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

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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Yamuna Singh ¹, Veena Krishna ¹

Affiliations
1 Atomic Minerals Directorate for Exploration and Research, Department of Atomic Energy, Begumpet, Hyderabad - 500 016, IN

Abstract

The Precambrian Chhotanagpur granite gneiss complex (CGGC) terrain covers more than 80,000 sq km area, and is dominated by granitoid gneisses and migmatites. Recent geochronological data indicate that the CGGC terrain has witnessed five tectonomagmatic thermal events at: (i) 2.5-2.4 Ga, (ii) 2.2-2.0 Ga, (iii) 1.6-1.4 Ga, (iv) 1.2-1.0 Ga, and (v) 0.9-0.8 Ga. Of these, the third and the fourth events are widespread. The whole-rock Rb-Sr isotopic analysis of twenty granite samples from the CGGC of Raikera-Kunkuri region, Jashpur district, Chhattisgarh, Central India, yields two distinct isochrons. The eleven samples of grey granites define an isochron age of 1005±51 Ma with moderate initial ⁸⁷Sr/⁸⁶Sr ratio of 0.7047±0.0065, which corresponds to the fourth tectonomagmatic event. On the other hand, the nine samples of pink granites indicate younger isochron age of 815±47 Ma with a higher initial ⁸⁷Sr/⁸⁶Sr ratio of 0.7539±0.0066 that matches with the fifth phase of the thermal event. The data suggest emplacement of large bodies of grey granite at ∼1005 Ma that evolved possibly from precursors of tonalitic-granodioritic composition. Furthermore, the younger age (∼815 Ma) suggests the age of metasomatism, involving isotopic resetting, that resulted in genesis of pink granite bodies of limited areal extent. By analogy, the age of metasomatism (∼815 Ma) may also be taken to represent the age of Y-mineralisation in the Raikera-Kunkuri region of the CGGC terrain.

Keywords

Rb-Sr Geochronology, Petrogenesis, Grey and Pink Granitoids, Precambrian, Chhotanagpur Granite Gneiss Complex, Central India.

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