Magma recharge processes of the Yandangshan volcanic-plutonic caldera complex in the coastal SE China: Constraint from inter-grain variation of Sr isotope of plagioclase

摘要

In situ Sr-isotope of plagioclase can be used to elucidate the details of magmatic processes. Here we report in situ Sr isotopic compositions of plagioclase from the intra-caldera porphyritic quartz syenite and mafic microgranular enclave of the Yandangshan caldera, SE China. Plagioclase phenocrysts from porphyritic quartz syenite show consistent initial Sr-87/Sr-86 ratios (0.7088-0.7091). Plagioclases in groundmass of mafic microgranular enclave have low initial Sr-87/Sr-86 ratios (0.7080-0.7081). In contrast, the plagioclase phenocrysts of mafic microgranular enclave show relatively variable initial Sr-87/Sr-86 ratios of 0.7081-0.7088, which exhibit distinct inter-grain variation with high values similar to the plagioclase phenocrysts in porphyritic quartz syenite, and low values similar to the plagioclase in groundmass. However, the initial Sr-87/Sr-86 ratios of individual plagioclases from phenocrysts and groundmass show insignificant intra-grain variation. Furthermore, initial Sr-87/Sr-86 ratios (0.7080-0.7088) of plagioclase from phenocrysts and groundmass of mafic microgranular enclave are within and lower than the whole-rock initial Sr-87/Sr-86 ratios (0.7082-0.7088), while the values of plagioclase phenocrysts from the porphyritic quartz syenite are within and higher than the whole-rock initial Sr-87/Sr-86 ratios (0.7086-0.7089). Therefore, we propose that the inter-grain Sr isotope variation of plagioclase is related to magma recharge events, which induce chemical and physical exchanges between the recharge magma and the host magma and the remobilization of crystal mush in magma reservoir. Furthermore, our study shows that whole-rock Sr isotopic compositions could not represent the end-members and they may be a result of the assemblage of rock-forming minerals such as plagioclase with different initial Sr isotopic ratios.