Volume : 2, Issue : 2, February - 2013

Petrographic Characteristics of Basement Rocks in A few Bore Holes of Upper Assam Basin

C. Taye, P. Bhattacharyya

Abstract :

Detail Petrographic studies of fourteen borehole samples of the basement rock of Diugarh, Tinsukia and Sivasagar District of Upper Assam have been made. Two subsurface samples of gneissic rocks of Sylveta area of Karbi–Anglong district of Assam which are supposed to be the exposed equivalent of the hidden basement rock of Upper Assam basin were also studied. Petrographic study reveals that the basement rock of the borehole samples are Granite gneiss, Biotite–gneiss and Porphyritic Granite types. The granite gneisses are predominantly composed of quartz, feldspars with or without hornblende and some biotite flakes while the biotite gneisses are dominated by biotite, quartz and minor amounts of feldspar with or without hornblende. The granites are mostly porphyritic in nature and exhibited by myrmekitic texture. Large phenocrysts of microcline are common. The mineralogy and texture of all the borehole samples resemble the characteristics of the gneissic and granitic samples of Sylveta area, the northeastern most part of Assam–Meghalaya Plateau. A very important feature in the sample of borehole– Lohali–1 is that a patch of fine grained volcanic rock is found to occur in the granitic rock. This volcanic rock is a part of metavolcanics and composed mainly of actinolite, pyroxene (feathery), sodic plagioclase with sub–schistose texture.