Metamorphic record of the NW Himalayan orogeny between the Indian plate-Kohistan Ladakh Arc and Asia: Revelations from foliation intersection axis (FIA) controlled P–T–t–d paths

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Our recent Foliation Intersection Axes (FIAs) data from porphyroblasts in a metapelitic sequence, exposed south of the Indus suture of the NW Himalaya, revealed a fascinating relationship between the trends of three FIA sets versus the plate motion of India since its collision with the Kohistan–Ladakh Arc. The established FIA trend sequence, ESE–WNW (set 1), E–W (set 2) and NNE–SSW (set 3), matches successive vectors of relative India–Asia plate movement between magnetic anomalies 25–22 (55–50Ma), 21–20 (48–44Ma) and 13–8 (36–29Ma), on the premise that FIAs develop normal to the direction of bulk crustal shortening. The correlation allows us to precisely model the pressure (P)–temperature (T) path followed by this portion of the NW Himalayan orogen from 55 to 29Ma. Representative samples preserving FIA sets in garnet porphyroblasts are modeled in the chemical system MnNCKFMASH using THERMOCALC. A steep continuous P–T path is obtained from garnet porphyroblasts for sample preserving FIA set 1 that brackets metamorphic conditions during early NNE–SSW shortening (anomalies 25–22) between 0.4–0.87GPa and 495–535°C (M1 metamorphism). Garnets preserving FIA set 2 revealed 0.46–0.8GPa and 507–565°C (M2 metamorphism) during N–S (anomalies 21–20) shortening. Subsequent ESE–WNW shortening parallel to the orogenic arc (anomalies 13–8) peaked at 0.9 to 1GPa and 595±19 to 618±28°C (M3 metamorphism) and was followed by high-temperature/low-pressure conditions of 0.8±0.2GPa at 747±32°C that accompanied north-side-down decompression (M4 metamorphism) after 29Ma. The M3 and M4 P–T estimates are based on average P–T conditions of garnet inner rim with the matrix mineral phases, which we suspect may not be in equilibrium due to later reactivation of the matrix foliation. The orthogonal orientation of FIA 1 and the NNE movement of India between anomalies 25 and 22 (55–50Ma) suggest that the Indian plate collided with the Kohistan–Ladakh Arc around this time.