The monazite record of pluton assembly: Mapping manaslu using petrochronology

Abstract

The Manaslu pluton in the Nepal Himalaya presents an ideal opportunity to examine the history of pluton assembly as recorded by the accessory mineral monazite. A combined dataset of 1407 individual U-Th/Pb and trace element and 795 Sm/Nd spot analyses from 29 specimens enables a detailed comparison with both existing whole rock isotope data and structurally lower metamorphic and anatectic rocks. A systematic decrease in monazite age and Gd/Yb along with a concomitant increase in εNdi, Y, and heavy rare earth elements with decreasing structural level indicates emplacement and crystallization of the pluton occurred from the top down in two broad episodes, one at ∼22 Ma and the other at ∼19 Ma. Monazite in structurally lower specimens contain a greater range in inherited age and Nd isotopic components than structurally higher specimens indicating less efficient monazite dissolution and melt homogenization in these rocks. We infer the difference in age and isotopic variation to result from a change in melt source and/or melt forming reaction. Variations in monazite 143Nd/144Nd (0.5116 – 0.5122) and Sm/Nd (0.1 – 0.2) within the sample suite reflect the isotopic heterogeneity of the source region and differences in modal abundance of apatite, respectively. This study illustrates the potential for combining isotopic and trace element information retained in monazite to extract information on the source region and assembly of plutonic rocks; insight that is otherwise lost in bulk rock isotopic measurements. Combined in-situ monazite U-Th/Pb and Sm/Nd, therefore, presents a means to better understand the history of minimum temperature melts in collisional orogenic systems.