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

Abstract

Spatially associated A- and I- type granites and mafic dyke rocks from the Jiuhuashan region (South China) have been geochemically studied with a special emphasis on their halogen (F, Cl, Br, I) contents to better understand the role of halogens during their genesis. The A-type granites and mafic dyke rocks (125–130Ma) intruded later than the I-type rocks (140–143Ma) and whole-rock Nb/Ta ratios increase from I-type granites (12–14) via A-type granites (14–18) to mafic dykes (19–20). The εNd(t) values for the two rock series are indistinguishable (about −4 to −8), whereas mafic dykes reach εNd(t) values of up to +2. We suggest that I- and A-type granites generate from a common source dominated by lower crust, whereas the associated mafic dykes representing mantle-derived magma. We propose that the I-type rocks were emplaced during subduction of the paleo-Pacific slab, whereas subsequent tectonic extension triggered upwelling of mantle-derived magmas that provided sufficient heat to re-melt the residual lower crust, and crystallized to the A-type granites.Relative to I-type granites, the A-type rocks are F-rich (mean of 1230ppm vs. 540ppm) but poor in Cl (mean of 25ppm vs. 50ppm) and Br (mean of 130ppb vs. 200ppb), with indistinguishable I contents (from <1 to 30ppb). 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.