studied. The stretch, approximately between 20°30′S and 23°07′S, was disturbed by two major tectonic features—Egeria transform fault in the north and the Gemino Fracture Zone in the south—besides eight other discontinuities of variable dimensions. This stretch was chosen to test the petrological variations and mechanisms of magma supply in four distinct geomorphic and tectonic regimes: a Ridge-Crest-Flank-Valley (RCFV), a Neo-Volcanic Zone (NVZ), a large transform discontinuity (LTD), and an overlapping spreading centre (OSC). The major and trace element geochemistry of 44 glass and 47 whole rocks, extent and depth of melting (Na8 and Fe8, respectively), and melt pristinity (Mg#) of the magma indicate that rocks along this stretch were probably sourced from a reasonably primitive melt generated at a relatively greater depth and later got accumulated in pockets at a shallower level before eruption. Petrochemical analysis, and isotopic composition and ratios suggest that in contrast to largely N-MORB type of rocks at RCFV and OSC areas, the rocks from LTD and NVZ locations show signatures of enrichment to transitional (T) and enriched (E) basalts. A model explaining possible processes of enrichment and ascending framework of the melt at different tectonic regimes along SCIR are discussed.