5.3.3 High-grade Metamorphism along the Kamirup valley
As mentioned above, the mouth of the Kamirup valley is marked by the presence of the Sarchu Fault. This high-angle normal fault brings into direct contact the Triassic rocks of the Lilang Group with the Cambrian rocks of the Karsha Formation (Fig 4.10). The rocks in the hanging wall of this fault have undergone an anchizonal metamorphism, whereas the rocks in the foot-wall of this fault are of epizone to mesozone grade. Garnet was found in a single location at the entrance of the valley in a muscovite rich metapelite of the Karsha Formation (Fig 5.6). The very limited occurrence of garnet in this area might be the result of a chemical control. Upstream and away from the Sarchu fault, the metapelitic rocks at the floor of the valley contain the mineral assemblage: biotite + chlorite + plagioclase + muscovite + quartz, typical of the Barrovian biotite zone (Fig 5.7). Seven kilometres from the mouth of the Kamirup valley, the metamorphic grade increases abruptly. Over a very short distance of less than one kilometre, one can observe the successive apparition of garnet, staurolite and finally kyanite in the metapelitic rocks. On the left side of the valley, these metamorphic zones are wrapped around a dome-shaped leucogranitic intrusion. This intrusion represents the easternmost occurrence of Tertiary leucogranites in the Zanskar-Lahul area. The zone of high-grade metamorphism is very restricted both laterally and vertically. Indeed, further upstream, the metamorphism decreases as quickly as it increased to reach again biotite zone grade. It thus appears that peak metamorphic conditions in the Kenlung Serai Unit are not found directly at the contact with the Sarchu Fault as was proposed by Spring (1993) but several kilometres to the south of the fault (Fig 5.4).
Deformation within these high-grade rocks is intense, the kyanite and staurolite prisms are systematically reoriented in a SW-NE direction. Microboudinage affects both minerals and the growth of muscovite in the necks of the boudins testifies of deformation under retrograde metamorphic conditions. The presence of late sillimanite, andalusite and cordierite crystals also testifies to a late retrograde metamorphic event. Shear sense criteria indicate nearly horizontal top to the northwest movements. Late high-angle brittle normal faults dipping 50-60° separate the high-grade rocks from the biotite zone Karsha metasediments. This small outcrop of high-grade rocks in the Kamirup valley is very similar to the restricted outcrop of kyanite grade rocks that was described by Steck et al. (1993), Spring (1993) and Fuchs and Linner (1995) several kilometres south of the Sarchu Fault along the Manali-Leh road.
We interpret these two occurrences of high-grade rocks within the Kenlung Serai Unit, both in the Kamirup valley and south of Sarchu as representing outcrops of the summit of the Gianbul dome. The combination of ductile shearing along the ZSZ and late high-angle normal faults brought the top of the HHCS dome into narrow contact with the overlying low- to medium-grade Kenlung Serai Unit. More to the west, along the Kurgiakh valley, the exhumation of the HHCS was more intense and the top of the HHCS reached higher structural level such as that the equivalent of the Kenlung-Serai unit was totally eroded. The metamorphic zonation observed along the Kurgiakh valley is the subject of the next paragraph.