A-type granites generally have much lower water but higher temperature and incompatible element concentrations than I-type granitoids. Yet it remains unclear why I-A-type granitic complexes occur in convergent plate margins. Here we conduct geochemical analyses on apatite and mafic minerals from the late Cretaceous I-A-type granitic complex in Fuzhou area, SE China, aiming to decipher differentiation, fluid metasomatism and degassing that primarily control the compositional diversity of felsic magmas. Apatites in both rock types are F-rich and show large H 2 O and δD variations, i.e., 341 − 3892 ppm H 2 O and −325 − +336‰ δD in I-type granitoids; 67−1366 ppm H 2 O and −251 − +1439‰ δD in A-type granites. H 2 O in apatite is negatively correlated with La/Sm and Sr/Y in the I-type granitoids, whereas it is positively correlated with Ce and total rare earth element (REE) concentrations in the A-type granites. Once H 2 O decreases up to hundreds of ppm, both rock types show a rapid decrease of H 2 O/Ce, an increase of F/Cl and extensive H isotope fractionation. Arfvedsonite occurs as a late crystallizing mineral in the A-type granite and has much higher contents of Na 2 O, K 2 O, F and high field strength elements (HFSE) than hornblende in the I-type granitoids, indicating addition of F-HFSE-rich alkaline fluids during its magmatic evolution. The consumption of arfvedsonite and formation of aegirine further indicate the role of fluid metasomatism and H 2 degassing via a reaction of 3Na 3 Fe 5 Si 8 O 22 (OH) 2 + 2H 2 O = 9NaFeSi 2 O 6 + 2Fe 3 O 4 + 6SiO 2 +5H 2 .The combined geochemical data demonstrate that the systematic differences in mineral assemblage, whole-rock composition, magma temperature, H 2 O and δD of This is the peer-reviewed, final accepted version for American Mineralogist, published by the Mineralogical Society of America.The published version is subject to change. Cite as Authors (Year) Title. American Mineralogist, in press.