An Experimental Study on F-bearing Fluid-fluxing Partial Melting of South China’s Gneiss: Constraints on Formation of REE-mineralized Granites

To clarify the effect of fluorine-bearing fluids on the geochemical behaviors of rare earth elements (REE) during magmatic processes linked to the origin of granitic magmas, this paper conducted a series of piston-cylinder experiments on the fluid-fluxing partial melting of gneiss at 0.8 GPa and 1000 ℃ using representative high-REE and low-REE gneiss powders and ~4% solutions (i.e., pure water, 1.5 mol/L HF, and 1.0 mol/L NaF) as the starting materials. The experimental results show that the melting degree of gneiss for the experiments is more than 40%, and the residue minerals include plagioclase+quartz+clinopyroxene±orthopyroxene+ilmenite+apatite±biotite. All the melts originated from fluid-fluxing partial melting of gneiss are of granodiorite and granite in compositions, displaying high fluorine (0.11%-0.27%) and water (0.38%-1.86%) contents as well as analogous geochemical characteristics of A2-type granitoids. Specifically, the melts derived from melting of the high-REE gneiss are shoshonitic and metaluminous-weakly peraluminous, whereas those derived from the low-REE gneiss are high-K calc-alkaline and strongly peraluminous. All the melts also display positive anomalies of K, Rb, Th, Ce, Sm, Y, and Yb and negative anomalies of Ba, Nb, Ta, Zr, Hf, and Ti in the spider diagrams, whereas the REE patterns exhibit LREE-enriched features and slightly negative Eu anomalies. Compared with the protolith, addition of F-bearing fluids enhanced the enrichment of Si, K, Na, Rb, Sr, Ba, Th, U, LREE, and F in the melts and raise the A/CNK and LaN/YbN ratios of the melts. The experimental results also demonstrated that the REE contents in the melts were controlled by the protolith, which implies that the high-REE gneiss could more readily fuse out the melts with high REE contents. Besides, the partial melting with fluid assistance could cause REE enrichment in the melts to varying extents; for example, melts derived from HF-bearing fluid-fluxing partial melting have REE contents almost 1.5 times as high as that of the protolith. This paper thus highlights the significant role of F-bearing fluids in the formation of LREE- or HREE-type mineralized granites in South China. Furthermore, this paper suggests a new petrogenetic model of A2-type granite, i.e., F-bearing fluid-fluxing partial melting of crustal basement rocks, which could plausibly explain the formation of the late Mesozoic inland A2-type granites associated with REE, rare-metal, and fluorine mineralization in South China. © 2024 Science Press. All rights reserved.