[SGL30-P04] Cooling profile of the southern slope of Mount Everest since 15 Ma based on zircon and apatite fission-track and muscovite Ar/Ar analyses of the Higher Himalayan Crystallines
キーワード：冷却プロファイル、エベレスト山塊、フィッショントラック年代、アルゴンアルゴン年代
We performed zircon and apatite fission-track (FT) and muscovite Ar/Ar analyses of the Higher Himalayan Crystallines (HHC) including leucogranite along the southern slope of Mount Everest, E. Nepal. Seventeen samples are from the metamorphosed Yellow Band under the Qomolangma Detachment (QD; 8,300 m a.s.l.) down to the Main Central Thrust (MCT; 2000 m a.s.l.) near Jubing. In 17 km of the structural distance between QD and MCT, we found three stages of cooling history based on the relationship between zircon and apatite FT ages. First, rapid cooling zone at the topmost (QD to ~1.5 km below) of HHC has a simultaneity of zircon and apatite FT ages (15~14 Ma) indicating a cooling rate of >> 100 °C /m.y. This is supported by unimodal distributions of confined-track length in zircon. Second, slow cooling zone (1.5 to 7 km below from QD) has a significant difference of FT ages up to 10 million years among the two minerals from the same rock sample. A cooling rate of 12 °C / m.y. is given in this zone. The resulting broad and skewed track-length distributions in zircon support the slow cooling history. Third, relatively fast cooling zone (40-50 °C/ m.y.) corresponds to the lower half of HHC in which the 10 km-thick massif has constant zircon and apatite FT ages being 5-3 Ma and 1.5-0.8 Ma, respectively. The cooling pattern of the third zone cannot be explained by uplift. In other words, the exhumation of Mount Everest massif has occurred earlier at least than 5 Ma. From interpolation between muscovite Ar/Ar ages and apatite FT ages, ~250°C is estimated as the closure temperature for the retention of fission tracks in zircon at the cooling rate of 50 °C/m.y. In this presentation, we will show temporal changes of cooling profile since 15 Ma and discuss exhumation history of HHC in the Mount Everest massif.