Connected volcanic and plutonic association by crystal-melt segregation in the Daiyunshan volcanic field, SE China

Abstract

The Daiyunshan volcanic field is one of the largest volcanic fields in the volcanic-plutonic complex belt along the coast of SE China, which records a long volcanic activity history from the Late Jurassic to the Late Cretaceous. In this paper, we conduct comprehensive petrological, geochemical, isotopic and zircon trace element analyses for two representative calderas (Shiniushan and Yunshan) to reveal the crystal-melt segregation and magma recharge processes of the Daiyunshan volcanic field. Systematic LA-ICP-MS zircon U-Pb dating and isotope analyses reveal that the porphyritic quartz monzonite, porphyritic granite and rhyolite of the Shiniushan caldera have identical crystallization ages (97–94 Ma) and consistent Nd-Hf isotope compositions [εNd(t) = −3.6 to −2.8; zircon εHf(t) = −2.7 to 1.8], which are consistent with those of the porphyritic quartz monzonite and rhyolites in the nearby Yunshan caldera, indicating that the magmas of the two calderas were derived from a common magma source region. Volcanic-plutonic rocks from two calderas display metaluminous to peraluminous features and have low MgO, FeOT, Ni and Cr contents. We thus suggest that their parental magmas were derived by remelting of pre-existing crust with contributions of juvenile components. The Shiniushan porphyritic granite and rhyolite and the Yunshan rhyolites are characterized by distinctly negative Eu anomalies and depletions of Ba, Sr, P and Ti, while the porphyritic quartz monzonites from both calderas show complementary geochemical signatures, such as positive Ba anomalies and neglectable Eu anomalies. The back-scattered electron (BSE) images and energy-dispersive X-ray spectrometry (EDS) phase mapping of the Shiniushan porphyritic quartz monzonite reveal textural indicators of crystal accumulation, such as imbrication and synneusis of plagioclase phenocrysts. Their interstitial assemblages of K-feldspar, quartz and plagioclase commonly resemble the phenocryst assemblages of porphyritic granite. We believe that the Shiniushan porphyritic granite and rhyolite and Yunshan rhyolites represent extracted melts from the mush reservoir, while porphyritic quartz monzonites in both calderas are inherent cumulates that have lost interstitial melts. Therefore, our study shows that the Late Cretaceous volcanic-plutonic rocks in the Daiyunshan volcanic field should be connected by crystal-melt segregation processes. Furthermore, a typical zigzag variation in zircon εHf(t) values is shown by the Late Jurassic to Late Cretaceous volcanic-plutonic rocks from the Daiyunshan volcanic field, indicating variable contributions of asthenospheric mantle-derived melts and the multistage slab rollback of the subducting paleo-Pacific plate beneath SE China.