A STREET MAP NAMED TOURMALINE: EXPLORING THE POTENTIAL OF THIS ACCESSORY MINERAL IN THE EVOLUTION OF THE HIMALAYAN MAIN CENTRAL THRUST

The Main Central Thrust (MCT) is a 2-10 km thick shear zone that closely parallels the entire topographic front of the Himalayas. The structure accommodated a significant fraction of shortening absorbed by Indo-Asia collision, and juxtaposes high-grade gneisses (Greater Himalayan Crystallines) in its hanging wall against footwall phyllites, metaquartzites, and mylonitic augen gneisses (Lesser Himalaya). An apparent inverted metamorphic sequence characterizes the shear zone that underlies the thrust, making precise placement of the MCT problematic because of the lack of a break in metamorphic grade across the structure. Tourmaline is a common accessory mineral in Himalayan rocks, thus the objective of this investigation is to determine whether a correlation could be made between tourmaline grain size, abundance, and composition with the structural location of a rock within the MCT. Samples from the Marysandi River transect across the MCT in central Nepal were analyzed using a petrographic microscope and electron microprobe. The grain size and distribution data from samples of the Lower Lesser Himalaya, the Upper Lesser Himalaya, and the Greater Himalayan Crystallines Formation I, display a consistent pattern. Rocks from the footwall of the MCT are characterized by a frequent occurrence of tourmaline grains with a small area and perimeter. Those taken from the hanging wall contain larger, but less frequent tourmaline grains. In general, Greater Himalayan tourmaline grains are less compositionally zoned and have lower Fe/(Fe+Mg) contents. The scientific motivation for this work revolves around several key issues, including (1) testing existing generalizations about correlations between tourmaline composition and petrogenesis, (2) ascertaining information about the sedimentary environment of the protoliths of rocks juxtaposed by the MCT, and (3) seeking further geochemical constraints to aid in placement of the MCT.