Journal of the Virtual Explorer

Abstract

The Paleoproterozoic Bomdila granites emplaced close to Main Central Thrust in the amphibolite-grade metasedimentary rocks in the Arunachal Pradesh, North East Lesser Himalaya. On the basis of field and petrological studies, two principle types of granites have been distinguished: porphyritic biotite-muscovite (two-mica) granites and tourmaline-bearing leucogranites. Both the types are subalkaline, strongly peraluminous (A/CNK > 1.1) and have >70% SiO₂, Na₂O + K₂O = 5.5 – 8.5%, K₂O/Na₂O = 0.8 – 2.9. The two-mica granites show relatively more abundance of Rare Earth Elements (REE) concentrations (up to 294 ppm) when compared to the tourmaline granites, which have a lesser amount of REE contents (up to 67 ppm). However, both the types exhibit similar LREE enriched and HREE depleted patterns with consistent moderate Eu negative anomalies (Eu/Eu* = 0.3 – 0.6). Comparison with experimental data and Q-Ab-Or-H₂O phase relations in the haplogranite system indicate the magmas for two-mica suite were generated by dehydration melting of biotite under water-undersaturated conditions at temperatures of > 8000C and pressures less than 5 kbar, with tourmaline-bearing samples representing near minimum melt compositions formed under low-extent dehydration melting of muscovite. The Bomdila granites with peraluminous geochemical characteristics and the presence of pelitic enclaves, probably exhibit the features of S-type granites, and thus it is proposed that the granites may have been derived by dehydration partial melting of the upper crust. Having high Rb contents and low Y + Nb abundances the granites plot mostly within the Syn-Collision field, indicating that the granites in Bomdila formed in a continental collision tectonic setting. These results support the hypothesis that the peraluminous S-type granites are generally derived from partial melting of pelitic protoliths and further document a regional late Paleoproterozoic collisional tectonic event, which is probably related to the welding of two ancient continental blocks along this region.