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

D. K. Paul ¹, J. H. Crocket ², T. A. K. Reddy ³, N. C. Pant ⁴

Affiliations
1 Department of Geology, Presidency College, College Street, Calcutta - 700 073, IN
2 School of Geography and Geology, McMaster University, Hamilton, Ontario, L8S 4K1, CA
3 Geological Survey of India, Bandlaguda, Hyderabad - 500 068, IN
4 Geological Survey of India, Nh5p, Nit, Faridabad, IN

Abstract

A suite of potassic ultramafic rocks occurs in the Krishna district in the southern Indian craton along with clusters of kimberlite. The principal minerals include olivine, pyroxene, amphibole and phlogopite. Compositional variation exists in the phlogopite of the different occurrences of the region. Mg number varies from 0.37 to 0.56 and the groundmass phlogopites are enriched in Fe and Ti and poor in Al compared to the phenocrysts. Major and trace element data indicate enrichment of LILE and LREE (La/Yb = 24 - 104; Gd/Yb = 3.8 - 8.02). Ba contents vary from 132 to 5327 ppm, which is rather low compared to literature values for lamproite. However, the abundances of Zr (> 500 ppm), Sr (> 1000 ppm) and La (>200 ppm) are similar to those in the lamproite. The potassic ultramafic rocks have variable but high U/Pb (2-37, average 17.2) and Th/U (4.6-10.7) ratios. Concentrations of Platinum Group Elements (PGE) are quite variable with Ir, Pt and Pd varying by >9, 21 and 15 times respectively over the suite. Significant features include differences in fractionation between PGE and enrichment of Cu relative to the mantle source region. Some of the samples are not ultrapotassic and peralkaline. However, combining their mineralogical and geochemical properties, these rocks of the Krishna region may be considered as a member of the lamproite clan. There is no evidence of significant crustal contamination, but fractional crystallisation of olivine and perovskite is suggested from the geochemical relations. Available isotopic age data indicate that the ca. 1220 Ma old Krishna lamproite preceded kimberlite emplacement in the region by about 160 million years. Partial melting of a mantle metasomatically enriched in Ti and Fe would yield the Krishna lamproite magma. Amphibole contributed to the melt, but the HREE abundance data preclude garnet involvement. The depth of melt formation was likely to be shallower than that of kimberlites.

Keywords

Lamproite, Proterozoic, Krishna District, Southern India, Platinum Group Elements, Petrogenesis.

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Santanu Kumar Bhowmik ¹, Taraknath Pal ¹, Abhinaba Roy ¹, N. C. Pant ²

Affiliations
1 Regional Petrology Laboratory, Geological Survey of India, Central Region, Nagpur - 440 006, IN
2 EPMA Laboratory, Geological Survey of India, Faridabad - 121 001, IN

Abstract

Distinct lithological associations, structural relationships, and petrographic and geothemobarometric results indicate the presence of reworked granulites of Pre-Grenvillian age from the northern margin of the Sausar mobile belt (SMB) in Central India. In rare residual zones, the mafic granulite-felsic migmatite gneiss association preserves early structures and high-P (∼ 10.5 kbar at ∼ 775°C) granulite histories which appear to pre-date and bear no relation to ∼ 1000 Ma old Sausar orogeny. An isothermal decompression through ∼ 5.5 kbar at 750°C from maximum recorded P-T condition, possibly signifies extensional collapse of the crust, previously thickened through collision. During the Sausar orogeny, the granulites were further tectonically interleaved with younger lithological associations. Widespread retrogression in amphibolite and finally in greenschist facies conditions have all but obliterated granulite mineralogy and structure except in rare low-strain domain.

Keywords

Metamorphic Petrology, Granulites, High-P Metamorphism, Sausar Mobile Belt, Central India.

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R. Ravindra ¹, N. C. Pant ¹, M. J. D'souza ¹

Affiliations
1 Geological Survey of India, Antarctica Division, Faridabad, IN

Abstract

The Humboldt region of the Wohlthat Mountains in East Antarctica, depicts both polar and alpine type of morphological set-up. While the former is represented by vast stretch of antarctic ice sheet with hills and nunataks, the latter is marked by glacially fretted uplands with cirques, aretes, horns and moraines. These and other landforms associated with denudation and depositional processes have been used to evaluate the regional landscape of Humboldt area, which conforms with the composite erosional landscape of Sugden and John (1976).

The glacial geomorphological features are largely, due to Somovken and Humboldt glaciers flowing on either side of the Humboldt fiat. The interference of the movement between the two glaciers is reflected in the depositional pattern of moraines. The polar ice evidently covered much greater portions of the area in the past with ice level covering the ridges at 1900 m - 2000 m above sea level.

Keywords

Geomorphology, Antarctica, Wohlthat Mountains.

