Fission track ages of some coexisting minerals in granites of AssamMeghalaya plateau

Abstract

This thesis embodies the results of (i) the geochronological investigation on the co-existing zircon, sphene and apatite minerals from the granite plutons viz. of Hamren, Umroi and Nongkhlow areas of the Assam - Meghalaya plateau by the Fission Track Dating(FTD) technique (ii) track annealing studies of the minerals along with the correction of the observed minerals ages ; (iii) the estimation of the uranium content of the whole rock samples as well as of the dated minerals and (iv) the cooling and uplift rated of the rocks, The calculated fission track ages of the co-existing apatite, sphene and zircon collected from the Hamren, Umroi and Nongkhlow granite plutons of the Assam-Meghalaya plateau are given below in table 1. Table 1 Fission Track Ages of co-existing apatite, sphene and zircon According to the above data, the co-existing minerals of the Umroi and the Nongkhlow granites of Megha-laya show very close ages but lower than those shown by the minerals of the Hamren granite of Assam. The discrepancy of FT ages between co-existing minerals can be explained by different cooling ages of these minerals and the slow crystallization of the rock or by part annealing of apatite and sphene. Further, it may be mentioned that the zircon, sphene and apatite are the minerals having annealing temperatures in the decreasing order and thus their ages reflect meaningful successive geothermal events of gradually lower intensity in the history of these rocks. The results on study of annealing behaviour of the minerals collected from the Umroi, the Nongkhlow and the Hamren granites is shown in the table II. Table II Annealing behaviour of the co-existing apatite and sphene. The study of annealing behaviour of the co-exist-ing apatite and sphene of Umroi and Nongkhlow granite reveals that the apatite and the sphene must be heated upto 3800C and 7030C respectively for one hour for complete fading of tracks. It is seen that the activation energy ranges from 1.48 eV to 2.81 eV for apatite and 2.42 eV to 5.37 eV for sphene as...