Halogen geochemistry of I- and A-type granites from Jiuhuashan region (South China): Insights into the elevated fluorine in A-type granite

Relative to I-type granites, the A-type rocks are F-rich (mean of 1230 ppm vs. 540 ppm) but poor in Cl (mean of 25 ppm vs. 50 ppm) and Br (mean of 130 ppb vs. 200 ppb), with indistinguishable I contents (from < 1 to 30 ppb). Apatite and biotite from the two rock series show very similar relationships in terms of their F and Cl contents. A potential role of F for the genesis of A-type rocks is indicated by positive correlations with whole-rock HFSE and HREE contents and many other trace elements (e.g., Ga, Tl, Rb, Be). Calculated F and Cl abundances for primary melts are much higher than the respective whole-rock F and Cl contents, supporting late-stage volatile release during magma evolution. We suggest that the elevated F contents in A-type granites may initially originate from a F-rich melt and further elevated during prolonged magma differentiation.