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

S. F. Sethna ¹, Priya Javeri ¹

Affiliations
1 Department of Geology, St. Xavier's College, Mumbai-400001, IN

Abstract

Major, trace and rare earth element analyses of spilitic basalts and some dolerite dykes and microprobe mineral analyses of a spilitic basalt are reported. It is postulated that these rocks represent subaqueous flows erupted under fairly shallow water which was insufficient for complete spilitisation of the basalt. The process of metasomatism under subaqueous conditions is considered to have been responsible for the alteration of tholeiite to spilite.

Keywords

Petrology, Geochemistry, Spilites, Deccan Traps, Daman.

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S. F. Sethna ¹, Priya Javeri ¹

Affiliations
1 Department of Geology St. Xavier's College Bombay - 400001, IN

Abstract

The occurrence of a trachyte intrusive into the Deccan Trap basalts at Sutrakar near Talasari is being reported for the first time. Major, trace and rare earth element analyses are given along with the petrography. It has been speculated that the origin of the trachytic magma is similar to that at Bombay-Salsette area where it has been postulated earlier by Lightfoot et al. (1987) that the trachytic magma could be generated as a partial melt from a gabbroic sill complex in the lower part of the crust due the crustal thinning and sagging during the formation of the Panvel Flexture.

Keywords

Trachyte, Petrology, Deccan Trap, Maharashtra.

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S. F. Sethna ¹, Prajakta Kothare ¹, Priya Javeri ¹

Affiliations
1 Department of Geology, St. Xavier's College, Mumbai - 400 001, IN

Abstract

Distinct vertical rhythmic layering is seen in one of the dykes 68 km southeast of Rajkot. The dyke extends for almost 50 km in length but has a limited width of 10 to 15 metres. The vertical layering is due to alternating layers rich in plagioclase and pyroxene, which is encountered mainly in the central part of the dyke. The layering is formed due to molecuIar diffusion at the front of cooling and crystallization with the temperature gradient catching up gradually to form the next layer. Alternatively it may be possible that layering is due to multiple injections of melt into the fracture that was responsible for disturbing the equilibrium from time to time.

Keywords

Petrology, Deccan Trap, Olivine Dolerite Dyke, Layering, Saurashtra.

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S. F. Sethna ¹, Prajakta Kothare ¹, B. S. Sethna ¹, Priya Javeri ¹

Affiliations
1 Department of Geology, St. Xavier's College, Mumbai - 400 001, IN

Abstract

Forty-five major dykes intruding the Deccan Trap were studied in the central and south-eastern parts of Saurashtra. There are some distinct structural differences seen between the area north of Amreli and that south of Amreli. The lava flows north of Amreli are practically horizontal, while south of Amreli they seem to be forming a major southerly-plunging antiform. In the area north of Amreli, dolerite dykes are invariably enriched in fresh olivine and are associated with one ankaramite dyke, a hybrid monzodolerite and a stock-like body of essexite (foid monzogabbro). In the area south of Armeli, dolerites are generally olivine free but in a few samples altered olivine is present. These dolerites are associated with rhyolitic dykes, which invariably contain microphenocrysts of quartz and are thus described as quartz porphyries.

Keywords

Deccan Trap, Intrusives, Petrography, Structure, Saurashtra.

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