Fission track studies on some accessory minerals in granites of Meghalaya

Abstract

Fission track ages of apatite and sphene minerals from Mylliem granite (MG), South-Khasi batholith (SK) and Nongpoh granite (NG) have been studied in the Departments of Physics and Geology, Gauhati University during 1979-83. The estimated ages are as follows. The results indicate a difference of age between the co-existing apatite and sphene of about 200 m.y. which may be explained by early crystallization of sphene and slow crystallization of the rock (as also evidenced by the very coarse-grained, porphyritic nature of the granites) and/or part annealing of apatite. The annealing behaviour of the minerals is also studied. It is seen that the apatite and sphene collected from Mylliem granite must be heated to temperature above 3980C and 7070C for one hour respectively for complete fading of tracks. It is found that the activation energy ranges from 1.2 eV to 2.4 eV for apatite and from 2.5 eV to 5.4 eV for sphene. The mean fission track ages for muscovites of the pegmatines from Nengkhra, Garo Hills and from Nongkhlow, Khasi Hills are found to be (531+_15) m.y. and (501+_25) m.y. respectively. Study on dependence of track density on anneal-ing time and temperature on muscovite indicates that muscovite will lose all tracks if it remains at a temperature 5270C for one hour. Activation energy of muscovites ranges from 1.0 eV to 2.8 eV. The present investigation uranium concentration in the three granites of Meghalaya ranges from (1.64+_0.04) to (2.94+_0.04) ppm by weight. Uranium concentration in water samples collected from different parts of Assam are measured by using SSNTD. Nothing abnormal in the U-content of various water sources of the region is observed. The results are compared with those of other workers wherever available.