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Evidence for Mio-Pliocene retrograde monazite in the Lesser Himalaya, far western Nepal

¹ Laboratoire de détection et de Géophysique, CEA, BP 12, F-91680 Bruyères-le-Châtel, France
² Laboratoire de Géologie, UMR 8538 du CNRS, Ecole normale supérieure, 24 rue Lhomond, F-75005 Paris, France
^* Now at: IGS, University of Bern, Baltzerstrasse 3, CH-3012, Bern, Switzerland

Corresponding author, e-mail: Laurent.bollinger{at}cea.fr

A multichronometric study involving ⁴⁰Ar/³⁹Ar and ²⁰⁸Pb/²³²Th ion-microprobe dating was performed on the two northernmost windows of Lesser Himalayan rocks in far western Nepal. Both regions were sampled for their monazite-rich series. Metamorphic peak temperatures range from 540°C to 370°C. In the highest-grade rocks (T_max 540°C), ⁴⁰Ar/³⁹Ar chronology on hornblende, biotite and muscovite gives ages of 12.9 ± 1.9 Ma, 8.9–11.7 Ma and 4.8 ± 0.4 Ma, respectively. Monazite grains yield two different ²⁰⁸Pb/²³²Th age populations of 9.3–11.4 Ma and 3.3–5.8 Ma range, respectively. The oldest monazites are found in garnet-rich samples whereas the youngest monazite grains texturally replace allanite in sample retrogressed under greenschist-facies conditions. The lowest-grade sample (T_max 370°C) bears also young monazites at 9.0 ± 1.0 Ma, as replacement products of allanite. The chronological results as well as the clear textural relationships between allanite and monazite (which furthermore show identical REE patterns) indicate a monazite growth at the expense of allanite at low temperature (< 370°C) during exhumation. This study shows that young Mio-Pliocene Himalayan monazite should not be considered systematically as a prograde or metamorphic-peak mineral.

Key-words: geochronology, monazite, allanite, ion-microprobe dating, Lesser Himalaya, black shales.

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