U-PB constraints on oligocene-miocene deformation and anatexis within the central Himalaya, Marsyandi valley, Nepal

In this study U-Pb geochronology provides direct age constraints on deformation and anatexis within the upper and lower Greater Himalayan sequence. From the lower Greater Himalayan sequence, metamorphic monazite ages from mylonitic migmatitic schists within the Main Central thrust zone indicate amphibolite-grade metamorphism during top-to-the-south thrust faulting at 22 Ma, possibly lasting until 18 Ma. Within the upper Greater Himalayan sequence, foliated migmatitic leucogranites yield complicated U-Pb systematics but are interpreted to have formed at similar to 22 Ma. Monazite data from the similar to 22 migmatite have a similar to 35 Ma inherited component which probably reflects Oligocene metamorphism or magmatism. Undeformed leucogranite dikes transect all foliations and intrude the contact between the Greater Himalayan sequence and the overlying Tibetan sedimentary sequence; they yield similar to 18 Ma ages, placing a lower bound on the age of regional metamorphism and high-temperature deformation associated with the South Tibetan detachment Field relationships and the ages of migmatization and leucogranite intrusion are consistent with models for melt-enhanced deformation (Hollister and Crawford, 1986) along the South Tibetan detachment system but also allow for late syntectonic granite melting to be enhanced by decompression. Previous suggestions that leucogranite generation within the upper Greater Himalayan sequence is temporally unrelated to the formation of migmatites or high-temperature deformation associated with the MCT are inconsistent with the age constraints presented here. Although fundamental distinctions are made between the migmatites and the leucogranites based on trace-element geochemistry, it seems unlikely that they are not genetically related considering their close proximity and timing.