Volcanism-induced generation of high-silica A-type granite: A snapshot from Yandangshan caldera, southeastern China

Abstract

Silicic magmatism was extensively developed in the coastal regions of southeast China. The abundant volcanic fields and granitic plutons in this region provide exceptional opportunities to investigate upper crustal magmatic processes and their evolution. We investigate the petrogenetic connections among alkali feldspar granites (AFGs), porphyritic enclaves, porphyritic rocks (including porphyritic monzonites and porphyritic syenites), and rhyolites from the Yandangshan caldera and surrounding area using zircon UPb dating, trace elements, Hf isotopic ratios, and bulk-rock geochemistry. Zircon UPb dating reveals that the plutonic and volcanic rocks of the Yandangshan caldera crystallized concurrently (98–102 Ma), with consistent Hf isotopic compositions and trace element trends in low-U zircons further supporting that lithological variations within the caldera result from crystal-melt segregation. The porphyritic rocks and enclaves are rich in Sr and Ba, display high Zr/Hf ratios, and show positive to weakly negative Eu anomalies, suggesting they represent cumulate residues from crystal-melt segregation. The AFGs and rhyolites are enriched in Rb but exhibit depletions in Sr, Ba, and Eu, characterized by low Eu/Eu^⁎ and high Rb/Sr ratios. Both units are interpreted as evolved silicic melts extracted from a crystal-rich magma reservoir. Compared to the erupted rhyolites, the AFGs display higher silica content and Rb/Sr ratios, along with lower Eu/Eu^⁎ ratios. Moreover, high-U zircons are exclusively found in the AFGs and exhibit the most evolved trace element signatures within the entire suite, characterized by elevated Hf and U concentrations, as well as low Eu/Eu^⁎ ratios. These characteristics indicate that the AFGs, which exhibit A-type features, are more evolved than the rhyolites, reflecting extended in-situ crystallization and differentiation following the eruption of the rhyolitic magmas. We propose that the magma responsible for the AFGs originated from the underlying feldspar-rich mush, following the rhyolite eruption. These melts then underwent in-situ crystallization and melt differentiation, leading to the formation of A-type granites that are more evolved than the rhyolites. By comparing the geochemical characteristics of contemporaneous silicic plutonic and volcanic rocks from southeastern China, we demonstrate that large volumes of high-silica granitic magma accumulated after silicic melt eruptions. Our study further reveals that the compositional diversity of A-type granites primarily results from crystal-melt segregation processes rather than distinct magma sources, emphasizing the importance of shallow crustal differentiation in granite formation. Recognizing these processes enhances our understanding of how similar geological mechanisms generate diverse granite compositions across various tectonic environments, ultimately deepening our insight into the evolution of silicic magmas within the Earth's crust.