Effect of F-Rich Fluids on the A-Type Magmatism and Related Metal Mobilization: New Insights from the Fogang-Nankunshan-Yajishan Igneous Rocks in Southeast China

About 45% of tungsten, similar to 20% of tin, and similar to 9% of fluorite of known world reserves are associated with Late Mesozoic igneous rocks, Southeast (SE) China. Here we demonstrate that Fogang granite, the largest inland batholith, is mainly of A2-type that is commonly found in post-orogenic settings and experienced plate subduction induced metasomatism. In contrast, the Yajishan syenite and Nankunshan granite intruding the Fogang granite similar to 20 Ma later are of A1-type formed in intraplate settings. We found that F-rich fluid fractionation, which could make the decline of Ga/Al ratio, total (Nb + Y + Ce + Zr) and Zr concentrations, Nb/Ta and Zr/Hf ratios, leads to chemical variations of a few Fogang granites changing from A2-type to highly fractionated or I- and S-type granitoids. Crystal and F-rich fluid fractionations, as well as crustal contamination most likely derived from the Fogang granite, result in some Nankunshan granites developing from A1-type into A2-type. These late- or post-magmatic processes should be taken into account carefully when discriminating the petrogenetic types of igneous rocks, especially for the A2-type suites. Combining with the distribution of 180-140 Ma A1- and A2-type igneous rocks, rare metal deposits, and fluorite deposits in SE China, we highlight the significant role of slab-released F-rich fluids in formation of A-type suites and subsequent chemical differentiation and rare metal and fluorine mineralization. A model of flat-slab northeastward rollback is thus proposed, in which the subduction front reached somewhere near Fogang and then started to roll back at similar to 165 Ma. The inland Jurassic granites of SE China represent a unique locality for formation of A-type suites and their associated mineralization. These granites are not anorogenic, but they are the result of slab rollback from a flat slab, founding of that slab at shallow levels, and metasomatism of by F-rich fluids related to slab heating by the asthenosphere.