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

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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B. S. Sethna ¹, Prajakta Kothare ¹

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

Abstract

The occurrence of ankaramite in the Deccan Volcanic Province of Saurashtra is reported here. The ankaramite occurs as a small dyke intruding into horizontal basalt flows. Petrographically it is characterised by the presence of large phenocrysts of ctinopyroxene (chromian diopside) and olivine (Fo₈₃) in a groundmass of plagioclase (An₆₀), iron oxides and accessory brown amphibole (kaersutite). Chemically it is characterised by enrichment in high field strength and light rare earth elements as compared to the picritic olivine dolerites of the area. It is concluded that the ankaramite magma represents a primitive sample of High-Ti type of tholeiitic basaltic magma while majority of the olivine dolerites and dolerites found in the area are representative of Low-Ti type of tholeiitic magma.

Keywords

Deccan Traps, Ankaramite, Rajkot, Saurashtra, Gujarat.

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

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

Abstract

The monzodolerite dyke with a width of 25 to 30 meters, extends nearly east-west for almost 30 kms. The dyke intrudes horizontal lava flows of the Deccan Volcanic Province. Major, trace and rare earth element analyses for three samples of the monzodolerite have been given. SEM-EDXA analyses for feldspars and pyroxenes have also been given. On the basis of field evidence (in the form of unassimilated blebs of more basic magma surrounded by the monzodolerite), mineralogical variation and whole rock geochemistry, the rock is presumed to have formed by the mixing of basic and intermediate magmas rather than by a process of pure differentiation. The intermediate magma is presumed to have formed by differentiation of basaltic magma at some depth.

Keywords

Petrology, Deccan Traps, Monzodolerite, Magma Mixing, Saurashtra.

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