REMOTE SENSING OF GNEISS DOMES AND GRANITE IN SOUTHERN TIBET

South of the Indus-Tsango suture, in southern Tibet, the Tethyan sedimentary belt is marked by granitic intrusions and gneissic domes that coincide with the axis of the Northern Himalayan antiform (NHA). In a previous study, short-wave infrared ASTER sensor images from the Earth Observatory System Terra satellite were used to map the gneiss domes and Tertiary granite intrusions along the NHA from the approximate latitudes of 28°N to 30°N and longitudes of 86°E to 90°E using ASTER bands 6, 5, and 4, loaded in red, green, and blue layers, respectively. These bands show the domes as a white to blue area with the surrounding metasediments as dark purple. ASTER band ratios 7/6, 6/5, and 6/4 loaded in red, green and blue layers, respectively, were used to distinguish muscovite-rich granite from muscovite-poor gneiss. The core of the well studied Kangmar dome consists of granite gneiss with less than 1% muscovite. Field study reveals the Mabja granite as a two-mica granite with muscovite content of more than 5%. These bodies were used to calibrate the band ratio images as the muscovite-rich granite appears red, while the gneiss appears blue. We applied these techniques to ASTER images covering the area between latitudes of 28°N to 30°N and longitudes of 84°E to 86°E. The images produced from bands 6,5,4 reveal the outlines of two domes flanked by metasediments in the area south of the Tsangpo suture. Images of these two domes examined using ASTER band ratios of 7/6, 6/5, and 6/4 predominately display the characteristic red signature associated with muscovite-rich granites. However, preliminary field work on the northern margin of the Changgo dome, located at approximately 29.15°N and 84.78°E, reveals a biotite-rich granite with only minor amounts (~5%) of muscovite. Spectral analysis of samples from this area, assisted by field work and geochronology, will allow further calibration of the band ratio method.