Geochemical and isotopic characteristics of the Cretaceous Orikabe Plutonic Complex, Kitakami Mountains, Japan: magmatic evolution in a zoned pluton and significance of a subduction-related mafic parental magma

Abstract

The Orikabe Plutonic Complex, northeast Japan, is a zoned pluton and one of the Cretaceous intrusions in the Circum-Pacific area. In the Main body, K-rich calc-alkaline rocks composed of marginal gabbro and a large amount of monzodiorite–quartz monzonite–monzogranite are intruded successively by innermost calc-alkaline rocks of granodiorite. The gabbro and monzodiorite–monzogranite have a continuous chemical variation, while the granodiorite has lower concentrations of K, Rb, Y, Zr, Nb and F at the same SiO2 content. The gabbro and monzodiorite–quartz monzonite have a Rb-Sr whole-rock age of 119±12 Ma with an initial 87Sr/86Sr ratio of 0.70392±0.00007. The initial 87Sr/86Sr ratio of the innermost granodiorite is estimated to be about 0.7042. The δ18O values of fresh rocks range from +6.7 to +8.3‰, indicating a positive correlation with SiO2 contents. The K-rich calc-alkaline rocks were derived through fractional crystallization from a mafic parental magma with a slightly high δ18O value, implying a major contribution of a sub-arc mantle at a continental margin. Trace element modeling indicates that the source could have been a fertile lherzolite enriched in LILE and depleted in HFSE. The innermost granodiorite was the differentiation product of a distinct parental magma, suggesting the involvement of a small amount of crustal component in the source and partial melting under a more hydrous condition.