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

A multichronometric study of monazite-rich rocks from the two northernmost windows of Lesser Himalayan rocks of eastern Nepal is reported using (super 40) Ar/ (super 39) Ar and (super 208) Pb/ (super 232) Th ion-microprobe dating. Metamorphic peak temperatures range from 540 to 370 degrees C. In the highest-grade rocks , (super 40) Ar/ (super 39) Ar chronology on hornblende, biotite and muscovite gives ages of 12.9 + or - 1.9, 8.9-11.7 and 4.8 + or - 0.4 Ma, respectively. Monazite grains yield two different (super 208) Pb/ (super 232) Th age populations of 9.3-11.4 and 3.3-5.8 Ma, the older being from garnet-rich samples and the younger texturally replacing allanite in samples retrogressed under greenschist-facies conditions. The lower-grade sample also has young monazites dated at 9.0 + or - 1.0 Ma as replacement products of allanite. These results and textural relationships between monazite and allanite (which also have identical REE patterns) indicate the growth of monazite at the expense of allanite at low temperatures (< 370 degrees C) during exhumation. Young Mio-Pliocene Himalayan monazite should not be considered as only a prograde or metamorphic-peak mineral.