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

U. K. Pandey ¹, Tikam Chabria ¹, P. Krishnamurthy ¹, R. Viswanathan ¹, B. Kumar ²

Affiliations
1 Atomic Minerals Directorate for Exploration and Research, Department of Atomic Energy, Begumpet, Hyderabad - 500 016, IN
2 National Geophysical Research Institute, Hyderabad - 500 007, IN

Abstract

Carbon isotope studies on calcite - graphite system yield narrow Δ Cat - Gr values, varying from 3.9 to 4.3, indicating equilibrium between coexisting calcite and graphite. Calcite - graphite geothermometry has assigned temperatures ranging from 630° to 700°C for the calc - granulites found to the NE of Usilampatti area. The inferred temperatures correspond to the retrograde temperatures obtained by cation exchange thermometry on the granulites from other areas in the Madurai Block. These temperatures may correspond to the late isobaric cooling event. X-ray diffraction studies indicate well ordered nature of these graphites and corroborate the findings of isotope geothermometric inferences.

Keywords

Geothermometry, Calc-Granulite, Calcite-Graphite, Carbon-Oxygen Isotopes, X-Ray Diffraction, Usilampatti, Tamil Nadu.

Full Text

     

K. K. Pandey ¹, K. S. Mishra ¹, U. K. Pandey ², P. V. Ramesh Babu ²

Affiliations
1 Atomic Minerals Directorate for Exploration & Research, CR, Nagpur - 440001, IN
2 Atomic Minerals Directorate for Exploration & Research, Hyderabad - 500016, IN

Abstract

Whole-Rock isochron dating of granite gneiss, hosting rare element-Bearing pegmatites from Jharsuguda district, Orissa defines an isochron age of 2762±±200Ma with initial ⁸⁷Sr/⁸⁶Sr ratio of 0.7082 ± 0.0027 (MSWD = 36.8). High ⁸⁷Sr/⁸⁶Sr ratio indicates involvement of crustal source for its formation. Petrography of the granitoids characterizes them as 'hornblende granite gneiss'. The ASI of the granite gneiss reveals its meta-Aluminous nature. It is a significant host to the zoned and complex type of 'Rare Elements-Bearing Pegmatites' , containing columbite-Tantalite (oxide of Nb + Ta) and beryl. Based on the whole-Rock isochron data, the granite gneiss of the Jharsuguda area can be equated with the Bamra and Tamparkola granite and phase III of the Singhbhum granite. Further, it postdates the Bonai and Kaptipada granite but is older to the Nilgiri and Ramapahari granite, with all these forming part of the Singhbhum North Orissa Craton.

Keywords

Rb-Sr Geochronology, Granite Gneiss, Rare Metals, Jharsuguda, Orissa.

Full Text

     

U. K. Pandey ¹, B. K. Pandey ², P. Krishnamurthy ³

Affiliations
1 Atomic Minerals Directorate Geochronology Lab , Begumpet, Hyderabad - 500 016, IN
2 Atomic Minerals Directorate, Pratap Nagar, Sanganer, Jaipur - 303 906, IN
3 Atomic Minerals Directorate, Khasmahal, Tatanagar - 831 002, Jharkhand, IN

