Crystal-melt separation and mush rejuvenation in the Sanbaishan volcanic system, Southeast China: Evidence from geochemistry and Sr-Nd-Hf-Ba isotopes

Abstract

The precise identification and understanding of crystal-melt segregation and mush rejuvenation have long been central themes in volcanological research. The Sanbaishan tuff (ST) in Southeast China consists of high-silica rhyolite (HSR; ∼20 vol% phenocrysts, >70 wt% SiO₂) and crystal-rich trachyte (CRT; ∼40 vol% phenocrysts, <67 wt% SiO₂). These two lithologies exhibit complementary compositional variations and distinct compositional gaps. LA-ICP-MS zircon dating reveals that the ST formed at 138–140 Ma, and both the HSR and the CRT share nearly identical Sr-Nd-Hf isotopic compositions (ε_Nd(t) = −8.1 to −7.7; (⁸⁷Sr/⁸⁶Sr)_i = 0.71133 to 0.71411; ε_Hf(t) = −11.3 to −7.0), indicating that the magma originated from the same shallow crustal mush reservoir. Zircon saturation thermometry and rhyolite-MELTS simulations suggest that the high-silica melts were extracted from a crystal-rich cumulate mush at depths of 13–14 km (340–420 MPa) under temperatures of 800–900 °C. Subsequently, the residual crystal cumulates underwent reheating, partial dissolution, and eventual rejuvenation. This evolutionary history is recorded in the disparities between the HSR and the CRT in terms of elemental compositions of phenocrysts and matrix, crystal-fraction differences, and temperature conditions. The HSR has slightly heavier Ba isotopic compositions (δ^138/134Ba = 0.01 to 0.03 ‰) compared to the CRT (δ^138/134Ba = −0.04 to −0.01 ‰), further reinforcing the interpretation that the HSR represents melt extracted from the crystal mush, whereas the CRT corresponds to eruptive material derived from the cumulates. The Sr-Nd-Hf isotopic homogeneity and the absence of mafic microgranular enclaves suggest that the recharge magma primarily supplied heat and volatile components. Our findings suggest that mush rejuvenation likely played a crucial role in triggering the eruption of crystal-rich volcanic rocks.