An Ion Microprobe Study of Single Zircons from the Amet Granite, Rajasthan

We have lIsed an ion microprobe to measure ²⁰⁷Pb/²⁰⁶Pb ages on carefully selected ∼20 μm domains of single zircons from two samples 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 (1σ). From a second sample, collected from an aplite dyke which cross-cuts the foliation of the augen gneiss, we calculate a minimum emplacement age of 1766±16 Ma; both these ages are based on a dynamic background correction of 0.01 c/s.

As part of this study we conducted 30 analyses of an internal standard which has a published, concordant age of 2631±4 Ma. Our data suggest that a low intensity dynamic background (≤0.01 c/s), comparable to the ²⁰⁴Pb count rate, is present within our system. This background could, in principle, lead to considerable uncertainties when calculating ²⁰⁶Pb/²⁰⁴Pb ages for young samples which have low radiogenic ²⁰⁶Pb concentrations. We believe an inaccurate background correction is the most likely explanation why the cross-cutting dyke yields an apparent 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 statistically indistinguishable from the age of the augen gneiss. The use of small geometry ion microprobes for obtaining ²⁰⁷Pb/²⁰⁶Pb age is best suited for older and radiogenic Pb-rich zircon, when such a low intensity background becomes less significant.

Our results indicate that the crystallization of the Amet porphyritic granite, the tectonic emplacement of the nearby mid-Proterozoic Sand Mata granulites and the subsequent deformation are the products of a single tectonic cycle which was completed by ∼ 1.64 Ga.