Fractionation process of high-silica magmas through the lens of zircon crystallization: A case study from the Tengchong Block, SW China

Abstract

The generation of high-silica magma is generally considered to occur as a result of liquid-crystal segregation from a crystal mush. Yet whether high-silica melts could undergo significant fractionation process after the segregation remains unclear. This study contributes to this topic through the lens of zircon geochemistry of the ~130 Ma high-silica granite in the Tengchong block, SW China. The high-silica granite, with SiO2 ~76%, displays low Zr/Hf and EuN/Eu*N, and high Rb/Sr ratios. The zircons crystallized from the granite can be divided into two groups. The group 1 zircons are characterized by lower Hf (<12,200 ppm), and show relatively lower U (710–1103 ppm, averaging 891 ppm) and Yb (688–1106 ppm, averaging 903 ppm), and higher EuN/Eu*N (0.11–0.18). By contrast, the group 2 zircons have higher Hf (>12,200 ppm), and display elevated U values (936–2429 ppm, averaging 1557 ppm) and Yb values (755–1556 ppm, averaging 1134 ppm), as well as lower EuN/Eu*N (0.01–0.1). The increasing Hf content from group 1 to group 2 zircons, presence of fluorite inclusions in high-Hf zircon, and similarity in Hf isotope between the two groups of zircon consistently support the high-silica granite has witnessed a highly fractionated crystallization process. The presence of these two groups of zircons is considered to reflect two-stage in-situ fractionation processes, which display distinct REE (rare earth elements) compositions in the magma system in response to differing precipitated mineral assemblages. In light of the change of zircon REE within each group, the percentage of minerals co-precipitating with zircons in each stage was estimated. The results showed that 35.24% quartz, 18.60% feldspar, 0.25% apatite, 0.14% zircon, 0.12% allanite, and 0.05% titanite fractionated in the early stage, and 67.50% quartz, 23.41% feldspar, 0.15% zircon, 0.10% allanite, 0.07% apatite, and 0.06% titanite fractionated in the second stage. The discovery of different zircons and corresponding estimation in this study provide a new line of evidence for the fractionation of high-silica melt.