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S. F. Sethna ¹, S. G. Viladkar ¹

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

Abstract

The temperatures of crystallization of 23 calcitic carbonatites have been estimated on the basis of the amount of MgCO₃ going into solid solution with calcite. Majority of the carbonatites have a temperature of crystallization between 350° to 475° C., while the one at Sevattur shows a rather high temperature of crystallization (about 600° C). There is a distinct relation between grain size and temperature of crystallization for the Amba Dongar carbonatites.

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V. Subramaniam ¹, S. G. Viladkar ², R. Upendran ¹

Affiliations
1 Office of the State Geologist, Madras 32, IN
2 St. Xavier's College, Bombay 400001, IN

Abstract

Dunites, pyroxenites, alkali syenites and carbonatites form the complex. The complex intrudes in a discordant fashion, hornblende gneisses which form the country rock.

The carbonatites are distributed mainly in pyroxenite as fracture fillings and to a lesser extent as dykes in syenites. The carbonatite bodies form a distinct but discontinuous ring. The calcite-dolomite carbonatite and para-ankerite carbonatite are the main types. Varieties having mica, monazite, riebeckite and ilmenite are identified. Quartz-baryte-galena veins, pegmatites of syenitic composition and albitites are associated with the carbonatites. Evidences for fenitisation are not much. Feldspathoids, normally associated with carbonatite activity are absent. Xenoliths of gneisses, pyroxenite and syenite in carbonatites, compositional banding and primary foliation, are some of the features of the carbonatites. On the basis of mineral assemblage, the temperature of formation of this complex is assigned 325°C. Analyses indicate relatively high values for barium, cerium, yttrium, zirconium, vanadium and boron.

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R. N. Sukheswala ¹, S. G. Viladkar ¹

Affiliations
1 Geology Department, St. Xavier's College, Bombay 400 001, IN

Abstract

Potash fenitization of sandstones of Amba Dongar carbonatite-complex was described earlier by Deans et al. in 1972. The present work mainly deals with newly found soda-fenites in carbonatites, rich in aegirine-augite, aegirine, albite and orthoclase. The chemistry of potash fenites and soda fenites (new analyses) reveals two trends in Amba Dongar fenites; (i) 'normal fenitization' in which both soda and potash have been added and (ii) predominantly potash fenitization leading to the development of potash fenites. The fenitlzing solutions were supplied by the carbonatite magmas (both sovite and ankeritic carbonatite).

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S. G. Viladkar ¹, R. K. Avasia ¹

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

Abstract

The Amba Dongar, Sirivasan and Panwad-Kawant carbonatite-alkalic complexes have close spatial relation. In Amba Dongar olivine-free nephelinite is the predominent type of alkaline rock with a few phonolite dykes; in Sirivasan tinguaite is more abundant than nephelinilte while in Panwad-Kawant region bulk of the alkaline rocks is made up of phonolite and tinguaite. The chemistry of the pyroxenes of these alkaline rocks has been interpreted and their evolutionary trends are discussed. The pyroxenes of Amba Dongar and Sirivasan complexes show early acmitic trend with phenocrysts being more calcic. The groundmass pyroxenes are more acmitic. The pyroxenes of the Panwad-Kawant complex, on the other hand, are less evolved diopsidic-salite type, showmg only slight initial hedenbcrgite enrichment and the final aemitic enrichment. These two trends are attributed to varying crystallizing conditions of the two batches of the same magma.

Keywords

Amba Dongar, Sirivasan, Nephelinite, Phonolite, Carbonatite, Alkaline Rocks Gujarat.

