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A. R. Nambiar ¹

Affiliations
1 Geological Survey of India, Marine Wing, Mangalore - 575 001, IN

Abstract

A swarm of alkaline and mafic dykes occurs in the Archaean Gneissic Complex of western Meghalaya, Northeastern India, spatially associated with a N-S trending deep-seated fault - Nongchram Fault. Lamprophyre constitutes the dominant member among the alkaline dyke rocks; other petrological variants include tinguaite, ijolite, trachyte and orthoclasite. Rare carbonatite is also present. Lamprophyre contains country rock fragments, as well as mantle derived ultramafic nodules, aggregated megacrysts and lapilli like structures. A feature characteristic of most lamprophyre dykes of the area is the presence of ocelli, commonly zoned, with carbonate-rich (rarely analcime) inner zone and an outer zone rich in alkali felspars, mica, pyroxenes and opaque minerals. Petrographically, the lamprophyres of the area are classified as camptonite, but they have some affinity towards ultramafic and carbonatitic lamprophyres. Field and petrographic studies suggest that the lamprophyre and carbonatite of Nongchram Fault Zone (NFZ) are genetically linked to a common parental magma, which differentiated by the process of liquid immiscibility to carbonate and silicate fractions, leading to the emplacement of discrete bodies. The best candidate for the parental magma appears to be mantle-derived alkali basalt magma. Lamprophyre and other sodic alkaline rocks and carbonatite in NFZ indicate rift-controlled magmatism triggered by a mantle plume. The spatial and temporal distribution of lamprophyre dykes (107 Ma) of the region with ultramafic-mafic-alkaline-carbonatite complexes of Assam-Meghalaya plateau and Kerguelen-related Sylhet-Rajmahal- Bengal Basin basalts (105-118 Ma) suggest their genetic link with early Kerguelen hotspot magmatism.

Keywords

Lamrophyre, Petrography, Alkaline-Carbonatite Complexes, Nongchram Fault Zone, Kerguelen Hotspot, Assam-Meghalaya Plateau.

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A. R. Nambiar ¹, G. P. Srivastava ²

Affiliations
1 Geological Survey of India, Marine Wing, Mangalore-575001, IN
2 18/144, Indira Nagar, Lucknow-226016, IN

Abstract

No Abstract.

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A. C. Dinesh ¹, N. Maran ¹, A. R. Nambiar ¹, E. Unnikrishnan ¹, C. Jayaprakash ¹

Affiliations
1 Marine Wing, Geological Survey of India, Mangalore, IN

Abstract

No Abstract.

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A. R. Nambiar ¹

Affiliations
1 Geological Survey of India, Op: Karnataka & Goa, Bangalore - 560 078, IN

Abstract

No Abstract.

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K. T. Vidyadharan ¹, A. R. Nambiar ¹

Affiliations
1 Geological Survey of India Operations, Karnataka & Goa, Vasudha Bhavan Kumarasway Layout Bangalore - 560 078, IN

Abstract

No Abstract.

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A. R. Nambiar ¹, T. Ajit Kumar Reddy ², M. Sridhar ², S. Ravi ², V. Chakravarthi ², S. Neelakantam ²

Affiliations
1 Geological Survey of India, Op.: Karnataka and Goa, Vasudha Bhavan, Bangalore - 560 078, IN
2 3-5-17Q/G, Narayanguda, Hyderabad - 500 029, IN

Abstract

No Abstract.

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A. R. Nambiar ¹, P. V. Sukumaran ²

Affiliations
1 Geological Survey of India, Op, Karnataka & Goa, Bangalore-560078
2 Geological Survey of India, Central Region, Seminary Hills, Nagpur-44006, IN

Abstract

Many sediment cores raised from the deep Arabian Sea, west of Lakshadweep Ridge, record the occurrence of 10-38 cm thick tephra layers, 35 to 195 cm below sea floor. These layers arc difficult, mainly due to admixing with biogenic debris, The non-carbonate coarse fraction of these tephra beds consists mostly of transparent fresh glass shards of K-rich, high silica rhyolitic composition, with some pumice fragments and crystals. The glass shard morphology is dominated by bubble wall type that often shows triple junctions. Individual analyses of glass shards from these volcanic ash layers is comparable to those of the proximal (Malaysia) and distal (Bay of Bengal and Indian subcontinent) tephra beds associated with the 75 ka Toba caldera cruption in north Sumatra. Characterisation on the basis of shard morphology and chemical composition demonstrates that all the tephra layers identified in the sediment cores from this study are correlative to the Youngest Toba Tuff. This new occurrence of youngest Toba ash layers in Arabiiin Sea, together with its identification further north in Murray Ridge and recent discovery of YTT glasses in the South China Sea and Central Indian Ocean Basin expand the previously known dispersal of Toba ash to much longer distances, both towards east and west from the source, and suggest that the volume of erupted magma was larger than the previously interpreted.

Keywords

Tephra, Glass Shards, Chemistry, Toba Tuff, Arabian Sea.

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S. Shivanna ¹, J. K. Srivastava ¹, A. R. Nambiar ¹

Affiliations
1 Geological Survey of India, Bangalore -560078, IN

Abstract

A large kimberlite body has been located for the first time in the Raichur District of Karnataka near Undraldoddi village. This is significant in view of ancient records of diamond recovery from Raichur, but primary source could not be identified so far in the area. The pipe is being investigated for the possible presence of diamonds.

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A. R. Nambiar ¹

Affiliations
1 Geological Survey of India, Karnataka and Goa, Vasudha Bhavan, K S. Layout, Bangalore 560 078, IN

Abstract

No Abstract.

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V. Madhavan ¹, A. R. Nambiar ², S. Shivanna ², J. K. Srivastava ²

Affiliations
1 Department of Geology, Kakatiya University, Warangal- 506 009, IN
2 Geological Survey of India, Bangalore - 560 078, IN

Abstract

No Abstract.

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A. R. Nambiar ¹

Affiliations
1 Geological Survey of India, Op.: Karnataka and Goa, Vasudha Bhavan, Kumaraswamy Layout, Bangalore 560 078, IN

Abstract

No Abstract.

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P. V. Sukumaran ¹, A. R. Nambiar ²

Affiliations
1 Geological Survey of India, Geodata Division, Nagpur - 440 006, IN
2 Geological Survey of India, Op. Karnataka and Goa, Kumaraswamy Layout, Bangalore - 560 078, IN

Abstract

This communication reports the new find of chromiferous lodestone near Chalingal village in Kasaragod District of Kerala. The chemistry and mineralogy of the lodestone suggest that the band may have segregated during the early phase of magmatic differentiation.

Keywords

Lodestone, Magmatic Differentiation, Ilmenite, Magnetite, Cumulus Texture.

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