RADIOELEMENT DISTRIBUTION AND RADIOACTIVE HEAT-GENERATION IN THE CENTRAL GNEISSIC COMPLEX, KAMENG DISTRICT, ARUNACHAL-PRADESH

The Central Gneissic Complex (CGC) in the Kameng distruct, Arunachal P radesh comprises the oldest Sela Group, followed by the Bomdila Group and the youngest Salari Group. The Sela Group consists of migmatite ho rnblende granite gneiss, two-mica granite and profuse intrusions of to urmaline granite (Tertiary ?). Along the Main Central Thrust (MCT), th e Sela Group overlies the Bomdila Group consisting of augen gneiss, qu artzite, quart-mica schist, marble and basic intrusive rocks. The Bomd ila Group, in turn, overlies the Salari Group that comprises black sha les and intrusive high-Ca granites. Radioelement (U, Th and K) content s, determined by gamma-ray spectrometry, of 28 granitic and migmatitic samples of CGC indicate that the tourmaline granite and two-micagrani te, augen gneiss have the highest U values as compared to that of othe rs. 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 gra nite have higher contents than the rest. Radioactive heat, computed fr om each of these radioelements, is highest in the tourmaline granite a nd two-mica granite, with the other rock types having low to intermedi ate values. The average total heat generation of CGC is of the order o f 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 crus t in the Arunachal Himalaya. The radioactive heat generation and the p ublished high heat flow values in the CGC indicate steep geothermal gr adients in the area, which may help in remobilisation and subsequent p recipitation/concentration of U in favourable locales.