Geochemical insights from clinopyroxene phenocrysts into the effect of magmatic processes on petrogenesis of intermediate volcanics

The contributions of magmatic processes such as fractional crystallization and magma mixing to the composition of intermediate volcanics are enigmatic. This is mainly attributed to the difficulty in deciphering the composition of primary melts. Here, we address this issue through a combined geochemical and petrological study of clinopyroxene phenocrysts in intermediate volcanics from the Middle-Lower Yangtze Valley in South China. Clinopyroxene phenocrysts in these volcanics exhibit various types of textural features and compositional zones. Three types of clinopyroxene domains can be identified according to their distinct REE distribution patterns. Type I clinopyroxene domains are characterized by smooth REE distribution patterns with significant negative Eu anomalies, Type II clinopyroxene domains exhibit enrichment in LREE and MREE over HREE with moderate negative Eu anomalies, and Type III clinopyroxene domains show depletion in MREE with significant negative Eu anomalies. All the clinopyroxene domains together defined two types of compositional trends for primary melts. While Type I clinopyroxene domains were crystallized from a less REE-enriched primary melt, Types II and III clinopyroxene domains were crystallized from a common more REE-enriched primary melt. The compositional transition from Type II to Type III clinopyroxene domains indicates fractional crystallization of significant amounts of LREE- and MREE-rich minerals, respectively, such as plagioclase and amphibole from the evolved magmas. Nevertheless, the host rocks show the absence of not only strong negative Eu anomalies but also significant MREE depletion, suggesting that the majority of those LREE- and HREE minerals were not separated from the host magmas. This leads to insignificant influences on the composition of the final magmas. In addition, there is recharge mixing between different batches of cogenetic magmas, which is recorded by patchily and reversely zoned clinopyroxene phenocrysts. Such mixing does not significantly change the composition of mixed magmas because of the compositional similarity between the mixing endmembers. As a consequence, both fractional crystallization and magma mixing would be operated during the evolution of intermediate magmas, but these two magmatic processes have insignificant influences on the final composition of intermediate volcanics. In this regard, the geochemistry of clinopyroxene phenocrysts provides insights into the contributions of magmatic processes to the petrogenesis of intermediate volcanics.