TEXTURAL AND CHEMICAL ANALYSIS OF WHITE MICA FROM THE TSO MORARI UHP GNEISS

The Tso Morari UHP (ultra-high pressure) terrane in the Himalaya of northwestern India is characterized by eclogite boudins entrained within quartzofeldspathic gneiss. Coupled chemical and textural analyses of white mica were performed on three gneiss samples collected in proximity to a large eclogite boudin to investigate possible correlation between grain size and chemical signatures. Chemical analyses show that there is both muscovite and phengite present in these samples. Petrologic analyses were then conducted to characterize deformation structures and the size ranges of white mica grains. Back-scatter electron (BSE) images of sample thin-sections were examined using the ENVI image analysis software package to measure major and minor axis lengths and calculate aspect ratios and approximate cross-sectional areas for each grain for which chemical data had been previously collected. Grain aspect ratio, major axis length, and cross-sectional area data for each sample were plotted against silicon (Si) abundance per formula unit (p.f.u.), a pressure proxy, and titanium (Ti) abundance p.f.u. and the total-iron to magnesium ratio (FeT/Mg), both temperature proxies, to identify the relationship between textural and chemical characteristics of white micas. Sample TM-2 has more grains containing high Si (>7 Si p.f.u.) and low Ti (<0.025 Ti p.f.u.) than the other samples, indicating that those grains likely grew at higher pressures and lower temperatures; no correlation with grain size is apparent. TM-11 has more grains containing high Ti (>0.03 Ti p.f.u.) and FeT/Mg (>8.0) with low Si (<6.55 Si p.f.u.), suggesting that those grains grew at higher temperatures and lower pressures than TM-2; they were also some of the smallest grains in the sample. TM-3 contains similar Ti and FeT/Mg values as TM-2, but lower Si values, meaning that those grains grew at similar temperatures but likely not at the same pressures. These results suggest white mica grew at different stages along the pressure-temperature path, and that conditions favored the growth of large micas at multiple points along the path. Large grains with phengite composition suggest significant mica growth at high pressure and UHP conditions. Identifying the periods of mica growth can better constrain metamorphic history of UHP terranes.