DETAILED STUDY OF UHP PHENGITE FROM A 10-METER TERRANE, TSO MORARI TERRANE, INDIA

Phengite, the high-pressure form of muscovite, can preserve pressure-temperature conditions and chemical signatures of the fluids present during its growth. The Tso Morari Ultra-high Pressure (UHP) Terrane in NW India is largely made up of white-mica-bearing, quartzo-feldspathic gneiss, which have not experienced retrograde recrystallization, post-UHP, in a uniform manner. On the scale of a 10m traverse perpendicular to the dominant fabric direction, samples in close proximity do not have the same phengite composition. Phengite samples > 5 m from the contact with the eclogite preserved the highest silicon concentrations (6.98 Si p.f.u.) while samples at the contact (TM11) and 2-3 m away preserve intermediate compositions between muscovite and phengite (6.45 Si p.f.u.). If we consider the Si and FeT/Mg values as markers of pressure and temperature respectively, we have grains that grew both at near-peak P-T conditions and at lower pressures during exhumation. Phengite in TM2 have an average Si of 6.88 p.f.u., suggesting preservation of the grains that grew in the UHP event. The compositions of mica grains in TM3 and TM11 have lower counts of 6.42 Si p.f.u. suggesting recrystallization during exhumation. FeT/Mg data shows increasing values from TM2 to TM11, from 2.67 to 6.1 with TM3 at an intermediate value of 4.05. The meter-to-centimeter scale heterogeneity coupled with the strongly expressed banded fabric suggests that the gneiss experienced heterogeneous strain and incomplete recrystallization on its return to the crust. The samples with the UHP phengite preserved will be used in conjunction with another study of boron isotope and trace element data measured in situ on the same grains. White mica (phengite) is the primary mineral used because it is a hydrous, high-pressure phase that characteristically contains boron when tourmaline is absent and is sensitive to P-T changes and in UHP terranes is often preserved even after exhumation. Initial electron probe data has confirmed both high-pressure phengite and retrograde lower-pressure phengite in the samples. The next steps will compare this data to the boron data and see if the micas have distinct in situ δ11B concentrations. Previous studies suggest that the phengite would have low boron concentrations and highly negative δ11B values that are below the range of values expected by MORB basalts and the mantle.