Abstract

Rb-Sr, Sm-Nd and Pb-Pb systematics on mafic granulites, intermediate-charnockites, charnockites, calcgranulites, orthogneisses, leptynites, granites and two (2) mineral samples (microcline and muscovite) from pegmatite have been studied Sm-Nd model ages (T_DM) for most of the rocks cluster around 2 1 Ga Calc-Granulites gave Rb-Sr whole rock isochron age of 1339 ± 110 Ma with initial ⁸⁷sr/⁸⁶sr ratio(I R ) = 0 709 Mesoproterozoic age for the Metasedimentary. Group of rocks (calc-granulites) has been inferred based on their ⁸⁷sr/⁸⁶sr ratio also. Granites and leptynites have yielded Rb-Sr whole rock isochron ages of 823 ± 38 Ma, with I R = 0 713 and 894 ± 82 Ma, with I R = 0 708 respectively which are younger than the calc-Granulites. Microcline and muscovite from pegmatite gave Rb-Sr model ages as 532 and 491 Ma respectively. Granites, leptynites and calc-Granulites are derived from the crustal source as indicated by their high initial ⁸⁷sr/⁸⁶sr ratios. Most probably the protolith of the granitic and granulitic rocks is of Palaeoproterozoic age in this part of the Madurai Block. The minimum age of granulite grade of metamorphism has been inferred at c 850 Ma, indirectly on the basis of Rb-Sr ages of leptynites, which normally form during the evolution of granulite facies assemblages. Both granulite facies metamorphism and granitic magmatism probably took place during Neoproterozoic period correlatable to the early phase of Pan-African orogeny. Later decompression, causing mineral scale resetting of the Rb-Sr and Sm-Nd systematics, around 450-550 Ma, may correspond to the final exhumation, which brought the middle to lower crustal granulites to upper crustal levels, during the last phase of Pan-African activity. As per ages obtained on various lithounits in the present study the Metasedimentary Group of rocks (Mesoproterozoic) are younger than the Charnockite Group of rocks (Palaeoproterozoic) followed by the rocks belonging to the Migmatite Complex (Neoproterozoic).

Keywords

Rb-Sr, Sm-Nd, Pb-Pb Geochronology, Proterozoic, Granulites, Age, Lithounits Usilampatti, Madurai, Tamil Nadu.

Full Text

     

A. K. Dwivedi ¹, U. K. Pandey ¹, C. Murugan ¹, A. K. Bhatt ², P. V. Ramesh Babu ¹, Mallikarjun Joshi ³

Affiliations
1 Atomic Minerals Directorate for Exploration and Research (AMD), Hyderabad - 500629, IN
2 Atomic Minerals Directorate for Exploration and Research (AMD), Jamshedpur - 831 002, IN
3 Banaras Hindu University, Varanasi - 221005, IN

Abstract

The Barabazar granite, exposed at the northern margin of Singhbhum craton, Eastern India, occurs along the South Purulia Shear Zone (SPSZ) and is emplaced into the Palaeoproterozoic metapelites and felsic volcanics of Singhbhum Group. Geochemical, petrographical and geochronological studies on the Barabazar granite addressed in the work have wide implications on understanding the geodynamics of SPSZ during Palaeoproterozoic to Mesoproterozoic. Geochemically, Barabazar granite displays limited range of major oxides, alkali enrichment and highly fractionated features (SiO₂ > 75%; Eu/Eu^* = 0.16-0.33; enrichment of K, Rb, Th, U and Nb; depletion of Ba, Sr, P and Ti). It is predominantly peraluminous (molar Al₂O₃/CaO+Na₂O+K₂O (A/CNK) = 1.14-144) and contains abundant alkali feldspar, perthite, and minor plagioclase, biotite and accessory minerals. Geochemical and petrological data indicates that it is A-type granite, which formed in 'Within plate granite' tectonic set up. The Barabazar granite was emplaced at ca. 1771 Ma (Pb-Pb) in rift related environs and evolved by partial melting of stabilized lower/middle crust (initial ⁸⁷Sr/⁸⁶Sr = 0.7302 ± 0.0066 and μ₁ = 8.5 ± 0.5). Subsequently, the shear zone (SPSZ) developed during the closure of the riftogenic basin and was reactivated during the Grenvillian orogeny (Ca. 900-1300 Ma), resulting in rehomogenisation of the strontium isotopes and thereby yielding younger whole-rock Rb-Sr isotope age of c. 971 Ma for the Barabazar granite. Probably during this tectonic event, the Singhbhum craton (Southern India Shield) would have finally juxtaposed with Northern Indian Shield along Central Indian Tectonic Zone (CITZ) during the global Grenvillian orogeny.