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A. Joshi ¹, N. C. Pant ¹, M. L. Parimoo ¹

Affiliations
1 Antarctica Division, Geological Survey of India, Faridabad-121 001, Haryana, IN

Abstract

The hypersolvus granitic rocks of the Petermann Ranges in East Antarctica reveal high alkali content, generation from a relatively dry magma as shown by the field and petrological characters and development in an anorogenic selling as indicatcd by an indirect genetic linkage with the spatially associated Gruber anorthosite massif. Hence, their resemblance to A-type granites and generation in a tensional tectonic environment is suggested.

The petrochemical characters indicate that the partial melting of a heterogenous but largely metasedimentary source generated the melt for crystallisation of the Petermann granite. The heat required for melting was possibly released by the fractionating basic magma which produced the juxtaposed Gruber anorthosite massif.

Keywords

Petermann Granite, East Antarctica.

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N. C. Pant ¹, P. K. Verma ²

Affiliations
1 Geological Survey of India, Shillong, IN
2 Department of Geology, University of Delhi, Delhi, IN

Abstract

Certain aspects of regional metamorphism of East Antarctica granulite terrain in the Humboldt Mountains are revealed through a study of the grain size of garnets and chemical zoning present in them. The Ferry and Cashman approach to grain size (Crystal size Distribution-CSD) development and Walther and Wood approach of grain growth are utilised to draw intersting conclusions. The growth of garnets was slower in the deeper levels but produced large, chemically unzoned grains.

Keywords

Mineralogy, Crystallography, Metamorphism, East Antarctica.

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P. R. Golani ¹, R. S. Rajawat ¹, N. C. Pant ², M. S. Rao ³

Affiliations
1 Geological Survey of India, AMSE Wing, Central Zone, Seminary Hills, Nagpur - 440 006, IN
2 Geological Survey of India, EPMA Laboratory, NIT-5, Faridabad - 121 001, IN
3 Geological Survey of India, PPOD, AMSE Wing, Bangalore - 560 078, IN

Abstract

Primary gold occurs with sulfides in Paleoproterozoic Aravalli rocks in the Bhukia area of Banswara District, Rajasthan. It occurs as microscopic grains of various shapes, mainly in arsenopyrite and lollingite. With a combined approach of ore petrologic and EPMA studies, we report high purity gold, native bismuth and alloys of gold-bismuth and bismuth-tellurium which best approximate to the rare minerals maldonite and wehrlite respectivety. The fineness of gold ranges between 934 and 995. Preliminary estimates on temperature using arsenopyrite composition coupled with rare presence of spherulitic graphite indicate the temperature of equilibration of around 550°C for the coexisting mineral phases of pyrrhotite, arsenopyrite and lollingite.

Keywords

Economic Geology, Gold Mineralogy, Bismuth, Maldonite, Wehrlite, Lollingite, Bhukia, Aravalli, Rajasthan.

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D. K. Paul ¹, S. S. Nayak ², N. C. Pant ³

Affiliations
1 Department of Geology, Presidency College, College Street, Kolkata - 700 073, IN
2 Geological Survey of India, Training Institute, Bandlaguda, Hyderabad - 500 068, IN
3 Geological Survey of India, NH5P, NIT, Faridabad 121 001, Haryana, IN

Abstract

Potassic, ultrapotassic, ultramafic rocks occur mostly as dykes and few diatremes in the southern and central Indian cratons. The ultramafic diatreme in Majhgawan in central India is the only diamond-Producing primary source at the present time. Several occurrences in southern India also contain diamond but are not being commercially exploited. Available isotopic age data on these rocks suggest a Proterozoic age (Table I), a feature that is distinctive compared to other ages on similar rocks barring the Premier pipe in southern Africa.

We present petrographic, mineralogical and geochemical data on carefully selected samples from several localities. Mineralogically there are differences in the content of olivine, Phlogopite, perovskite and spinel among the localities. In common with occurrences elsewhere, the olivines show a range from 83 to 93 mole percent. The macrocrystal olivines are more magnesian. Among the phlogopites, a two-Fold division on the basis of Mg number is discernible. Most of the ilmenites are distinctly less magnesian than the southern African kimberlites. Geochemically the samples show a wide variation in major element abundances. Despite geographical separation and petrographic differences, Zr and Hf maintain a perfect positive correlation. In common with similar rocks, all the samples studied show an enrichment of both compatible and incompatible elements. Compared to established and accepted kimberlites of southern Africa and lamproites of Western Australia, the Indian samples show general similarity but also divergences in geochemical parameters. This is believed to suggest a transitional nature of the Indian potassic, ultramafic rocks between kimberlites and lamproites Petrogenesis of the occurrences is discussed.

Keywords

Indian Kimberlites, Review, Petrology, Geochemistry, Geochronology.

