AN ION MICROPROBE STUDY OF SINGLE ZIRCONS FROM THE AMET GRANITE, RAJASTHAN

We have used an ion microprobe to measure Pb-207/Pb-206 ages on carefu lly selected similar to 20 mu m domains of single zircons from two sam ples of the Amet Granite. A sample of the Amet Granite, consisting of a strongly foliated augen gneiss, yields a minimum crystallization age of 1641+/-14 Ma (1 sigma). From a second sample, collected from an ap lite dyke which cross-cuts the foliation of the augen gneiss, we calcu late a minimum emplacement age of 1766+/-16 Ma; both these ages are ba sed on a dynamic background correction of 0.01 c/s. As part of this st udy we conducted 30 analyses of an internal standard which has a publi shed, concordant age of 2631+/-4 Ma. Our data suggest that a low inten sity dynamic background (less than or equal to 0.01 c/s), comparable t o the Pb-204 count rate, is present within our system. This background could, in principle, lead to considerable uncertainties when calculat ing Pb-207/Pb-206 ages for young samples which have low radiogenic Pb- 206 concentrations. We believe an inaccurate background correction is the most likely explanation why the cross-cutting dyke yields an appar ent age which is older than that obtained from its host rock Assuming that the dynamic background was negligible at the time of its analysis , we recalculate an age of 1646+/-19 Ma for the dyke sample which is s tatistically indistinguishable ; from the age of the augen gneiss. The use of small geometry ion microprobes for obtaining Pb-207/Pb-206 age is best suited for older and radiogenic Pb-rich zircon, when such a l ow intensity background becomes less significant. Our results indicate that the crystallization of the Amet porphyritic granite, the tectoni c emplacement of the nearby mid-Proterozoic Sand Mata granulites and t he subsequent deformation are the products of a single tectonic cycle which was completed by similar to 1.64 Ga.