USING MULTIPLE GEOCHRONOMETERS TO BETTER CONSTRAIN GEOLOGIC EVENTS IN CENTRAL HIMALAYAN GRANITES

The central Himalayan Kathmandu Klippe preserves an over 500 million year record of tectonism along the northern margin of India, punctuated by two distinct tectonothermal events during the early Paleozoic and the Cenozoic. Among the rocks exposed within the thrust sheet are a series of peraluminous S-type leucogranite plutons. Previous geochronologic studies of these rocks—largely focused on U-Pb zircon dates from single exposures within individual plutons—suggested the granites were emplaced relatively rapidly as single bodies between 480 and 470 Ma. Recent reconnaissance mapping, however, has revealed the occurrence of at least 4 distinct compositional and textural varieties of granitic rocks within the region, suggesting that the previous geochronologic constraints may not accurately describe the true timescale of magmatism. To better understand the timing and duration of pre-Himalayan igneous activity, we took a multi-chronometric approach to date multiple rocks collected from three different plutonic bodies from the southern klippe margin: the Palung, the Sindhuli, and Dhobare Thumka 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.