K-Ar Dating of White Mica from the Lesser Himalaya, Tansen-Pokhara Section, Central Nepal: Implications for the Timing of Metamorphism

Abstract

The Lesser Himalaya in central Nepal comprises low-to medium-grade metasedimentary rocks. Metamorphic studies show that they have experienced at least two metamorphic events (M 1 and M 2 ). However, exact timing of metamorphism is still controversial. In the present study K-Ar dating of white micas in shales, slates, phyllites, schists and gneisses from the Tansen-Pokhara section was carried out to understand the timing of metamorphism. The muscovite in gneiss from the MCT zone shows an age of about 1255 Ma representing the age of crystallization of parent granite. Detrital mica from the Bhainskati Formation gives an age of about 2441 Ma. Probably this is the age of crystallization of muscovite in its provenance. The recrystallized white micas from slate and phyllite show older ages (Early Paleozoic) in the southern part (279 to 458 Ma). Most probably this represents the timing of M 1 . Age become gradually younger towards the north due to the mixing of older (M 1 related) and younger (M 2 related) white micas. Youngest age (10 Ma) was measured from the sample just below the Upper MCT. This age may be related to the M 2 which was due to the Late Miocene-Pleistocene reactivation of the Upper MCT.

Lithostratigraphy and metamorphism of the Khudi-Tal area, west central Nepal

“…The stages of metamorphic events are studied in the Higher and Lesser Himalaya by various researchers in Central Nepal Himalaya (Le Fort 1975;Le Fort et al 1986;Caby et al, 1983;Paudel and Arita, 1983;Hodges et al, 1996;Vannay and Hodges, 1996;Paudel and Arita, 2000;Larson and Godin, 2009;Martin et al, 2009;Paudel, 2011). They have also mentioned that the increase in metamorphic grade structurally towards upper section from chlorite grade-lower green schist facies to garnet grade-upper green schist facies in the Lesser Himalaya resulting the inverse metamorphism and medium temperature/ high pressure kyanite grade Barrovian type prograde metamorphism followed by the high temperature/ low pressure sillimanite grade metamorphism in the Higher Himalaya which is similar to the present study area.…”