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S. G. Viladkar ¹, V. Subramanian ²

Affiliations
1 Geology Dept., St. Xavier's College, Bombay, IN
2 State Geological Survey, Tamil Nadu, Madras, IN

Abstract

The Proterozoic carbonatites and associated pyroxenites and syenites of Tamil Nadu are emplaced in the Precambrian gneissic rocks. The carbonatite complexes are located along a major NE trending lineament. In Sevathur, dolomitic carbonatite is predominant over sovite and ankeritic carbonatite whereas in Samalpatti, sovite and silico-sovite form the major bulk of carbonatite mass with small dykes of dolomitic and ankeritic carbonatites. In different types of carbonatites, in addition to calcite, dolomite and ankerite, phlogopite, amphibole, magnetite, and apatite are found in varying amounts between 2% and 10% with accessory amounts of pyrochlore, perovskite, monazite, NbiImenorutile, zircon, baddeIeyite, pyrite, ilmenite, galena and thorite. Both mica and amphibole show change in composition from Fe-rich in syenite and sovite to Mg-rich in the dolomitic carbonatite. The carbonatites are enriched in Ba, Sr, Nb, REE's, Th and U in comparison to the primitive mantle values. Pyroxenites are characterized by high abundances of Mg, Cr and Ni and, low abundances of Nb and Zr. The petrographic evidence, with globules silicate rocks surrounded by carbonatitic matrix is sufficiently clear to suggest a genetic link between syenite and carbonatite by liquid immiscibility.

Keywords

Mineralogy, Geochemistry, Carbonatites, Igneous Petrology, Sevathur, Samalpatti, Tamil Nadu.

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S. G. Viladkar ¹, R. Ramesh ², R. K. Avasia ¹, P. B. Pawaskar ³

Affiliations
1 Department of Geology, St. Xavier's College, Mumbai - 400 001, IN
2 Physical Research Laboratory, Navrangpura, Ahmedabad - 380 009, IN
3 Bhabha Atomic Research Centre, Trombay, Mumbai, IN

Abstract

It is noteworthy that Amba Dongar carbonatite complex has both intrusive and extrusive phase carbonatites. The stratified tuff with components of both carbonatite and nephelinite have been recently traced in western part of Amba Dongar complex. In this preliminary report we present trace and rare earth elements data and C and O isotopes of these tuffs. The C and O isotopes of the least altered tuff falls within the mantle box while the remaining samples plot in the narrow field of 16-20% in oxygen. These values are explained using simple model of water-Rock interaction.

Keywords

Carbonatites, Extrusive Phase, Amba Dongar, Gujarat.

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N. V. Vladykin ¹, S. G. Viladkar ², T. Miyazaki ³, V. Ram Mohan ⁴

Affiliations
1 Institute of Geochemistry SB RAS, Irkutsk-33, Box 4019, RU
2 Geology Department, St Xavier's College, Mumbai-400001, IN
3 Graduate School of Science and Technology, Niigata University, Niigata 950-2181, JP
4 Department of Geology, University of Madras, Chennai-600025, IN

Abstract

Benstonite-Ba-Sr carbonatites are found only in two localities in the world the Murun massif in Siberia and Jogipatti in Tamil Nadu, South India. The specific feature of these carbonatites is high BaO (20-35%) and SrO (up to 10% as in the Murun carbonatites) concentrations. These carbonatites jointly with calciocarbonatite varieties occur in caibonatite-alkaline complexes of Siberia and South India. However, carbonatites of South India and Siberia differ in terms of the chemical composition, especially the trace and rare earth element contents. In South India (Tamil Nadu area), carbonatites are found in three alkaline massits Samalpatti, Sevathur, and Elagiii. Benstonite carbonatites are found only in the Samalpatti massif as a small dyke. In the Murun massif carbonatites are separated from the silicate rocks and occur as a residual silicate-carbonate complex. It comprises microclinites, pyroxene-microclines, quartz-microcline- pyroxene-calcites, which contain calcite carbonatites and charoite rocks as schleiren and dykes. Benstonite carbonatites form a horizontal body of 40 m thick in the southwestern part of the complex. We examined and compared the geological and geochemical features of these two areas where Bensonite occurs in association with other carbonatites and discuss their origin. The Indian carbonatites crystallized as the plutonic phase while Siberian ones formed in the near-surface area and effused on the Earth's surface.

Keywords

Benstonite, Carbonatites, Geochemistry, Sevathur, Samalpatti, Tamil Nadu.

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