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Masato Fukuoka ¹, A. Mondal ², Dipayan Guha ², G. Chattopadhyay ²

Affiliations
1 Department of Geology, Kyushu University, Fukuoka, JP
2 Department of Geological Sciences, Jadavpur University, Calcutta 700032, IN

Abstract

Piemontite-garnet bearing assemblages with or without braunite, occur in calc-silicate rocks in Sausar Group, India. The studied assemblages equilibrated under similar physical condition of metamorphism, at a high fO₂ regime. Chemically both piernontite and garnet are enriched in Mn in braunite-bearing assemblages, as compared to the braunite-free ones. petrographic features show that garnet+calcite±hematite are the late overprints in these assemblages, derived through a devolatilization reaction involving piemontite+precursor carbonate+quartz±braunite. The study shows that in the braunite-bearing assemblages, the 'maximum' Mn³⁺ content of the piemontite was not dictated by the mere presence of braunite, but it is rather correlatable with the modal abundance of braunite. Further, the X_Mn³⁺ in the piemontite and the modal abundance of braunite have a direct relationship with Al/Mn, Mn/Al+Si and Si/Al) ratios in the bulk composition of the assemblages. In the present situation the Al-Mn³⁺-Fe³⁺ distribution coefficients amongst equilibrating piemontite-garnet pairs show strong dependence on the bulk chemical composition.

Keywords

Geochemistry, Calc-Silicate Rocks, Piemontite, Sausar Group, Madhya Pradesh.

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Dipayan Guha ¹, Sanjay Das ², C. Srikarni ², S. K. Chakraborty ³

Affiliations
1 Geochronology and Isotope Geology Division, Geological Survey of India, 15 A & B, Kyd Street, Kolkata - 700 016, IN
2 Geological Survey of India, Operation Gujarat, Gandhinagar - 382 093, IN
3 Central Petrological Laboratories, Geological Survey of India, 15 A & B, Kyd Street, Kolkata - 700 016, IN

Abstract

Field evidences in Kachchh show that alkali basalts occur as flows over Mesozoic sediments. The top parts of Mesozoic Bhuj Sandstone often carry scoria and lapilli related to alkali basalt magmatism indicating that alkali magmatism and sedimentation went side by side. Tholente flows occur as blanket over both Mesozoic sediments and alkali basalt bodies.

Evidences are provided to show that alkali basalt magmatism in Kachchh was not only an earlier episode than tholente emplacement, it also predates the closing of Kachchh Mesozoic basin and seems to support a protracted period of Deccan magmatism (70 - 60 Ma). The study also shows that huge concentration of alkali basalt in the western Kachchh mainland is associated to a zone of transtension related to the rifting of Kachchh basin. Alkali basalt bodies and whole of Mesozoic sequence were affected by reverse faults related to closure of Kachchh basin with development of series of hanging wall anticlinal and footwall synclinal structures.

Keywords

Alkali Basalt, Tholente, Flow, Plug, Releasing Bend, Kachchh.

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V. Ravikant ¹, Dipayan Guha ²

Affiliations
1 G&IG Division, Geological Survey of India, Kolkata - 700 016, IN
2 D.D.G.(CGL) Cell, Geological Survey of India, Kolkata - 700 016, IN

Abstract

We report here petrographic and bulk geochemical characters of an ultrapotassic dyke from the Ladakh batholith, NW Himalaya, for the first time, and attempt to compare it with similar dykes from SW Tibet which are typical of post-collisional magmatism. Postcollisional shoshonitic and ultrapotassic dykes have been reported from north Karakoram (Pognante, 1990) and potassic and ultrapotassic lavas from SW Tibet by Miller et al. (1999). They have been linked to large scale tectonic reactivation involving the lithospheric mantle during the continued India-Eurasia collision since ∼50 Ma.

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Manas Roychowdhury ¹, T. K. Saha ¹, Dipayan Guha ¹, R. L. Munshi ¹

Affiliations
1 Operations Gujarat, Geological Survey of India, Gandhinagar, IN

Abstract

Sauropod nest-sites having well-preserved tubocanaliculate types of eggs have been found within the Lameta Formation at Ihalod taluka, Panchmahals district, Gujarat. Four morphotypes of eggs have been distinguished based on thickness and microstructure of eggshell. Dinosaur eggs have been found in three horizons in the same stratigraphic section indicating a site fidelity which is for the first time being reported from the Lameta Formation of India.

Keywords

Palaeontology, Dinosaur Eggs, Panchmahals, Gujarat.

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