The Fission Track Ages of Some Co–Genetic Minerals  in the Granitic Rocks From Nongstoin Area of Central  Assam– Meghalaya Plateau

P. Bhattacharyya; C. Bhagawati; C. Taye

Volume : III, Issue : IX, September - 2013

The Fission Track Ages of Some Co–Genetic Minerals in the Granitic Rocks From Nongstoin Area of Central Assam– Meghalaya Plateau

P. Bhattacharyya, C. Bhagawati, C. Taye

Abstract :

Some granitic rocks including porphyritic and nonporphyritic granites, aplites and pegmatites of Nongstoin area belonging to Central Assam– Meghalaya Plateau have been selected for the present study. The plateau is considered as an extension of peninsular India and consists essentially of quatzo feldspathic gneiss with NE–SW strike of foliation. The gneisses together with the enclaves of various rock units are intruded by several generations of granitic rocks of different types. Fission Track (F.T) ages of sphene and apatite in studied samples were found to be discordant. The apatite ages are significantly lower than those of sphene. These ages range from 595+43 Ma to 525+40 Ma for sphene and 132+12 Ma to 110+10 Ma for apatite respectively. The F.T. ages in sphenes show that the granitic, aplitic and pegmatite veins in the area under study belong to different generations. The sphene ages can be considered as the cooling ages of the rock and mark the ages of Indian Ocean Cycle orogeny. The F.T. ages of apatite showing early to middle Cretaceous age are taken to be indicative of resetting of tracks during a thermal event associated with the extensive igneous activities during the Jurassic Cretaceous time which post dated the time of granitic emplacement in the area. The temperature of this thermal event might not have raised high enough to affect the tracks in sphene.