Metamorphic evolution of the northern Himachal Himalaya: phase equilibria constraints and thermobarometry

In the northern Himachal Himalaya, the Himalayan orogen consists of three tectonic domains, the Tethyan Himalaya, the High Himalayan Crystalline and the Lesser Himalayan Sequence, from NE to SW. Along the transect from the Spiti valley through eastern Lahul to the Parvati valley, the metamorphic evolution of these domains was studied in pelitic rocks of the Precambrian to Lower Cambrian Phe Formation and the Proterozoic Berinag Series. In this area four ductile deformational phases occur, including a first phase D-1 of unknown geometry, the NE-verging D-2 stacking of the Shikar Beh Nappe, SW-verging D-3 folding and SW-directed D-4 extrusion of the Crystalline Nappe between the SW-directed Main Central Thrust at the base and NE-directed normal movement in higher crustal levels. Detailed microtextural analyses allow these deformational phases to be related to four stages of metamorphic crystallization M-1 to M-4. Phase equilibria based on discontinuous reactions allow to place constraints on the metamorphic conditions of each stage. The peak conditions of the main metamorphism M-2 in the High Himalayan Crystalline are estimated by thermobarometry. Metamorphism M-1 reached sillimanite zone peak conditions for metapelites in the High Himalayan Crystalline with temperatures ranging between 620 and 760 degreesC. From the Tethyan Himalaya downsection to the underlying High Himalayan Crystalline a gradual succession of chlorite, biotite, garnet and kyanite Barrovian metamorphic mineral zones are observed that formed during M-2 and M-3 respectively, indicating a gradual transition between these two domains. As a consequence, the Tethyan Himalaya and the High Himalayan Crystalline represent metamorphic zones rather than tectonic units in the studied area. M-2 is the main metamorphism in the High Himalayan Crystalline. It is a Barrovian-type metamorphism, reaching kyanite zone peak conditions of T approximate to 700 degreesC and P approximate to 850 MPa. By contrast, M-3 is the main metamorphism in the Tethyan Himalaya. In the High Himalayan Crystalline M-3 peak conditions were close to M-2 conditions within the kyanite zone. During D-4 thrusting of the Crystalline Nappe over the Lesser Himalayan Sequence along the Main Central Thrust, the M-2/M-3 Barrovian kyanite zone mineral assemblage was overprinted by a retrograde M-4 greenschist facies metamorphism at the base of the High Himalayan Crystalline. In the underlying Lesser himalayan Sequence D-4 shearing was related to a prograde M-4 greenschist facies metamorphism. Concurrently with greenschist facies M-1 overprint along the Main Central Thrust, the M-2/M-3 kyanite zone mineral assemblage was overprinted by a sillimanite-bearing assemblage in NE-dipping D-4 normal shear zones in higher crustal levels. Barometry indicates that D-4 normal shearing was associated with increasing decompression going upsection in the High Himalayan Crystalline. The overgrowth of the M-2/M-3 mineral assemblage by M-4 during extrusion of the Crystalline Nappe results in an apparent inverted metamorphic zonation, from greenschist facies at the base of the High Himalayan Crystalline grading upsection into kyanite- and then into sillimanite zone conditions. M-2 metamorphic peak conditions decrease downsection from T approximate to 700 degreesC and P approximate to 850 MPa to T approximate to 650 degreesC and P approximate to 720 MPa in the kyanite zone. These data suggest that, in addition to the apparent inverted metamorphic zonation caused by M-4 overprint, M-2 isogrades were passively deformed during SW-verging D-3 folding and/or during D-4 Crystalline Nappe extrusion, leading to a real inverted metamorphic zonation.