Keywords

A-Type Barabazar Granite, Geochemistry, Geochronology, Grenvillian Orogeny, Singbhhum Craton, SPSZ, West Bengal.

Full Text

     

U. K. Pandey ¹, D. V. L. N. Sastry ¹, B. K. Pandey ¹, Madhuparna Roy ², T. P. S. Rawat ², Rajeeva Ranjan ³, V. K. Shrivastava ³

Affiliations
1 Atomic Mineral Directorate for Exploration and Research, Geochronology Lab., Begumpet, Hyderabad - 500 016, IN
2 Atomic Mineral Directorate for Exploration and Research, Northern region, New Delhi - 110 066, IN
3 Atomic Mineral Directorate for Exploration and Research, Central Region, Nagpur - 440 001, IN

Abstract

The Dargawan gabbros intrusive into the Moli Subgroup of Bijawar Group, yielded Rb-Sr whole rock isochron age of 1967 ± 140 Ma. Based on the oldest age from overlying Lower Vindhyan (1.6Ga) and the underlying youngest basement ages (2.2 Ga), the time range of Bijawar sedimentation may be assigned as 2.1-1.6 Ga (Paleoproterozoic). Sm-Nd Model ages (T_DM), obtained, for Dargawan gabbros, is c. 2876 - 3145 Ma. High initial ⁸⁷Sr/ ⁸⁶Sr ratio of 0.70451 (higher than the contemporary mantle) and negative ε_Ndi (at 1.9 Ga) value of -1.5 to -4.5, indicate assimilation of Archaean lower crustal component by the enriched mantle source magma at the time of gabbroic intrusion. The dolerite, from Damdama area, which is intrusive into the basement and overlying sediments of Chandrapur Group in the central Indian craton, yielded Rb-Sr internal isochron age of 1641 ± 120 Ma. The high initial ⁸⁷Sr/⁸⁶Sr ratio of 0.7098 and ε^Ndi value of -3.5 to -3.7 (at 1.6 Ga) is due to contamination of the mantle source magma with the overlying sediments. These dolerites have younger Sm-Nd Model ages (T_DM) than Dargawan gabbros as c. 2462 - 2675 Ma, which is similar to the age of the Sambalpur granite, from which probably sediments to this part of Chattisgarh basin are derived. Hence mixing of sediments with the Damdama dyke during its emplacement, gives rise to high initial ⁸⁷Sr/⁸⁶Sr and low initial ¹⁴³Nd/¹⁴⁴ ratios for these dykes. The c. 1600 Ma age indicates minimum age of onset of the sedimentation in the Chandrapur Group of Chattisgarh basin. Both the above mafic intrusions might have taken place in an intracratonic rift related (anorogenic) tectonic setting. This study is the first reliable age report on the onset of sedimentation in the Chandrapur Group. The total minimum time span of Chandrapur and Raipur Group may be 1.6 Ga to 1.0 Ga (Mesoproterozoic). The unconformably underlying Shingora Group of rocks of Chhattisgarh Supergroup thus indicates Paleoproterozoic age (older than 1.6 Ga). Most part of the recently classified Chattisgarh Supergroup and Bijawar- Vindhyan sequence are of Mesoproterozoic-Paleoproterozoic age and not of Neoproterozoic-Mesoproterozoic age as considered earlier. Petrographic study of basic dykes from Damdama area (eastern margin of Chattisgarh Supergroup) indicated presence of primary uranium mineral brannerite associated with goethite. This is the evidence of mafic intrusive providing geotherm and helping in scavenging the uranium from the surrounding and later alterations causing remobilisation and reconcentration of pre-existing uranium in host rocks as well as in mafic dyke itself otherwise mafic rocks are poor source of uranium and can not have primary uranium minerals initially. It can be concluded that mafic dykes have role in uranium mineralisation although indirectly.

Keywords

Geochronology, Age of Sedimentation, Uranium Mineralisation, Northern and Central Indian Cratons.

Full Text