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, ∼20% of tin, and ∼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 ∼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 ∼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.