Geological Setting

Bomdila orthogneisses occurs in the Arunachal Himalaya, Arunachal Pradesh, which is the northeastern most state of India. The study area, situated in the Lesser Himalaya, West Kameng district of Arunachal Pradesh, is bounded by many important thrust faults on either side (Fig. 1). Although many workers have studied the area and proposed different stratigraphic names (Acharya et al., 1975, Verma and Tandon, 1976, Kumar, 1997), recently, on the basis of lithostratigraphy, grade of regional metamorphism and associated igneous intrusives, the Proterozoic rocks of Arunachal Himalaya have been grouped into three major tectono-stratigraphic units by Srinivasan (2001).They are the Sela Group, Rupa Group and the Bomdila Group. The Sela Group, which is considered to be the youngest sequence (late Proterozoic) among the three, is best exposed around Se La pass along Bomdila-Tawang road in western Arunachal Pradesh close to Bhutan border. It consists of calc-silicates, marble, kyanite-sillimanite ± staurolite polyphase deformed schists, migmatites, high-grade ortho-augen gneisses and amphibolites, etc. with younger intrusions of hornblende granite (481 ±23 Ma, Dikshitulu et al., 1995), tourmaline granite (29 ±7 Ma, Bhalla and Bishui, 1989), pegmatites and aplites.

Figure 1. Geological map

Geological map

Geological map of the Arunachal Pradesh (after Srinivasan, 2001) showing different lithostratigraphic units including Bomdila orthgneisses.

The Rupa Group of rocks belong to Mesoproterozoic which unconformably overlies the Bomdila Group in the Lesser Himalaya. The Rupa Group constitutes a thick sequence of low- to medium-grade garnetiferous biotite-muscovite schists, phyllites, sericite quartzites, calc-silicates and tremolite-actinolite marbles. Main Crystalline Thrust (MCT) separates the Sela group from Rupa group of rocks and thus the former group belongs to the Higher Himalayas. The Bomdila Group comprises essentially of low- to mediumgrade metasedimentary rocks (mainly phyllites, garnetiferous mica schists and quartzites) intruded by Paleoproterozoic Bomdila augen gneisses and mafic metavolcanics.

The Bomdila gneiss, which has been characterized as orthogneiss on the basis of textural properties, is a batholithic dimension body occupying ~500km2 area in the western Arunachal Pradesh, India. It is characterized by medium- to coarse-grained, well-defined porphyritic augen gneisses wherein the quartz/albite augens, measuring 1-10 cm, are wrapped with biotite and muscovite. The augen gneisses exposed around Bomdila town have been dated as 1914 ±23 by Rb/Sr technique by Dikshitulu et al. (1995) and hence are considered to be Paleoproterozoic in age. Mafic metavolcanics (amphibolite sills and dykes) occur within the gneisses. The discordant field relations and reaction zones noticed at number of places with the gneisses clearly demonstrate that the metavolcanics are younger than the gneisses and might have intruded into the basement rocks.

A weakly foliated tourmaline-bearing leucogranite is well exposed along Bomdila-Rupa road section. This granite shows an intrusive relationship with the augen gneisses, which further indicates that the leucogranites are younger than augen gneisses. In places, the tourmaline granites are associated with pegmatites and numbers of aplite along with quartz veins. Metasedimentary (pelitic schist) enclaves of different sizes have been found in the augen gneisses, which is in agreement with the already established hypothesis that the collision-related, S-type granites contain metasedimentary enclaves. This in turn suggests that the metasediments might have had a significant role in the petrogenesis of these granites. Efforts were made to collect fresh and unweathered samples of the gneisses and granites mainly from road cuttings in and around Bomdila town.