Journal of Geological Society of India (Online archive from Vol 1 to Vol 78)

S. D. Iyer ¹

Affiliations
1 Geological Oceanography Division, National Institute of Oceanography, Dona Paula, Goa 403 004, IN

Abstract

Textural, mineralogical and compositional characteristics of basaltic glasses from the Central Indian Ocean show them to be altered to varying extents through their interaction with the seawater, resulting in the fonnation of palagonite. The major chemical changes are the loss of Si, Mg and Ca and a gain of Na and K whilst, Fe and Ti remained immobile. It reflects that the basaltic glasses have undergone initial to intermediate stages of palagonitisation under low temperature oxidative alterations. Subsequent accretion of ferromanganese oxides over the glasses have halted the progress of alteration.

Keywords

Geochemistry, Basalts, Palagonitisation, Central Indian Ocean.

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R. Banerjee ¹, S. D. Iyer ¹

Affiliations
1 National Institute of Oceanography, Dona Paula, Goa, 403 004, IN

Abstract

The petrography of the basalts collected in dredge hauls from the Carlsberg Ridge (3° 37' N and 64° 07' E) show systematic variations in their textural and mineralogical assemblages, from an outennost vitreous glassy to a holocrystalline interior through a variolitic zone. The silica-alkalies relation show these basalts to be of sub-alkaline nature. Variable normative compositions and Mg number, increase in alkali index, differences in Al₂O₃/CaO and FeO*/MgO ratios, variable trace element concentrations, together with variable relations of P₂O₅ TiO₂ , Ni and Cr with MgO indicate a process of fractional crystallisation of a primary magma. Chemically and mineralogically these rocks have a closer affinity to those of the other parts of the Mid-Indian Ocean Ridge.

Keywords

Petrography, Carlsberg Ridge, Trace Element Geochemistry.

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S. M. Karisiddaiah ¹, S. D. Iyer ¹

Affiliations
1 National Institute of Oceanography, Dona Paula, Goa 403 004, IN

Abstract

Rocks dredged in the vicinity of the 79°E fracture zone, in the Central Indian Basin, are sub-alkaline basalts, which are regarded as precursors to spilites. The Minerals identified are mainly albitic plagioclase, augite, olivine, and less of chlorite, epidote and opaques. The oxide variation plots indicate a mid-ocean ridge basalt trend for the samples, which have been spilitised to varying degrees. It is suggested that the basalts formed as a result of a fissure type of eruption along the fracture zone. Subsequently, some of the basalts were spilitised probably due to hydrothermal solution, possibly the result of tectonic activity along the fracture zone.

Keywords

Basalts, Spilitisation, Fracture Zone, Central Indian Basin, Petrology.

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S. D. Iyer ¹, M. Sudhakar ¹

Affiliations
1 Geological Oceanography Division, National Institute of Oceanography, Dona-Paula, Goa - 403 004, IN

Abstract

Based on various lines of evidence such as the widespread occurrence of basalts. pumice. volcanic glass shards and their transformational products (zeolites, palagonites, and smectite-rich sediments), we suggest the presence of a volcanic province in the Central Indian Basin (CIB). In addition to the rocks studied, the occurrence of many morpho-tectonic features such as seamounts, abyssal hills and major fracture zones at 73°E, 75°45'E and 79°E, have helped in correlating and in deciphering the source of the rock types. Further, the large manganese nodule fields in the CIB are seen to occur in conj unction with the volcanic materials, since the latter forms nuclei and substrates for ferromanganese deposits. It is concluded that a spatial relation exists between the occurrence of the volcanic materials and the ferromanganese deposits in the CIB.

Keywords

Marine Geology, Ferromanganese Deposits, Volcanism, Central Indian Basin.

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R. Banerjee ¹, S. D. Iyer ¹

Affiliations
1 National Institute of Oceanography, Dona Paula, Goa - 403 004, IN

Abstract

Pillow basalts from the Carlsberg Ridge at 3°35'N contain disseminated chalcopyrite, pyrite, and magnetite. The euhedral shape of the pyrite grains indicate them to be early formed and grown unobstructed while magnetite occurs as skeletal crystals. These opaque minerals indicate their origin from a rapidly cooled silicate melt and probably resulted by a process of liquid immiscibility during lava emplacement.

Keywords

Carlsberg Ridge, Basalts, Pelrology, Indian Ocean.

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B. Nagender Nath ¹, S. D. Iyer ¹

Affiliations
1 National Institute of Oceanography, Dona Paula, Goa 483 004, IN

Abstract

Two cores recovered from the flanks of Mid-India oceanic ridge in the vicinity of Vityaz fracture zone consist of discrete pyroclastic layers at various depths. These layers are composed of coarse-grained, angular basaltic microlapilli in which labradorite is the major mineral set in a glassy mesostasis. Chemically the microlapilli are similar to Mid-Indian oceanic ridge basalts. The microlapilli are suggested to have been formed by the process of granulation, transportation and deposition, during Late Quaternary when lava of low viscosity erupted from small vents on a segment of Vityaz fracture zone.

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