The Ashizuri granite-alkaline gabbro complex in the forearc of a Paleogene accretionary complex, Shikoku, Japan: Constraints on evolution by zircon U-Pb age and trace element composition

In order to better understand the tectono-magmatic history of the Miocene Ashizuri igneous complex in Shikoku, SW Japan, we conducted U-Pb isotopes and trace element analyses of zircons from an alkali gabbro, a dolerite and A-type granites in this complex by laser-ablation inductively-coupled plasma-mass spectrometry (LA-ICP-MS). The weighted mean U-Pb ages of zircons in all samples are in the range of ca. 14–13 Ma. The chondrite-normalized REE patterns show positive Ce anomalies, negative Eu anomalies, and enriched HREE relative to LREE and MREE. On the other hand, a sandstone in the Shimizu Formation intruded by the Ashizuri complex contains detrital zircons older than 35 Ma. These results suggest that the ca. 14–13 Ma Ashizuri felsic magma was less affected by crustal contamination and/or assimilation during its emplacement and solidification. Moreover, the gabbros, in spite of their mafic composition, contain igneous zircons most likely derived from a felsic magma. This indicates that magma mixing/mingling took place between the mafic and felsic magmas. Integration of our results with previously published data suggests that the Ashizuri granitic magma underwent fractionation from an OIB-type magma.