GEOCHEMICAL ANALYSIS OF TOURMALINE AS A TOOL FOR DETERMINING THE LOCATION AND CHARACTERISTICS OF THE MAIN CENTRAL THRUST, CENTRAL NEPAL

Understanding the location and characteristics of the Main Central Thrust (MCT) in Central Nepal is of major importance because it separates the Lesser Himalaya (LH), which consists of low-grade metamorphic rocks, from the Higher Himalaya (HH), comprised primarily of gneisses and leucogranites. Determining the location and extent of the MCT is essential to understanding the collision of the Indo-Eurasian plates; as well as determining the cause of inverted metamorphism, and the source of granitic bodies, both of which occur across the LH and HH. Determining the location of the MCT could also aid in predicting earthquakes, landslides, and glacial lake outburst floods (GLOFs), and guide the search for economic mineralization. Determining the location of the MCT is difficult due to the lack of fossils or other geochronologic markers, the gradational nature of the MCT, the array of proposed definitions of the MCT, and the inaccessibility of outcrops. Tourmaline is a mineral that may add significant insight to the important issues associated with the MCT. With the advent of sensitive micro-analytical techniques, tourmaline has recently shown promise as a recorder of geologic processes over a broad range of pressures, temperatures and compositions due to its complex structure (sectored and zoned), variable chemical substitutions, and resistance to alteration. In May of 2014 samples were collected from 114 sites along a north-south transect across Central Nepal from Langtang Valley to Kathmandu Valley. Tourmaline-bearing samples were primarily collected in leucogranites of the HH and LH. Elemental maps of 100-200 individual tourmaline grains using Scanning Electron Microscopy are being carried out, which will allow for careful analysis and comparison of inter-sector and inter-zonal major and trace element chemical signatures using Electron Microprobe (EMP) and Laser Ablation - Inductively Coupled Mass Spectrometry (LA-ICPMS). The tourmalines unique chemical “fingerprint” will help establish the actual location of the MCT because of the difference between the LH and HH. Tourmalines with unique chemical signatures will further delineate the source of individual granites. Lastly, trace chemistry can provide constraints on pressure, temperature, and composition, of the melts and fluids at the time of formation.