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

V. B. Bhanot ¹, Suresh K. Kwatra ¹, Ashok K. Kansal ¹, B. K. Pandey ¹

Affiliations
1 Physics Department, Panjab University, Chandigarh, IN

Abstract

Rb-Sr isochron age of 1430±150 m.y. has been obtained for the gneissic member of Chails exposed near Nirath and Baragaon of Northwestern Himalaya.

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P. V. Ramesh Badu ¹, B. K. Pandey ², R. Dhana Raju ³

Affiliations
1 Atomic Minerals Division, Department of Atomic Energy, Nagpur- 440 001, IN
2 Atomic Minerals Division, Department of Atomic Energy, Bangalore - 560 072, IN
3 Atomic Minerals Division, Department of Atomic Energy, Hyderabad - 500 016, IN

Abstract

isotopic date on 12 samples of the Paliam and Darba granites of the Bastar Craton in Central India have defined an isochron age of 2308 ± 48 Ma. with initial Sr-isolopic ratio of 0.7354 ± 0.0097. Mineral ages on perthite, biolile and muscovite from tin- and rare metal-minearlised as well as non-minearlised pegmatites of the Bastar-Koraput Pegmatite belt within this craton have indicated two important dates of about 2050 and 1620 Ma. These dates are interpreted as (a) emplacement of the granite around 2300 Ma., (b) derivation of the pegmatites from this parent granite around 2050 Ma and (c) reselling of Rb-Srsystem at about 1620 Ma., concomitant with the metamorphic/uplift event in the nearby Eastern Ghats. The very high initial ⁸⁷Sr/⁸⁶Sr ratio of 0.735 for granite indicates thai they are crustal derived, possibly from a metasedimentary source.

Keywords

Rb-Sr ages, Geochronology, Pegmatitic Minerals, Bastar-Koraput Belt, Madhya Pradesh, Orissa.

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G. R. Dikshitulu ¹, B. K. Pandey ², Veena Krishna ², R. Dhana Raju ¹

Affiliations
1 Atomic Minerals Division, Department of Atomic Energy, Bangalore-560 072, IN
2 Atomic Minerals Division, Department of Atomic Energy, Hyderabad-500 016, IN

Abstract

Precambrian Central Gneissic Complex (CGC) in the Kameng district of Arunachal Pradesh comprises the oldest Sela Group thrusting over the Bomdila Group, with the Salari Group being the youngest. Augen gneiss of the Bomdila Group, granite of the Salari Group and hornblende granite gneiss of the Sela Group define Rb-Sr isochron ages of 1914 ± 23, 1536 ± 60 and 481 ± 23 Ma, respectively. The 481 Ma age on the hornblende gneiss from the oldest Sela Group is attributed to the resetting of Rb-Sr clock due to tectonic imprint of the Main Central Thrust (MCT). The 1914 Ma age of the augen gneiss, the oldest reported so far from the Arunachal Himalaya, is either the emplacement age or latest metamorphic event. The 1536 Ma old emplacement age of the granite intruding the black shale of the Salari Group disproves the hitherto considered Gondwana age for the black shale, and, thus, necessitates revision in its chronostratigraphy. (⁸⁷Sr/⁸⁶Sr)_i of the hornblende granite gneiss and Salari granite is high (0.719 and 0.709) indicating a crustal source for these, whereas it is low for the augen gneiss (0.703) suggesting an inherited signature of the upper mantle source together with some crustal contamination. Rb-Sr whole-rock isochron ages from the Arunachal Himalaya are correlatable with those reported from other parts of the Indian Himalaya. These cumulatively point to at least three major periods of activity in the Himalaya at ca. 2060 ± 250, 1530 ± 90 and 530 ± 75 Ma.

Keywords

Rb-Sr Whole-Rock Ages, Geochronology, Central Gneissic Complex, Arunachal Himalaya.

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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.

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Navin Goyal ¹, P. C. Pant ², P. K. Hansda ³, B. K. Pandey ⁴

Affiliations
1 Atomic Minerals Directorate For Exploration and Research, Jamshedpur-831 002, IN
2 Atomic Minerals Directorate For Exploration and Research, Jamshedpur-831 002
3 Atomic Minerals Directorate For Exploration and Research, New-Delhi-110 066, IN
4 Atomic Minerals Directorate For Exploration and Research, Hyderabad-500 016, IN

Abstract

The Precambrian Godhra Granite occurring in parts of central Gujarat, intrudes the metasediments of Lunavada and Champaner Groups. RbSr isochron based on 14 whole rock samples of the Godhra Granite gives an age of 965±40 Ma, with an initial ⁸⁷Sr/⁸⁷Sr ratio of 0.7109±0.0015. Geochemical data indicate its origin by partial melting of crustal rocks in post-orogenic environment.

Keywords

Godhra Granite, Peraluminous Granite, Rb-Sr Isochron, Crustal Source, Gujarat.

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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.

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