Abstract:Leucogranite is an ideal object to study the composition of shallow crustal and geodynamic process of the collision orogenic belt, and also an important carrier of rare metal mineralization. The Pagele two- mica granite studied in this paper are developed in the southwest of the compound granitic batholith in the Nianqingtanggula Mountain, Gangdese. It is produced in the form of stocks and apophyses, and occasionally contains beryl. Precise zircon U- Pb dating shows that its crystallization age is 18.4 Ma and 17.6 Ma, corresponding to Miocene. The rock has high content of SiO- 2(72.78%~75.53%), K- 2O(4.83%~5.52%)and A/CNK(1.04~1.12), and low content of Al- 2O- 3(13.42%~14.60%), MgO(0.10%~0.24%), TFe- 2O- 3(0.86%~1.65%), and belongs to the peraluminous high- K calc- alkaline and shoshonitic granite. (87Sr/86Sr)- i=0.712139~0.713545, ε- Nd(t)=〖CD*2/3〗9.0~〖CD*2/3〗8.3, tDM- 2=1211~1468 Ma, 206Pb/204Pb=18.797~18.841, 207Pb/204Pb=15.755~15.785, 208Pb/204Pb=39.425~39.519, ε- Hf(t)=〖CD*2/3〗6.8~2.0, tDM2=970~1529 Ma, showing that the source was composed of main crust- derived melts (70%~80%) originating from the Proterozoic Nianqingtanggula Group metamorphic rocks in the middle and upper crust, and a small amount of mantle- derived melts (20%~30%). LREE/HREE=4.30~9.61, (La/Yb)- N=3.61~13.42, δEu=0.16~0.34, Differentiation Index (DI)=95~97, K/Rb=76.51~116.39. Rb, Th and U are relatively enriched. P, Zr, and Ti are strongly depleted and the Rb- Ba- Sr differentiation is obvious. They indicate that the magma may have undergone strong crystallization of K- feldspar, plagioclase, zircon, apatite and biotite. Combined with regional geological data, it is believed that the Miocene granite in the Nianqingtanggula area, including the Pagele mica granite, probably formed under the background of the tectonically weak belt in the upper- middle crust caused by tearing of the subducted Indian continent plate. The upwelling of asthenosphere induced the mantle magma to invade along the weak zone, and caused the dehydration and melting of muscovite in the pelitic metamorphic rocks in the upper- middle crust to form the crust- derived magma. Subsquently, the mantle and crust- derived magmas mixed and underwent different degrees of crystal differentiation. The rare occurrence of beryl in the Pagole two- mica granite may be related to the enrichment mechanism of partial melting of beryllium- rich upper- middle crust, strong crystallization differentiation and F- Be complexation. The beryllium- rich granitic pegmatite, evolved from the Pagole beryllium- rich two- mica granitic magmatic system, is likely to have great beryllium mineralization capacity.
