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

K. Gopalan ¹, J. R. Trivedi ¹, M. N. Balasubrahmanyan ², S. K. Ray ³, C. Anjaneya Sastry ⁴

Affiliations
1 Physical Research Laboratory, Ahmedabad 380009, IN
2 Geological Survey of India, 29, Jawaharlal Nehru Road, Calcutta 700 016, IN
3 Geological Survey of India, Bhutan Circle, Samchi, BT
4 Geological Survey of India, Geochronology and Isotope Geology Division, 29, Jawaharlal Nehru Road, Calcutta 700016, IN

Abstract

Rubidium and strontium determinations are reported for the Udaipur and Saladipura granites from the Khetri copper belt. The whole-rock data define a Rb-Sr isochron corresponding to an age of 1480 ± 40 m.y. and initial Sr ratio of 0.7131 ± 0.006. This age is older than previously believed, but consistent with the generally high age of post-Delhi intrusive rocks so far dated in the region. Biotites associated with these rocks give younger ages around 700 m.y, This indicates mobilisation of Sr isotopes in a later metamorphic event. It is significant that this secondary event is almost synchronous with the intrusion of granites to the west and south of the Aravalli mountains.

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S. N. Sarkar ¹, J. R. Trivedi ², K. Gopalan ²

Affiliations
1 Indian School of Mines, Dhanbad 826 004, IN
2 Physical Research Laboratory, Ahmedabad 380 009, IN

Abstract

Six whole rock samples of the Tirodi gneiss belonging to the basal part of the Sausar Group rocks in the Dongri Buzurg area yield a good Rb-Sr isochron corresponding to an age of 1525±70 Ma and an initial Sr ratio of 0.7148±0.0033. Mineral isachrons for two of these rocks give concordant but younger ages at 860 Ma in good agreement with the previously reported K/Ar ages on minerals separated from the Sausar Group rocks. The 1525 Ma event is interpreted to mark the main phase of regional metamorphism of the Sausar Group of rocks including the granitization of the basal Tirodi unit. The deposition of the sediments of the Sausar Group and their basement must therefore be even older. The good agreement of the RblSr and K/Ar mineral ages at about 860 Ma indicates the latest thermal overprint on the Sausar - rocks, The regional metamorphism of the Tirodi gneiss at about 1500 Ma ago is also reflected in the Rb/Sr and K/Ar mineral ages of the Amgaon gneiss and Dongargarh granite to the east of the Tirodi gneiss.

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J. R. Trivedi ¹, K. Gopalan ¹, K. S. Valdiya ²

Affiliations
1 Physical Research Laboratory, Ahmedabad 380 009, IN
2 Geology Department, Kumaun University, Nainital 263 001, IN

Abstract

Whole-rock Rb-Sr isochron analyses of granitic components of the two prominent tectono-stratigraphic units, the Ramgarh and Almora Groups, in the Kumaun Lesser Himalaya yield two relatively coherent age clusters around 1800 Ma and 550 Ma, respectively. The older age corresponds to the porphyritic granites, quartz-porphyry and augen gneisses from the Ramgarh Nappe, the basal parts of the synformal Almora Nappe and the Askot-Dharamghar Klippen in the inner Lesser Himalaya, whereas the younger age is restricted to the massive Champawat Granodiorite - Almora Granite emplaced in the upper part of the Almora Nappe. The older rock seems to occur also in the ischolar_main zone at the base of the Great Himalaya. The early Palaeozoic granites appear to be very widespread, extending both into the Himachal Pradesh and Nepal sectors. The spatial distribution of the two age components has been used to infer the possible relationships between the two lithotectonic units.

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