Precambrian Crustal History Unraveled from the Geochemical Studies of Post-Archean Rocks, Arunachal Pradesh, NE Lesser Himalaya

Abstract

In an endeavor to document the composition and evolution of the Precambrian crust from the Northeast Lesser Himalaya from where much less information is available, Paleoproterozoic (1900 ± 100 Ma) metasedimentary (pelites and quartzites) rocks and granites from the Bomdila Group, Arunachal Pradesh were thoroughly examined. The integrated approach adopted in the study including field, petrography and major and trace element geochemistry indicate that the metasediments are felsic in composition and may have been derived from a granitic source, which may have undergone moderate to intense chemical weathering. The associated basement granites, on the basis of field and mineralogy, consists of two types of magmatic phases; porphyritic gneisses containing biotite and muscovite without tourmaline (referred as two-mica granites) and a weakly to non-foliated leucogranite having abundant tourmaline (referred as tourmaline granite). The geochemical signatures such as high peraluminosity (A/CNK > 1.1), S-type nature, normative corundum, presence of metasedimentary enclaves, enrichment in incompatible elements (Rb, Ba, K, Th, La) and depletion in high field strength elements (HFSE) and their respective ratios (such as Zr/Sc, Ti/Zr, Th/Sc) suggest that both suites are derived from a pelitic source, similar to that of the associated metasediments. This study further suggests the unroofing of felsic material which has supplied detritus to the Bomdila basin, thus establishing the felsic composition of the Precambrian crust in the region.