K-Ar ages of biotite and muscovite from Pangong metamorphic complex, Shyok suture zone, India: Implications for the youngest post-collision metamorphic event in Ladakh Himalaya

Abstract

The garnet -kyanite -staurolite and garnet -biotite -staurolite gneisses were collected from a locality within Lukung area that belongs to the Pangong metamorphic complex in Shyok valley, Ladakh Himalaya. The kyanitefree samples have garnet and staurolite in equilibrium, where garnets show euhedral texture and have flat compositional profile. On the other hand, the kyanite -bearing sample shows equilibrium assemblage of garnetkyanite -staurolite along with muscovite and biotite. In this case, garnet has an inclusion rich core with a distinct grain boundary, which was later overgrown by inclusion free euhedral garnet. Garnet cores are rich in Mn and Ca, while the rims are poor in Mn and rich in Fe and Mg, suggesting two distinct generations of growth. However, the compositional profiles and textural signature of garnets suggests the same stage of P -T evolution for the formation of the inclusion free euhedral garnets in the kyanite -free gneisses and the inclusion free euhedral garnet rims in the kyanite -bearing gneiss. Muscovites from the four samples have consistent K -Ar ages, suggesting the cooling age (~ 10 Ma) of the gneisses. These ages make a constraint on the timing of the youngest post -collision metamorphic event that may be closely related to an activation of the Karakoram fault in Pangong metamorphic complex.

Metamorphic P – T conditions and CO 2 influx history of medium-grade metapelites from Karakorum, Trans-Himalaya, India

“…Dating of monazite (Streule et al, 2009) provided Cretaceous age (108 ± 0.6 Ma) for the sillimanite-grade metapelites whereas staurolite grade metamorphism is considered to be younger, of Tertiary age, although the precise timing is not very well constrained. Earlier studies suggest this metamorphism as a result of shear heating under fault-guided magmatic advection within the KFZ (Rolland and Pêcher, 2001;Rolland et al, 2009;Valli et al, 2008;Thanh et al, 2009Thanh et al, , 2011. However an alternative explanation states that the metamorphism was the result of ongoing crustal thickening (Searle et al, , 2011Streule et al, 2009;Wallis et al, 2013Wallis et al, , 2014.…”