LINKING THE MANASLU GRANITE TO ITS SOURCE USING MONAZITE PETROCHRONOLOGY

The Manaslu granite of central Nepal is one of the best-studied leucogranite plutons in the Himalaya. Previous studies involving field mapping, structural analysis, bulk and trace element geochemistry, isotopic analysis of O, Sr, and Nd, as well as Rb/Sr, Sm/Nd, and (U-Th)/Pb geochronology have produced a wealth of information regarding the source, generation, and assembly of the composite body. This study exploits the well-constrained structural and petrogenetic framework of the Manaslu region and employs combined (U-Th)/Pb and trace element petrochronology of monazite to resolve the spatiotemporal relationships of leucogranites from within the Greater Himalayan Series (GHS) source, through the injection complex transport network, and into the main granite body. A preliminary dataset consisting of over 3000 LASS-ICPMS individual monazite spot analyses from 58 granite samples indicates complex internal trace element zoning that corresponds to age variation within and between individual monazite grains. ²⁰⁸Pb/²³²Th ages range from 15-24 Ma with evidence for monazite inherited from earlier metamorphism and/or magmatism (24-36 Ma) as well as from Cambro-Ordovician (450-490 Ma) orthogneiss protoliths. While broadly consistent with previous (U-Th)/Pb studies, this dataset reveals a previously unrecognized 15-18 Ma pulse of granite magmatism that coincides with metamorphic monazite ages in the upper GHS. Furthermore, late-stage magmatism is identified only in the source region and injection complex, but not in the main granite body, indicating limited melt transport after 18 Ma. Monazite crystallization ages provide support for a model involving semi-continuous melt production in the source region punctuated by 1-2 Ma pulses of magma transport and granite emplacement.