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Fareeduddin ¹, N. C. Pant ², S. Neogi ¹

Affiliations
1 Geological Survey of India, PPOD Division, A M S E Wing, Bangalore - 560 078, IN
2 Geological Survey of India, EPMA Lab , Op P, H & HP, Faridabad - 12 1 001, IN

Abstract

Thc Kodomali body is one among the six pipes, reportedly of kimberlitic composition in Mainpur area, district Raipur, Chhattisgarh. It occurs within the Bundelt granitoid that is emplaced in the lower part of the Khariar/Pairi group of rocks. Recent age determinations have indicated that the Kodomali has intruded into Bundeli granitoids at about 500 Ma. The Kodomali diatreme exhibits a distinct macro crystal texture with coarse olivine occurring in a very fine grained groundmass made of diopside, phlogopite, spinel and secondary serpentine. Two generations of olivine occur in the rock. Macrocrystal olivine is distinctly xenocrystic whereas microphenocrystic olivine, which is zoned with more forsteritic cores than their margins, show chemical parameters suggestive of crystallization from a melt Fe-poor diopside and T₁-rich, Al and Cr-Poor phlogopite constitute the bulk of the groundmass. Phlogopite cores are I richer in T₁ and Al compared to rims and show crystallisation trends typical of orangaeites. Spinel group minerals (Magnesio-Chromite) occur as (1) coarse xenocrystic zoned grains with T₁ poor, Al and Mg rich cores relative to the rims and (11) as line size grains in groundmass. Some coarse spinel group mineral grains are enveloped by corona of sphene against macrocrystic perovskite, inferred to result from low temperature hydrothermal alteration through interaction with Si-Bearing fluids. Groundmass spinel, also represented by magnesian chromite, is sparse, has no perceptible zoning and exhibits T₁- magnetite crystallization trend that is charactenstic of orangeites. The petro-Mineralogical attributes of the Kodomali diatreme show close affinities with the South African orangeites. Recent studies on another diatreme in the area is also suggestive of an orangeitic affinity for the rock. These studies imply that the Mainpur area represents an orangeitc field of Palaeozoic age.

Keywords

Kimberlite, Orangeite, Petrography, Mineral Chemistry, Khariar, Pairi, Kodomali, Mainpur, Chhatisgarh.

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A. Joshi ¹, N. C. Pant ², S. Neogi ³, J. K. Nanda ⁴

Affiliations
1 A-2, Pushpa Bhavan, Geological Survey of India, New Delhi - 110 049, IN
2 EPMA Laboratory, Geological Survey of India, Faridabad - 121 001, IN
3 PPOD, A M S E Wing, Geological Survey of India, Bangalore - 560 078, IN
4 OP Orissa, Geological Survey of India, Bhubaneshwar - 75 1 012, IN

Abstract

High-Al gabbro is reported here for the first time from three anorthosite complexes of Orissa sector of Eastern Ghats Granulite Belt (EGGB). It has distinctly different textural, mineralogical and chemical characters from the anorthosite within which it occurs, and also from the spatially associated Fe, Ti and REE enriched ferrodiorite suite.

The high-Al gabbro of EGGB is comparable to similar rocks from other anorthosite complexes in major, trace element compositions and Mg# range (50-58). However, in contrast to the high-Al gabbro of the Laramie Anorthosite Complex (LAC), it does not form the most primitive rock of this association and exhibits negative Eu anomaly (Eu/EU^* 0 2 0 6). The Mg enriched composition of the silicates of the high-Al gabbro and higher MgO contents of the associated anorthosite and ferrodiorite of the LAC are other critical differences.

Variable composition of the parental melts for these rocks at EGGB and LAC is interpreted Derivation of these aluminous melts by partial melting of basalt under high pressure i s suggested.

Keywords

Petiogenesis, High-Al Gabbio, Anorthosite, Ferrodiorite, Eastern Ghats Granulite Belt, Orissa.

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N. C. Pant ¹

Affiliations
1 Geological Survey of lndia, Faridabad, IN

Abstract

No Abstract.

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N. C. Pant ¹, Amitava Kundu ¹, Sonalika Joshi ¹

Affiliations
1 Geological Survey of India, NH-5P, N I T, Faridabad-121001, IN

Abstract

The subcalcic Fe-Mg amphibole rocks (anthophyllite-cummingtonite schist) are exposed in the Bayal area of Mahendragarh district in southern Haryana. These are considered to be hydrothermally altered mafic volcanic rocks metamorphosed to amphibolite facies. Monazite is present in association with cordienite and quartz in the anthophyllite-cummingtonite schist and texturally defines the metamorphic event in these rocks. In the present study two grains of Monazite from anthophyliite cummingtonite schist have been dated. Monazite grain in sample no 135 yielded an age 952±16 Ma while sample no 277 gave an age ot 945±14 Ma. This is inferred to be the age of metamorphism of rocks of Delhi Supergroup in Haryana.

Keywords

Delhi Supergroup, Monazite, EPMA Ages, Haryana.

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N. C. Pant ¹, M. C. Manoj ²

Affiliations
1 Delhi University, IN
2 NCAOR, Goa, IN

Abstract

No Abstract.

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