Rejuvenation of silicic magmatic systems: Insights from the formation of garnet-bearing, crystal-rich tuff in Southeast China

Crystal-rich volcanic rocks are important for understanding the evolution of large silicic magma systems. However, the magmatic evolution and rejuvenation processes that form the eruptible crystal-rich magma remain controversial. In this study, we conducted petrological, whole-rock major and trace element geochemical and zircon U-Pb and Hf-O isotopic analyses on the Banshan crystal-rich tuff from the large Cretaceous volcanic belt of southeast China. Systematic SIMS and LA-ICP-MS zircon U-Pb geochronology indicate that the Banshan tuff was formed at -114-112 Ma. Their zircon Hf isotopic compositions [epsilon Hf(t) = -10.9 - 4.2; TDMC = 2.18-1.68 Ga] indicate that the magma of Banshan tuff was derived from a combination of mantle-derived melts and partial melting of the Paleoproterozoic crust of southeast China. In addition, they have low zircon O isotopic compositions (delta 18OVSMOW = 4.39-5.36 parts per thousand) reflecting that the assimilation of wall-rocks which underwent hightemperature hydrothermally exchange with meteoric waters. Extremely Mn-rich garnets in the Banshan tuff (MnO = 30.48-35.82 wt%) suggest that the magma once reached a high degree of crystallization which was nearing consolidation. The subsequent mush rejuvenation caused by magma recharge is evidenced by partly resorbed alkali feldspar, quartz and garnet, as well as the CL-bright rims of zircon. Crystal-rich volcanic rocks are widely distributed in the lower sequences of Cretaceous volcanic strata from southeast China, but rarely developed in the upper sequences. We suggest this reflects that the magmas of lower sequences have higher volatile contents and were formed in a subduction tectonic setting.

Automated summary

This study conducted comprehensive geochemical and isotopic analyses on the Banshan crystal-rich tuff from the large Cretaceous volcanic belt of southeast China, revealing its formation process and subsequent magma rejuvenation. The results indicate that the magma of Banshan tuff was derived from a combination of mantle-derived melts and partial melting of the Paleoproterozoic crust of southeast China, influenced by high-temperature hydrothermal exchange with wall-rocks and volatile contents.