Genesis and mechanism of tourmaline granite in the Da Qaidam area, northern margin of the Qaidam Basin

B, as a volatile element, is widely developed in the global granite pegmatite mineralization system and may be a key volatile component that promotes rare metal mineralization. However, the origin and control mechanism of volatile components during the abnormal enrichment process of rare metals are still unclear. The northern margin of the Qaidam Basin is located in the northeastern Qinghai-Tibet Plateau. Influenced by multistage magmatic-tectonic activities, many intrusive rocks and rare metal deposits are present. Recently, we found many B-rich granites in the Da Qaidam area on the northern margin of the Qaidam Basin. Identifying the genesis of these tourmaline granites can provide a reference for solving the scientific problem of the origin and control mechanism of volatile components in the process of rare metal enrichment. Therefore, we investigated two tourmaline granites plutons, Tataleng and Juhongtu, and carried out detailed field geological surveys, element geochemistry, Hf isotopes and zircon U-Pb chronology and obtained the following findings: (1) The tourmaline granites in the Da Qaidam area belong to a highly differentiated granite rich in potassium, silicon, and peraluminium. (2) Tourmaline granites were formed similar to 438Ma in a continental collisional environment where the Qaidam block subducted under the South Qilian block after the end of northern Qaidam marginal ocean subduction. (3) The formation process of B-rich granites is as follows: serpentinisation occurs during the subduction of oceanic crust, in which fluid-mobile elements are enriched, and then continuous subduction causes the dehydration of ophiolite. The dehydration process promotes the B-rich fluid to enter the mantle wedge and causes the remelting of crustal materials. The B-rich magma is contaminated by upper mantle crystals to form the ore-bearing parent magma. In the subsequent evolution process, the magma experiences the fractional crystallization of K-feldspar, plagioclase, hornblende, allanite, and monazite.