Unusual Hydrothermal Rims on Igneous Zircons, Tso Morari Complex, Ladakh, India

Understanding how mountain belts form requires identifying the ages of the rocks within them. The granitic rocks of the Tso Morari massif are thought to have formed ~450-500 million years ago (Ma), but there are few studies, and granites elsewhere in the Himalaya are as young as ~425 Ma. Here, we image and date zircon (ZrSiO4) grains from a metamorphosed granite of the Tso Morari massif to gain a better understanding of the age of the intrusion and origin of the zircon. Zircon crystals were separated, mounted in epoxy, polished, and imaged using cathodoluminescence (CL), Field Emission Scanning Electron Microscopy (FESEM) and Raman spectroscopy to identify zoning patterns (including inherited cores) mineral inclusion patterns, and mineral IDs. Lastly, we used the ICAP-RQ quadrupole ICP-MS to measure U-Pb ages and trace element compositions. Most zircon crystals exhibit unusual, high-porosity rims with micro-inclusions of chemically exotic, U-, Th-, REE-rich minerals. These inclusion-rich rims overgrow inclusion-poor cores. The ages of the rims and adjacent cores are indistinguishable at ~430 Ma, younger than previously reported ages. The association of high porosity and exotic mineral inclusions has been interpreted in other studies to reflect hydrothermal replacement of prior zircon. If so, hydrothermal dissolution-reprecipitation must have occurred about the same time as the original igneous crystallization