In situ U-Pb zircon micro-geochronology of MCT zone rocks in the Lesser Himalaya using LA-MC-ICPMS technique

Abstract

A multi-collector (MC) inductively coupled plasma mass spectrometer (ICPMS) was used in combination with an Excimer (193 nm) laser to carry out in situ U-Pb dating of zircons. High performance twovolume sample cell provided unmatched laser ablated aerosol transportation efficiency resulting in reducing laser-related down-hole fractionation. Three wellcharacterized natural zircon reference standards (Harvard zircon 91500, GJ-1 zircon, Plešovice) were repeatedly measured in different sessions to evaluate the analytical figures of merits. Precision of < 1% was achieved for spot sizes 20 μm with accuracies well within 2% of the reference values for these standards. Zircons from MCT Zone in the inner Lesser Himalaya reveal a highly discordant Palaeo-proterozoic (1901 ± 11 Ma) magmatic crystallization age inferred from the upper intercept in the concordia plot. The 207Pb/206Pb ages are also internally consistent with the disconcordia age with a weighted mean of 1900 ± 10 Ma and in turn suggest a major phase of Palaeo-proterozoic magmatic activity along the northern margin of Indian craton, while Early Miocene (~25 Ma) Pb loss in zircon inferred from lower intercept in disordria may be related to tectono-thermal activity along MCT.