Crystal-rich enclaves in rhyolitic tuffs of the Yandangshan caldera (Southeast China): A window into the compositional heterogeneity of silicic subvolcanic reservoirs

Crystal-rich magmatic enclaves in silicic volcanic rocks may carry critical information concerning the compositional heterogeneity and evolutionary processes in subvolcanic magma reservoirs. Here, we investigate crystal-rich enclaves occurring in rhyolitic ash-flow tuffs from the Yandangshan caldera complex in Southeast China. The trachytic, crystal-rich enclaves display high crystallinity (∼30−50 vol%) and contain resorbed K-feldspar and plagioclase phenocrysts that are significantly coarser than those in the host rhyolitic ash-flow tuffs. Geochemically, the crystal-rich enclaves have high Ba, Zr, Hf, Sr, and Ti contents; low Rb/Sr ratios; and positive Eu anomalies, which are similar to those of intracaldera resurgent porphyritic quartz syenites. However, they differ from those of the rhyolitic ash-flow tuffs that are characterized by high Rb/Sr ratios, negative Eu anomalies, and pronounced depletions of Ba, Sr, and Ti. Additionally, the crystal-rich enclaves exhibit zircon U-Pb ages and Hf isotopic compositions indistinguishable from those of the rhyolitic ash-flow tuffs and porphyritic quartz syenites, which indicates that the crystal-rich enclaves and their host rhyolitic ash-flow tuffs are syn-eruption products of different parts of the same magma reservoir. We suggest that the magma reservoir of the Yandangshan caldera was compositionally stratified, due to crystal-melt separation and crystal accumulation. The crystal-rich enclaves represent cumulate fragments entrained in the extracted rhyolitic melts, while the nonerupted residual cumulates solidified as the intracaldera porphyritic quartz syenite intrusions. This study provides direct evidence for the compositional heterogeneity of silicic magma reservoirs and the genetic link between volcanic and plutonic rocks in caldera systems.