Genesis of Andesitic Magma Erupted at Yufu Volcano, Kyushu Island, Southwest Japan Arc: Evidence from the Chemical Compositions of Amphibole Phenocrysts.

This article investigates the generation processes of andesitic magma from Yufu Volcano in northeastern Kyushu, Japan, through analysis of major- and trace-element compositions of amphiboles in andesite. Two distinct amphibole groups—pargasite (Prg) and magnesio-hornblende (Mhb)—were identified, crystallizing under different temperature, pressure, and melt composition conditions, indicating the presence of two magma chambers at different depths. Using multivariate methods to estimate equilibrium melt compositions and partition coefficients, the study infers that Prg crystallized from adakitic magma derived from slab melting of the subducted Philippine Sea Plate, while Mhb crystallized from felsic magma likely formed by partial melting of crustal material. By integrating mineral and melt compositions with whole-rock geochemical trends, the authors constrain the chemical compositions of mafic and felsic end-member magmas involved in magma mixing beneath Yufu Volcano, with the mafic end-member resembling gabbroids and the felsic end-member similar to Cretaceous granitic rocks. The study demonstrates that amphibole chemistry can effectively reveal magmatic evolution and magma mixing processes in arc volcanoes.