USING MULTIPLE GEOCHRONOMETERS TO BETTER CONSTRAIN GEOLOGIC EVENTS IN CENTRAL HIMALAYAN GRANITES
Using a combination of U-Pb depth profiling and in-situ LA-ICPMS measurements, we obtain dates from zircon, monazite, apatite, allanite, and xenotime. For each sample, monazite and zircon rims give similar dates interpreted here to represent the timing of igneous crystallization. In total these dates span over 50 million years (510 and 460 Ma), significantly longer than previous estimates of the duration of pre-Himalayan magmatism within the klippe. Monazite in these samples commonly display breakdown textures characterized by successive coronae of 1) fine grain apatite with thorite inclusions and 2) blocky allanite. Similar textures from other locations have been previously interpreted to form from post-magmatic hydrothermal alteration during subsequent metamorphism. Isotopic analysis of these alteration products, as well as accompanying xenotime, yields dates between 40 and 20 Ma, consistent with monazite breakdown during Cenozoic orogenesis. Together these results outline a prolonged history of crustal reworking in the central Himalaya and demonstrate the importance of analyzing diverse geochronometers, beyond zircon, when working in geologically complex areas.