PETROCHRONOLOGIC INSIGHT INTO THE ASSEMBLY OF HIMALAYAN PLUTONS

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 ~1500 individual U-Th/Pb and trace element spot analyses and a further ~800 Sm/Nd analyses from 30 specimens enables a detailed comparison with both existing whole rock isotope data and structurally lower metamorphic and anatectic rocks to interpret the emplacement history of this pluton and the associated behavior of monazite. A systematic decrease in monazite age and Gd/Yb along with a concomitant increase in εNd_i, Y, and REE with decreasing structural level indicates emplacement and crystallization of the pluton occurred from the top down in two broad episodes at ~22 Ma and ~19 Ma. Compared to structurally higher specimens, monazite in lower specimens contain a greater range in inherited age and Nd isotopic components, indicating less efficient monazite dissolution and melt homogenization in these rocks. We interpret the difference in age and isotopic variation to result from lower melt temperatures and/or decreased water availability in structurally lower samples. Variations in monazite ¹⁴³Nd/¹⁴⁴Nd and Sm/Nd both within samples, and across the sample suite reflect the isotopic heterogeneity of the source region and differences in modal abundance of other Nd-bearing phases, 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 understand the detailed history of minimum temperature melts in collisional orogenic systems.