Distribution of U-Th-K in Metamorphic Rocks of Siang Group in Parts of Baririjo-Maro-Gamkak-Kau-Darak Section, West Siang and Upper Subansiri Districts, Arunachal Pradesh

Metamorphic rocks of the Siang Group in West Siang and Upper Subansiri districts, Arunachal Pradesh, have anomalous concentration of intrinsic radioelements (viz., U, Th and K). The study reveals that psammopelites comprising paragneiss, carbonaceous phyllite and quartzite/phyllite/mica schist (av. 8.2-27.3 ppm U, 19.7-42.8 ppm Th, 2.2-4.0 % K); volcano-sedimentary garnet amphibolite (av. 14.3 ppm U, 23.1 ppm Th, 1.8 %K), and intrusive feldspar porphyry (av. 12.7 ppm U, 26.3 ppm Th, 5.9 %K) have higher radioelement contents, particularly U, in comparison to pelitic graphite schist, iron formation and metabasics (av. <4.2-4.7ppm U, 18.5-60.2 ppm Th, 3.3-4.9 %K). All rock types, except feldspar porphyry, exhibit noticeable difference in mean U and Ra (eU) values. Variation in U content has been conspicuous in carbonaceous rocks, with phyllitic units more enriched than graphitic schists. U and Th contents appear to be independent of K concentration in all rock types. Such features of variable radioelement concentration indicate differences in the composition of precursor sedimentary rocks and also differential mobility during metamorphism.