Fission track studies on some minerals and water of the northeast India

Abstract

The present work is aimed at the geochronological investigations of the minerals of some granitic rocks and estimation of uranium contents in minerals, rocks and water of some parts of the North-East India with the help of fission track (FT) technique. The study includes FT dating of minerals from granites of four different plutons and from pegmatites of two different regions of Meghalaya, annealing studies of the minerals including the correction of the observed mineral ages and calculation of the cooling and uplift rates of the rocks. Precision on the errors of the mineral ages has also been studied in detail in the present work. The experiment was carried out at the Research laboratories of the Physics and the Geology Departments, Gauhati University, Assam, India during 1981-1984. Experimental results show four different age groups of the minerals which would reflect meaningful geological events in the history of these rocks. Sphenes of the Nartiang and Dawki granites record the highest FT ages ~ 1120 m.y. which may be linked with a major plutonic event. Muscovites of the Nartiang and Jowai pegmatites measure ages ~ 690 m.y. which may correspond to the time of emplacements of these pegmatites. Apatites of the Nartiang granites show FT ages ~ 470 m.y. which falls within the period of Indian Ocean cycle event. But, apatites of the Jowai, Pynursla and Dawki granites record the youngest ages ~ 90 m.y. which lies within the period of extru-sion of cretaceous lavas (Sylhet traps). Apatite age of the Nartiang granite and muscovite ages of the Nartiang and Jowai pegmatites were corrected to ~ 510 m.y., ~ 743 m.y. and ~ 741 m.y. respectively by applying the plateau method of age correction. However, insignificant corrections were found in the sphene ages of the Nartiang and Dawki granites and apatite ages of the Jowai and Dawki granites. An interesting finding of the present work is the wide discrepancy between the apatite ages ~ 90 m.y. (Cretaceous) of Jowai, Pynursla and Dawki granites, and the sphene ages ~ 1120...