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

A multichronometric study involving 40Ar/39Ar and Pb-208/Th-232 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 degrees C to 370 degrees C. In the highest-grade rocks (T-max similar to 540 degrees 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 Pb-208/Th-232 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 similar to 370 degrees 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 degrees C) during exhumation. This study shows that young Mio-Pliocene Himalayan monazite should not be considered systematically as a prograde or metamorphic-peak mineral.