Arc building in the Abukuma Mountains of NE Japan: Insights from petrology, geochemistry and zircon U-Pb-Hf isotope data

Subduction zones along convergent plate margins are the major regions of continental crust building on Earth. The petrological and geochemical features of arc crust are therefore critical in understanding the formation and evolution of continental crust. Here we investigate the magmatic suites in the Abukuma Mountains of NE Japan, which document episodic accretionary events during the Cretaceous. The rock suite in the Abukuma Mountains mainly comprises hornblende (Hbl)-gabbro, diorite, granodiorite and monzogranite. Zircon U-Pb dating suggests that dioritic to granitic rocks in the Abukuma Mountains were formed during 110 to 115 Ma, followed by gabbroic intrusions (100 to 110 Ma) and large-scale high-T/low-P metamorphism. But combined with literature geochronological results, the gabbroic rocks in Abukuma are coeval with the formation of intermediate to felsic rocks with a peak age of similar to 108 Ma. Major and trace elements characteristics of these rocks including their calc-alkaline nature and negative Nb, Ta anomalies suggest their arc-related origin linked with the subduction of the Paleo Pacific (Izanagi) plate. About 200 km north of the Abukuma Mountains, the Kitakami magmatic rocks were also formed during the subduction of Izanagi plate, but previous studies show that parts of the Kitakami intermediate-felsic rocks have distinct adakitic features and more juvenile Hf signatures (epsilon Hf(t) = 7.9 to 10.6) than those in the Abukuma suite (epsilon Hf(t) = 1.5 to 8.8, average at 6.1 & PLUSMN; 1.8). The geochemical features of the magmatic rocks in NE Japan are consistent with slab-break off, mantle upwelling and partial melting resulting in the formation of adakitic rocks in the Kitakami region. However, subsequent to this event, the slab subduction was reestablished, leading to the formation of normal island arc rocks in the Abukuma area. Our study provides new insights into the geodynamic evolution of NE Japan.