Preservation of deep Himalayan PT conditions that formed during multiple events in garnet cores: Mylonitization produces erroneous results for rims

Abstract

The Kathmandu Thrust Sheet, which overlies the Lesser Himalayas along the southern part of the Main Central Thrust (MCT) and forms the leading edge of the Higher Himalayan crystalline rocks, is folded at a regional scale by the Gorkha-Kathmandu fold couplet in Central Nepal. Garnet porphyroblasts lying close to the MCT within this thrust sheet preserve structural and metamorphic history that predates mylonitization during thrust emplacement. The succession of five FIA sets preserved within these porphyroblasts formed due to changes in the direction of India's motion relative to Asia after they collided. The intersection of Fe, Ca and Mn isopleths for garnet cores reveals that FIA sets 1, 2, 3, 4 and 5 nucleated respectively at 6.2kbar and 515°C, 6–7kbar and 545–550°C, 6.6kbar and 530°C, 5.6–6.2kbar and 525–550°C and 6.8–6.9kbar and 520–560°C. The average PT mode of thermocalc, which relies on equilibrium being achieved between the garnet rims and the matrix, gives pressures around 11kbar that do not accord with the lengthy succession of lower core pressures. The many foliations in the matrix, which formed during top to the south thrusting plus subsequent deformations that eventually led to these rocks reaching the surface, truncate all foliations preserved within the porphyroblasts that are defined by inclusion trails. This has resulted in the garnet rims not being in equilibrium with the matrix and the anomalously high pressures. The garnet rims may have been affected by slow dissolution and solution transfer over the period of time that the matrix was deforming plastically at high strain rates as the rocks were uplifted. The assumption of equilibrium between garnet rims and surrounding silicates used by various rim geothermobarometric methods does not hold for these rocks.