COMBINING THERMODYNAMIC MODELING AND FOLIATION INTERSECTION AXIS (FIA) IN GARNETS TO DECODE PROGRESSIVE COLLISION AND METAMORPHIC HISTORY OF THE INDIAN PLATE WITH THE KOHISTAN-LADAKH ISLAND ARC, NW HIMALAYA (Invited Presentation)

Barrovian-type mineral assemblages typically result from regional- to plate-scale collision tectonics and resulting clockwise P-T paths. Inclusion trail patterns preserved within such assemblages can provide important information about the kinematics of an orogen and of orogenic processes in general. We have measured Foliation Intersection Axes (FIA) preserved in garnet porphyroblasts of a metapelitic sequence exposed south of the Main Mantle Thrust in the Swat region of the NW Himalaya, Pakistan. This area underwent multiply deformation and metamorphism from ca. 55 to 38 Ma as part of the Himalayan orogen. Metamorphic garnets in the area contain spiral, sigmoidal, millipede and straight inclusion trails that represent progressive stages of a complex succession of deformations during this period. Spiral trails are commonly truncated by the matrix foliation, whereas the other types can be followed in the matrix indicating an older age of the spiral garnets. Determination of FIA for these garnets using two complementary techniques reveals two main FIA sets with broadly E-W versus N-S trends. Relative timing criteria indicate an older age of the E-W trending FIA consistent with development during progressive N-S crustal shortening, thrusting and convergence between the Indian plate and Kohistan-Ladakh Island Arc (KLIA). Younger N-S trending FIA formed in response to E-W crustal shortening as a result of orogen-parallel compression. The trends of FIA sub-classes suggest a progressive rotation of about 125° in the orientation of collision between the Indian Plate and KLIA. In order to estimate P-T conditions during this orogenic evolution, representative samples preserving different geometric types of inclusion trails in garnets (FIAs) are modeled in the chemical system MnNCKFMASH using THERMOCALC 3.33. A sophisticated clockwise P-T-t-d path is obtained that brackets metamorphic conditions during the N-S shortening (M1 metamorphism) between 4.0–8.7 kbars and 495-565^oC. Subsequent E-W shortening (M2 metamorphism) peaked at 9.2±1.4 to 10±0.8 kbar and 595±19 to 618±28^oC followed by high-temperature/low-pressure conditions of 8.0±2.0 kbars at 747±32^oC during crustal north-side-down decompression (M3 metamorphism) and emplacement of late leucogranites around 38 Ma.