Petrogenetic Appraisal of Early Palaeozoic Granitoids of Kinnaur District, Higher Himachal Himalaya, India

The Early Palaeozoic felsic magmatism of Kinnaur district is represented by more-or-less equivalent Akpa granitoids (477±29 Ma) and Rakcham granitoids (453±9 Ma), which form an integral part of the Central Crystalline Zone in the southern portion of the Tethys Himalayan Tectogen. Both, Akpa granitoids (AG) and Rakcham granitoids (RG), intrude the rocks of the Vaikrita Group. Substantially low magnetic susceptibility values (c = 0.016-0.187 × 10⁻³ SI) of these Early Palaeozoic granitoids suggest their nature similar to as ilmenite series (reduced-type) granitoids emplaced in a syn-collisional tectonic setting. The modal compositions of AG mainly correspond to two-mica (muscovite-biotite) monzogranite belonging to the granitoids generated by crustal fusion. Based on mineral assemblage (Ms+Bt+Tur+Ap), associated skarn-type tungsten mineralization, occurrence of metasedimentary enclaves, and whole-rock geochemistry (SiO₂ = 70.75-72.95 wt.%, TiO₂ = 0.02-0.17 wt.%, K₂O/Na₂O = 1.05-2.32, molar A/CNK = 0.93 to 1.33, CIPW corundum 0.17 to 3.82 wt.%, Av. Sr = 304ppm, initial ⁸⁷Sr/⁸⁶Sr = 0.7206±0.00235), the AG can be typically characterized as peraluminous (S-type) granite as similarly noted for RG. Higher Rb/Sr ratio and initial ⁸⁷Sr/⁸⁶Sr = 0.737±0.002 of RG compared to that of AG however indicate involvement of relatively more crustal component in generation of RG. The AG is enriched in Ba and light rare earth elements (LREE). The chondrite normalized REE patterns of AG appear weakly fractionated (La_N/Lu_N= 1.06-6.43) and show pronounced negative Eu-anomaly (Eu_N/Eu^* = 0.03). The AG and RG represent typical S-type granite suites that largely evolved due to differential degree of anatexis (partial fusion) of sedimentary protoliths, prior to Caledonian but after the Pan-Indian thermal orogenic event.