Petrography

Petrography

In general, the Bomdila Orthogneisses are foliated, augen-bearing, medium to coarse grained, leucocratic and consists of quartz, plagioclase (albite to oligoclase), microcline, biotite and muscovite as essential minerals. The substantial amount of felsic mineralogy strongly suggests that they are leucogranite magmas, in agreement with the common occurrence of other such leucogranites in the Himalaya. On the basis of mineralogy (and field observations), two principle types of granites are identified. They are tourmaline-free biotite-muscovite (here after referred to as two mica) granites, which constitutes major part of the batholith and tourmaline-bearing granites (here after referred to as tourmaline granites). The only mineralogical difference between the two types is in the nature of the ferromagnesian phase, biotite in two-mica granites and tourmaline in tourmaline granites.

The gneissose structures with perfect alternate bands of mica and quartzo-fedspathic minerals are preserved in the two mica granites in which the augens are of quartz and sodic plagioclase. They show porphyritic texture with the development of K-feldspar and plagioclase phenocrysts set in a groundmass of quartz, biotite and muscovite. In addition to perthite, myrmekite and graphic, intergrowth textures were also developed in the two-mica granites. Inclusional relationships and crystal morphologies have been used to constrain the crystallization sequence, which is very similar for both types. Plagioclase showing tabular habit was perhaps the earliest mineral to crystallize, whereas anhedral K-feldspar phenocrysts, which contain abundant inclusions of other phases, occurred later. Biotite is usually interstitial and commonly intergrown with muscovite. However, albitic plagioclase (An12) phenocrysts with occurrence of abundant euhedral biotite and muscovite inclusions (along cleavage planes) suggest that the latter minerals began to crystallize early. The quartz is generally interstitial but also commonly occurs as rounded inclusions in plagioclase, K-feldspar, biotite and muscovite, thus indicating an early crystallization. Some samples, particularly those from the marginal part of the main pluton, show fine- to medium-grained cataclastic textures with abundant tectonic-related features. For example, occurrence of ribbon quartz with undulating extinction, bending in biotite, bending and cracks in plagioclase which are invaded by plastically mobilized quartz, etc. are some of the features indicative of ductile shearing of the rocks at the contact zone. In addition to these features, the two mica granites also exhibit mylonitization characteristics at the contact zone along with the development of tectonically fractured garnets where muscovite and quartz squeezed plastically into cracks of the brittle garnets. Among the accessory minerals, iron oxides occur along with apatite, zircon and garnet.

The tourmaline granite consists of predominantly quartz, K-feldspar, plagioclase, muscovite and tourmaline. Biotite is either absent or very rarely found in the thin sections. Tourmaline is euhedral to subhedral, slightly broken and contains abundant inclusions of quartz, muscovite and, in some cases, of apatite. Tourmaline is also found as inclusions in euhedral plagioclases and anhedral quartz. The occurrence of penetrative relation between tourmaline and muscovite suggest a magmatic origin for these two minerals. Foliation is completely absent in this granite, however few thin sections show weak foliation. Textures are mainly equigranular, with occasional plagioclase or K-feldspar megacrysts up to 4mm in length. Quartz recrystallization along cleavage planes of perthitic feldspar suggests that it has been subjected to metamorphism during Tertiary Himalayan orogeny and hence is considered to be older than Tertiary age. Zircon and tourmaline occur as common accessory minerals.