| Popis: |
The structural and metamorphic evolution of the Darjeeling-Sikkim Himalaya, a “classical” region of inverted metamorphism in the Himalaya, has been investigated by field studies combined with 2-D numerical modelling of the thermal evolution. In the Himalaya, an orogen-wide zone of inverted isograds is spatially associated with the Main Central Thrust (MCT). In the Sikkim-Darjeeling region, an inverted metamorphic field gradient is indicated by garnet-grade rocks in the upper Lesser Himalaya (LH), which increase in grade to sillimanite + K-feldspar assemblages in the middle to upper structural levels of the Higher Himalayan Crystallines (HHC). Metamorphic breaks in the “Barrovian sequence” have been established between the garnet- and sillimanite-bearing rocks in the Darjeeling region and between the kyanite-staurolite schists and biotite-sillimanite schists in Sikkim. Since the accurate location of the MCT is critical to constraining the metamorphic evolution of the Higher and Lesser Himalaya, a number of criteria are used in defining the MCT zone in this region. These include lithologic contrasts, increase in non-coaxial deformation features towards the MCT zone and geomorphology. The MCT forms a zone of distributed ductile deformation that has propagated southwards with time, resulting in a 3-10 km wide zone, containing rocks from both the Higher and Lesser Himalaya. Four episodes of deformation and two metamorphic events have been identified in the HHC. Textural evidence and garnet zoning profiles indicate a single episode of prograde metamorphism, but four deformation events in the MCT zone and the LH. Garnet zoning profiles from the HHC indicate retrograde equilibrium. M1 resulted in a peak assemblage of prismatic sillimanite + K-feldspar as well as muscovite dehydration melting resulting in millimetre to centimetre scale leucosomes, while M2 is associated with rapid exhumation of the HHC during simultaneous movement along the MCT and the South Tibetan Detachment System (STDS) forming decompression textures in metabasic boudins and pelites. |
