Genesis of Early Cretaceous A-type granites in Gan-Hang Belt: Generated by rejuvenile of shallow crustal magma reservoirs

A-type granites are peculiar rocks with mineralogical and geochemical characteristics that distinguish them from subduction-related calc-alkaline granites. Although many models have been proposed by previous studies, their genesis remains highly controversial. In this study, we synthesize the data (chronology, geochemistry, and isotopes) of Early Cretaceous volcanic-intrusive complexes, A-type granites, and related granitic rocks from the Gan-Hang belt and attempt to establish the genesis connection between the volcanic rocks and intrusive rocks, and thus to constrain the origin of these A-type granites. For the volcanic-intrusive complexes (e.g., Xiangshan volcanic-intrusive complex), our study indicates that the felsic volcanic rocks are more felsic than the felsic intrusive rocks. The felsic volcanic rocks represent the high silicic melts extracted from a magma reservoir and the felsic intrusive rocks represent residual crystal accumulation in the magma reservoir. Furthermore, A-type granite and related granitic rocks in the Gan-Hang Belt can be divided into porphyritic granite and equigranular granite. In addition, mineralogical and geochemical features of the porphyritic granite and equigranular granite indicate that they were generated by the mixing of crustal-derived felsic melts and mantle-derived mafic magmas. The porphyritic granite and equigranular granite have similar formation ages within analytic error, identical mineral assemblages, similar Sr-Nd-Hf isotopic compositions, and consistent variations in major and trace elemental compositions, which suggests that their parental magma should come from a common silicic magma reservoir and that the lithological differences are the result of melt extraction processes. Porphyritic granites are characterized by the low SiO2 and Rb/Sr, and high Sr and Ba and Eu/Eu*, suggesting they may represent residual crystal accumulation in the magma reservoir. In contrast, the equigranular granites, which show the features of A-type granite, are characterized by the high SiO2 and Rb/Sr, and extremely low Sr and Ba and Eu/Eu*, indicating they represent high silicic melts extracted from a magma reservoir. The wide occurrence of microgranular mafic enclaves within the porphyritic granites and miarolitic cavities in the equigranular granites reveals that the injection of mantle-derived hotter mafic magma into the magma reservoir and the exsolution of volatiles from the interstitial melt rejuvenated the pre-existing magma reservoir. Subsequent extraction and upward migration of silicic melt resulting from compaction of the magma reservoir formed the high silicic A-type granites at shallow crustal levels, which left the complementary crystal residue solidified as porphyritic granite at the bottom. Our study indicates that A-type granite can be generated in the shallow magma reservoirs via crystal-melt segregation.