A study of illite crystallinity and fluid inclusions in the Kathmandu Klippe and the Main Central Thrust zone, Nepal

Abstract

Abstract The Kathmandu Klippe is an outlier of the Tibetan crustal slab resting on the Main Central Thrust (MCT); it contains a 20 km thickness of Precambrian and Lower Palaeozoic metasediments and Ordovician granites folded into a large syncline. The regional metamorphism predates granite emplacement and, on the basis of illite crystallinity and index mineralogy, is lowest grade in the core of the syncline (epizone-chlorite zone) and highest on the north and south limbs (garnet zone). The distribution of metamorphic zones is ‘normal’ for a burial sequence. The klippe rests on the Benighat Slates of the Midland Formations of the Indian crustal slab. Fluid inclusion studies of the syn-MCT quartz veins in the Benighat Slates show H 2 O, CO 2 , CH 4 , Na + , K + , Cl′ fluids which were hotter than the country rocks. The MCT contact at Dolalghat does not show the effects of ‘reverse’ metamorphism expected from the Le Fort model of heat transfer from the hot klippe several kilometres down into the Midland Formations. A study of index minerals, illite crystallinity and illite b 0 indicates that the grade of regional metamorphism within the anticlinal Midland Formations varies between epizone-chlorite and garnet zone with the highest grades occurring in the core of the structure. Further to the N around Barahbise, the contact along the MCT between the Midland Formations and the Tibetan Slab is a 2 km thick zone of highly sheared Benighat Slates. The grade of metamorphism in the Benighat Slates is chlorite-epizone grade i.e. lower than either of the sequences above (kyanite zone) or below (garnet zone). The inference drawn from these relationships is that although the inverse metamorphic zonation in the base of the Tibetan Slab might be explained by cooling against the Indian Slab during the Tertiary Himalayan collision, metamorphism in the Indian plate was largely pre-collision.