Abstract:

The Palung granitic gneiss in Nepal is a part of the Kathmandu thrust sheet of which the major mineral compositions are: quartz, plagioclase, Kfeldspar, microcline and muscovite. Zircons from the granitic gneiss have corerim structures with inherited detrital cores and magmatic rims showing concentric oscillatory zoning. LAICPMS zircon UPb dating results of the rims gave weighted mean ages of 485.5±1.4 Ma and 455.1±3.1 Ma, hinting that the protolith of the gneiss was the early Paleozoic granite recording two episodes of magmatism. The εHf(t) values range from -8.7 to -3.5 and the calculated TCDM(twostage model age) vary from 2.01 Ga to 1.69 Ga. Combined with the petrological characteristics and compared with Hf isotope compositions of the metapelites and orthogneisses in the Greater Himalayan Complex(GHC)and the metasedimentary rocks in the Lesser Himalayan sequence(LHS), we propose that the source of granite was possibly generated from the partial melting of the metapelite in the GHC. Zircon UPb ages of the Palung granitic gneiss and the published geochronological results of the CambrianOrdovician granites/granitic gneisses demonstrate that the early Paleozoic orogeny was existed in the Himalayas. The early Paleozoic tectonic events preserved in the Himalayas are well compared with the contemporaneous ones in Lhasa terrane, Qiangtang terrane and BaoshanTengchong terrane located in the southeast of the Tibetan Plateau. Integrating the results from previous studies, we suggest that it is resulted from the Andeantype orogeny formed through the subduction of the protoTethys Ocean lithosphere along the northern margin of the Gondwana, rather than the PanAfrican orogeny which was related with the continentcontinent collision during the Gondwana assembly.

Key words: Himalayan orogen, granitic gneiss, zircon LAICPMS UPb dating, Hf isotope, early Paleozoic orogeny