The early Oligocene beryl-bearing pegmatite in the Cuonadong dome，southern Tibet: Its forming mechanism and geological significances

The large-scale tungsten-tin-beryllium mineralization was newly discovered in the Cuonadong Dome，which is located in the east of the Tethyan Himalaya. Cuonadong Dome was divided into the upper part (the edge)，the middle part (the mantle)，and the lower part (the core) by the upper and lower detachment faults. A set of syntectonic deformed beryl-bearing granitic pegmatite was occurred in the mantle，of which the rocks were strongly deformed. The zircon U-Pb isotopic data show that the pegmatite was formed at 33.7±0.4 Ma (MSWD=1.12)，which was the Early Oligocene magmatism and obviously older than the Miocene leucogranites(20-14Ma)in the Dome. The characteristics of the whole-rock geochemistry and isotopic geochemistry show that: (1) the Early Oligocene granitic pegmatite is the peraluminous granitic rock with high potassium and sodium. The characteristics of the major elements show that the granitic rocks have higher SiO₂ (>69.74%) and Al₂O₃ (>14.58%)，lower CaO，MgO，MnO and TiO₂than common granitic rocks; (2) high field strength elements(HFSE) and large ion lithophile elements (LILE) all exhibit high variation characteristics，enriching light rare earth elements and depleting heavy rare earth elements; (3) The values of the initial Sr isotope (0.696308～0.751604) and the initial Nd isotope (-11.48～-12.05) range between those of amphibolite and metapelites，and the values of zircon εHf(t) are between -5.4 and 0.1 (mainly concentrated in -5.4～-1.8). According to comprehensive research，the Early Oligocene beryl-bearing pegmatite was derived from the partial melting of source consisting dominantly of metapelites and subordinately amphibolite，which was related to the activities of the South Tibetan Detachment System (STDS)，and indicate that the primary partial melting source of the Cenozoic crustal anatexis in the Cuonadong Dome changed from amphibolite to metapelites at the Early Oligocene. The discovery of the syntectonic deformed beryl-bearing pegmatite reveals that the Cuonadong Dome began to form at least in the Early Oligocene. The initial enrichment of beryllium mineralization likely occurred in the Early Oligocene，whereas the giant enrichment occurred in the Miocene associated with Miocene leucogranites.