Magma evolution and mineralization of Longmenshan lithium-beryllium pegmatite in Dahongliutan area, West Kunlun

Several lithium-beryllium deposits of pegmatite type have been found in Dahongliutan area, West Kunlun, which is a new lithium resource base in China. The origin of these deposits was emphasized as crystallization differentiation of the two-mica monzonitic granite formed by anatexis of the crust, but the relationship between the biotite granite in the study area and mineralization has never been discussed and concerned. In order to explore this problem, we conducted geochemistry and chronology studies of biotite granite, two-mica monzonitic granite, granitic pegmatite, and aplitic granite related to mineralization in Longmenshan. The results show : 1) The biotite granite and two-mica monzonitic granite have similar geochemical features of S-type granite and are enriched in Rb, La and Nd, depleted in Ba, Nb, Sr, P and Ti; 2) From biotite granite, two-mica monzonitic granite to aplitic granite, the process exhibits a continuous differential evolution of the magma; 3) The zircon LA-ICP-MS U-Pb age of biotite granite is 216.8 +/- 0. 85Ma, the zircon SIMS U-Pb age of two-mica monzonitic granite is 216. 0 +/- 1. 5Ma, the zircon LA-ICP-MS U-Pb age of aplitic granite is 209. 5 +/- 1. 2Ma, and the cassiterite LA-MC-ICP-MS U-Pb age of granitic pegmatite is 211. 3 +/- 5. 0Ma. This means that the formation time of granite from biotite granite, two-mica monzonitic granite to aplitic granite is continuous and contiguous. Based on this, we believe that the two-mica monzonitic granite and the lithium-beryllium pegmatite in Dahongliutan area are formed by the crystallization differentiation of biotite granite. Experimental petrology showed that biotite dehydration-melting of meta-greywacke or biotite gneiss forms biotite granite, and it is speculated that the extensive biotite granite in Dahongliutan area originated from the dehydration-melting of biotite and some amphibole during granulite facies metamorphism in the lower crust.