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.

Full Text

     

G. R. Dikshitulu ¹, R. Dhana Raju ²

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

Abstract

The Central Gneissic Complex (CGC) in the Kameng district, Arunachal Pradesh comprises the oldest Sela Group, followed by the Bomdila Group and the youngest Salari Group. The Sela Group consists of migmatite, hornblende granite gneiss, two-mica granite and profuse intrusions of tounnaline granite (Tertiary ?). Along the Main Central Thrust (MCT), the Sela Group overlies the Bomdila Group consisting of augen gneiss, quartzite. quart-mica schist, marble and basic intrusive rocks. The Bomdila Group, in turn, overlies the Salari Group that comprises black shales and intrusive high-Ca granites. Radioelement (U, Th and K) contents, detennined by gamma-ray spectrometry. of 28 granitic and migmatitic samples of CGC indicate that the tourmaline granite and two-mica granite, augen gneiss have the highest U values as compared to that of others. Th content is more in the augen gneiss and hornblende gneiss than in other rock types. In case of K, two-mica granite and tourmaline granite have higher contents than the rest. Radioactive heat, computed from each ofthese radioelements, is highest in the tourmaline granite and two-mica granite, with the other rock types having low to intermediate values. The average total heat generation of CGC is of the order of 9.3 Heat Generation Units (HGU). A value of about 7 HGU is generally taken as the separating value between hot and cold crust and, hence, the higher value of 9.3 HGU for CGC points to the presence of hot crust in the Arunachal Himalaya. The radioactive heat generation and the published high heat flow values in the CGC indicate steep geothermal gradients in the area, which may help in remobilisation and subsequent precipitation/concentration of U in favourable locales.

Keywords

Radioactivity, Central Gneissic Complex, Arunachal Pradesh.

Full Text

     

G. R. Dikshitulu ¹, R. Dhana Raju ²

Affiliations
1 Atomic Minerals Division, Department of Atomic Energy, Begumpet, Hyderabad - 500016, IN
2 Atomic Minerals Division, Department of Atomic Energy, Begumpet, Nagarabhavi, nangalore - 560072, IN

Abstract

Petrology and geochemistry (major, minor and trace elements including REE) of the different granitoids from Central Gnessic Complex indicate that, (a) the dominant Palaeoproterozoic migmatite, hornblende granite gneiss, two-mica granite and augen gneiss are syn-kinematic, autochthonous, peraluminous, low-Ca granodiorite, with the first three being of S-type formed by highdegree (>50%) anatexis of sediments of psammopelitic composition, whereas the augen gneiss is I-type formed by partial melting of a basic rock of mantle affinity coupled with notable crustal contamination and affected by soda metasomatism; (b) the minor Mesoproterozoic Salari granite is late-kinematic, allochthonous, metaaluminous, epidote-bearing, high-Ca monzogranite of I-type formed by partial melting of basic rock from crustal depth; (c) the youngest (ca. 30 Ma) tourmaline granite is post-kinematic, allochthonous, strongly peraluminous, high-level syenogranite of S-type, with its parental melt derived from a peraluminous granitoid by anatexis induced by large volume of volatiles and fluids, and (d) tectonically, these granitoids are of 'syn-collision' type.

Keywords

Petrology, Geochemistry, Granitoids, Anmachal Himalaya.

Full Text