Geochemical and isotopic characteristics of the Cretaceous Orikabe Plutonic Complex, Kitakami Moutains, Japan: Magmatic evolution in a zoned pluton and significance of a

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 delta18O values of fresh rocks range from +6.7 to +8.3permil, 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 delta18O 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. [PUBLICATION ABSTRACT]