In situ U-Pb Zircon Micro-Geochronology of MCT Zone Rocks in the Lesser Himalaya Using LA–MC–ICPMS Technique

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 m 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 Pb/Pb 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.

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