MONAZITE IN HIMALAYAN LEUCOGRANITES: INVESTIGATING MELT SYSTEMATICS IN THE MID-CRUST
The Leo Pargil Dome (LPD) in NW India provides excellent exposure of anatectic rocks of the mid-crustal Greater Himalayan Series. An interconnected network of pre-, syn-, and post-kinematic leucogranites pervasively intrudes amphibolite-facies metapelites in the core of the dome. Leucogranite bodies range from thin (5-cm-wide) locally-derived sills to thick (2-m-wide) cross-cutting dikes extending >100 m. Three-dimensional exposures elucidate cross-cutting relations between different phases of melt injection and crystallization.
Twenty leucogranites representing different phases of emplacement were selected for detailed analysis. EPMA X-ray mapping of monazite grains reveals complex internal zoning of U, Th, and Y. Laser ablation ICP-MS targeting distinct chemical domains with a spatial resolution of 7 μm yields a large, internally consistent dataset of 800 U-Th-Pb and 400 trace element spot analyses. Intra- and inter-grain variation in isotopic age correlates with differences in trace element distribution, indicating 1) discontinuous crystallization of monazite under fluctuating melt compositions, and 2) incomplete resetting of restitic monazite cores. The youngest U-Th-Pb ages are consistent with field relations and older ages indicate a complex inheritance pattern reflecting monazite derivation from both partially crystallized melt as well as host metapelites. U-Th-Pb data indicate that melting occurred in a semi-continuous manner from 30 to 18 Ma with distinct pulses of magmatism lasting 1 to 2 Myr. We incorporate these observations into a model of mid-crustal magmatism in the LPD involving 1) steady equilibrium batch melting of one or more metapelitic sources, 2) pulses of increased melt mobility resulting in segregation of melt from its source and amalgamation into mixed magmas, and 3) rapid emplacement and final crystallization of leucogranites.