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Serpentinites from the inside comer high (6%.5'S/ 68'19.34'E) from the Northern Central Indian Ridge (NCIR) are comprised mainly of high Mg-rich lizardite and chrysotile pseudomorphs with varying morphologies. 'Mesh rim', 'window', 'hourglass' and 'bastite' are the most common textures displayed by both chrysotile and lizardite. Numerous chrysotile veins in association with cross cutting magnetite veins indicate an advanced stage of serpentinisation. The relatively high abundance of chrysotile and lizardite suggest their close association and formation a t a temperature below 250°C.Abundant 'mesh rim' and 'bastite' texture and variegated matrix reveal that the present serpentinites might have formed due to the interaction of harzburgite and seawater. Positive Eu anomaly (Eu/Eu* up to +3.38), higher L a m (up to 4.40) and NbLa (up to 6.34) ratios suggest substantial hydrothermal influence during the formation of the serpentinites.

To establish the depositional environment of the surface sediments in the Central Indian Basin, Indian Ocean, major and trace element compositions of different sediment types were studied. The study area composed of terrigenous–siliceous transition, calcareous, siliceous, and red clay sediments. The sediment type changed due to various physico‐chemical conditions with varying proportions of biogenic, hydrogenetic, early diagenetic, and detrital inputs. An oxidative environment is present throughout the basin with variable intensity. The terrigenous input was variable in all sediment types, while its signature was traceable up to 15.30°S latitude. The variable distribution pattern of Mnexcess and Feexcess indicate that apart from continental source, there were additional sources of supply for both these elements, with which most of the trace metals like nickel, copper, chromium, vanadium, yttrium, rubidium, and thorium were associated. The Al/Ti ratios of the sediments indicate a terrestrial influence of fluvial nature up to as far as 14°S latitude while, in the calcareous zone, the influence of volcanic precursors was observed. The present study envisages that Alexcess in the sediments adjacent to the ridges and fracture zones might indicate that Al in these areas is not entirely derived from the continental source.

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Mylonitic gabbro and altered gabbro were recovered from off-axis high and corner high locations at ridge-transform intersection, adjacent to Vityaz transform fault of the slow spreading (32-35 mm/yr, full spreading) Northern Central Indian Ridge. Both the varieties show signatures of extensive alteration caused due to interaction with sea water. Mylonitic gabbro represents high temperature metamorphism (∼ 700-800 • C) and comprised of hornblende mineral which exhibits well defined foliation/gneissic appearance along with dynamically recrystallised plagioclase grains frequently intercalated with magnetite-ilmenite. Altered gabbro from corner high generally includes low temperature greenschist grade (∼ 300 • C) mineralogical assemblages: chlorite, albite, quartz and locally magnesio hornblende. Crystal plastic deformation resulted in mylonite formation and often porphyroclasts of plagioclase and clinopyroxene grains, while altered gabbro locally exhibits cataclastic texture. Presence of Vityaz transform fault and adjacent megamullion at the weakly magmatic ridge-transform intersection and off-axis high locations prompted the present scenario very much conducive for hydrothermal circulation and further facilitate the exhumation of present suite of gabbro.

Abstract. Buried nodules from siliceous sediments in the central Indian Basin are morphologically variable and mineralogically consist of d-MnO~ and incipient todorokite. Compositionally they are weakly diagenetic. The sediment coarse fractions (>63 ~m) at different depths show variable abundances of micronodules, volcanic glass shards and biodebris. Dissolution of biodebris increases and abundance of micronodules decreases with increasing depth. Enrichment in Mn, Fe, Cu, Ni, Co, together with a decrease in organic carbon in the sediment column, may result from diagcnetic metal remobilization. Diagenetically remobilized trace metals might have been utilizcd for the growth of micronodules over the buried nodules.