Thermal History of Rajasthan Pegmatites of India as Revealed by Fission Track Studies: Discussion

Abstract

Nagpal and Nagpaul (1974) have inferred a cooling rate of m.y.-' for Rajasthan pegmatites and related this to ,the last major metamorphic event, that is, Delhi orogeny (860-735 m.y.) (p. 522). They bracket my work (Crawford 1970) with that of Holmes (1949), Holmes (1955), Sarkar et al. (1964), and Vinogradov et al. (1964), and imply that we all agree that ages of gneissic complex are of order of 2600 m.y., Aravalli orogeny from about 1500 m.y. to about 950 m.y., and Delhi from about 860 m.y. to about 735 m.y. We do not so agree. I attempted to show that picture is far more complex, that gneissic complex contains rocks of various ages, that Aravalli rocks must be younger than about 2500 m.y. and older than about 2000 m.y., and that Delhi rocks were first intruded by granites dated between 1700 and 1650 m.y. In other words, dates show that sequences are very much older than previous workers believed and the several orogenic cycles postulated by Holmes (1955) in his valuable pioneer study cannot be substantiated . (Crawford 1970, p. 108). I agree with Fermor (1950) who, in questioning conclusions Holmes drew about age of Delhi Systems from pegmatite mineral ages, stated that they gave information only about those pegmatites. Nagpal and Nagpaul have obtained fission-track ages for muscovites with a mean value of 787 + 42 m.y. It is relevant to state here that total-rock Rb-Sr analyses of Mount Abu granite and Idar granite fit an isochron giving an age almost exactly that of Malani Rhyolite and its associated granites, 745 + 10 m.y.