We present a new LA-ICP-MS system for zircon fission-track (FT) and U-Pb double dating, whereby a femtosecond laser combined with galvanometric optics simultaneously ablates multiple spots to measure average surface U contents. The U contents of zircon measured by LA-ICP-MS and standardized with the NIST SRM610 glass are comparable to those measured by the induced FT method, and have smaller analytical errors. LA-ICP-MS FT dating of seven zircon samples including three IUGS age standards is as accurate as the external detector method, but can give a higherprecision age depending on the counting statistics of the U content measurement. Double dating of the IUGS age standards gives FT and U-Pb ages that are in agreement. A chip of the Nancy 91500 zircon has a homogeneous U content of 84 ppm, suggesting the possibility of using this zircon as a matrix-matched U-standard for FT dating. When using the Nancy 91500 zircon as a U-standard, a zeta calibration value of 42-43 year cm 2 for LA-ICP-MS FT dating is obtained. While this value is strictly valid only for the particular session, it can serve as a reference for other studies.

This study is concerned with the tectono-thermal history of the Kathmandu nappe and the underlying Lesser Himalayan sediments (LHS) that are distributed in eastern Nepal. We carried out zircon fission-track(ZFT) dating and obtained 16 ZFT ages from the eastern extension of the Kathmandu nappe, the Higher Himalayan Crystalline, Kuncha nappe, and the Main Central Thrust (MCT) zone. The ZFT ages of the frontal part of the Kathmandu nappe range from 13.0 ±0.8 Ma to 10.7 ±0.7 Ma and exhibit a northward-younging tendency. These Middle Miocene ZFT ages indicate that the frontal part of the Kathmandu nappe remained at a temperature above 240 C until the termination of its southward emplacement at 12-11 Ma. The ZFT ages of the LHS range from 11.1 ±0.9 Ma in the southern part of the Okhaldhunga Window to 2.4 ±0.3 Ma of the augen gneiss in the northern margin and also exhibit a northward-younging age distribution. The ZFT ages show the northward-younging linear distribution pattern (−0.16 Ma/km) along the across-strikesection from the frontal part of the Kathmandu nappe to the root zone, without a significant age gap. This distribution pattern indicates that the Kathmandu nappe, the underlying MCT zone, and the Kuncha nappe cooled from the frontal zone to the root zone as a thermally united geologic body at a temperature below 240 C. An older ZFT age (456.3 ±24.3 Ma), which was partially reset at the axial part of the Midland anticlinorium in the central part of the Okhaldhunga Window, was explained by downward heating from the "hot" Kathmandu nappe. The above evidence supported a model that southward emplacement of the hot Kathmandu nappe resulted in a thermal imprint on the upper part of the LHS; however, the lower part did not reach 240 C.

We investigated the tectonothermal history of the Lesser Himalayan sediments (LHS), which are tectonically overlain by the Higher Himalayan Crystalline. Fissiontrack dating and the track length measurement of detrital zircons obtained from the Kuncha nappe and the Lesser Himalayan autochthonous sediments in western central Nepal revealed northward cooling of the nappe and possible downward heating of the autochthon by the overlying hot nappe. Nine zircon fission-track (ZFT) ages of the nappe showed northward-younging linear distribution from 11.6 Ma in the front at Tamghas, 6 Ma in the central at Naudanda, and 1.6 Ma in the northernmost point at Tatopani. Thermochronological invert calculation of the ZFT length elucidated that the Kuncha nappe gradually cooled down (30 C/Myr) at the front and rapidly cooled down (120 C/Myr) at the root zone. In contrast, the ZFT age of the Chappani Formation, located just beneath the Kuncha nappe in the central part, demonstrated a totally reset age of 6.8 Ma, whereas the Virkot Formation, structurally far from the nappe, yielded a partially reset age of 457.3 Ma. This suggests that the LHS underwent downward heating, resulting in a thermal print on the upper part of the LHS; however, the thermal effect was not sufficient to anneal ZFT totally in the deeper part. Presently, the nappe cover is eroded and denuded from this area. Detrital zircons from the Chappani Formation in Tansen area to the south of the Bari Gad Fault did not show any evidence of annealing, suggesting that nappe never covered the LHS distributed to the south of the fault.

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