Origin of highly fractionated peraluminous granites in South China: Implications for crustal anatexis and evolution

Abstract Voluminous Mesozoic intra-continental granitic rocks occur in South China, which provide insights into the processes and associated P-T conditions of crustal anatexis and evolution in an intra-continental setting. Most granitic plutons contain multiple phase intrusions, commonly with an early-stage main-phase granite and a late-stage minor-phase highly fractionated granite, and the latter is generally thought to result from extreme differentiation of the main-phase granite. We challenge this model based on our case study on the Tianmenshan composite pluton that is composed of an early-stage biotite granite and a late-stage two-mica granite. Zircon U-Pb dating yields similar emplacement ages of 157.3 ± 3.1 Ma for the biotite granite and 158.5 ± 3.4 Ma for the two-mica granite. The two rock types are peraluminous with A/CNK = 1.07 to 1.17 and show comparable Nd isotopic compositions, with eNd(t) = −9.4 to −11.5 for the biotite granite and −10 to −11.2 for the two-mica granite. The two-mica granite is characterized by high silica (SiO2 = 75.2–77.5%), low abundances of Sr (2.3–42 ppm) and Ba (10–114 ppm), large negative Eu anomalies in the chondrite-normalized REE patterns and typical REE tetrad effect, which, along with its high Ga/Al ratios, Fe-index and F contents, suggesting its affinity to highly fractionated granite. Nevertheless, the petrographic observations, zircon REE concentrations and biotite compositions consistently demonstrate that the late-stage two-mica granite formed under more water-deficient and reduced conditions than the early-stage biotite granite. This, along with a modeling study of trace elements (Rb, Sr, Ba,Y and Nb) and the lack of continuum in terms of whole-rock chemistry and biotite compositions between the two rock units, suggests that the two-mica granite cannot be formed through differentiation of the main-phase biotite granite. We suggest that the parental magma to the late-stage two-mica granite could have been derived from partial melting of a granulitic residue after extraction of granitic melts to form the biotite granite from the Neoproterozoic metavolcano-sedimentary rocks, followed by a significant feldspar-dominated fractionation during magma ascent. This model may apply to most other composite granite plutons in South China, and is of significance to better understand the process of crustal anatexis and evolution in an intra-continental setting.