Delamination-induced conjunction of sub-oceanic and sub-arc mantle peridotites in the Hokkaido, northern Japan

In Hokkaido, northern Japan, the westward migration of the Kuril forearc sliver (KFS) that started in the late Miocene due to the oblique subduction of the Pacific Plate along the Kuril Trench results in a "collision" between the KFS and the western part of Hokkaido, the northern extension of the Northeast Japan Arc. The "collision" rapidly uplifted the arc crust, forming the present-day Hidaka Mountains, and tectonically forced delamination occurred beneath the mountains. Based on the depth conversion of seismic wave velocities and geological observations, the delamination has occurred in the upper lower crust, ~23 km depth of the original crustal section. The central to eastern Hidaka Mountains interpret an Eocene-Miocene island-arc crustal section that shallows eastward (Hidaka metamorphic belt; HMB). In the western part, the Poroshiri ophiolite extends approximately 70 km long and <2 km wide, exposing a nearly complete oceanic crust-mantle section that shallows to the west. Both units are bounded by a thrust at the deepest lithologies.

The Uenzaru peridotite complex is a steeply dipping sheet approximately 800 m wide. It lies between the metagabbro of the Poroshiri ophiolite and the pelitic granulites of the HMB. The western part consists mainly of harzburgite, showing metamorphism with abundant amphiboles and complete absence of clinopyroxene. The eastern part consists mainly of fresh spinel lherzolite and plagioclase lherzolite along with pyroxenite and gabbro veins/bands similar to lithologies found in the Horoman peridotite complex, the largest peridotite body in the HMB. The compositional relationship between spinel Cr#[= Cr/(Cr+Al) in atomic ratio] and olivine Fo suggests that the western peridotites are petrogenetically related to the gabbro of the Poroshiri ophiolite. The eastern sample showed a wide range of spinel Cr# consistent with Horoman peridotites. The REE pattern of amphiboles throughout the area shows significantly low abundance and a leftward decreasing pattern in the western part, a spoon or U-shaped pattern at the boundary to the eastern part, and relatively high abundance with an LREE-depleted pattern in the easternmost part. Comparing these patterns with those of the clinopyroxene, the western pattern is consistent with that of the mafic cumulate of the Poroshiri ophiolite, while the eastern part has a similar spoon- or U-shaped pattern. From the plagioclase lherzolite of the Horoman peridotite body, clinopyroxene with spoon- or U-shaped patterns has been reported for spinel lherzolite and harzburgite. Therefore, the trace elements of amphiboles in the Uenzaru Complex reflect the REE pattern of clinopyroxene, indicating that the eastern part belongs to the HMB.

In the HMB, metamorphic pressure and temperature conditions of <970 MPa and <890˚C have been estimated for partial melting of exposed crustal parts. Therefore, the delaminated materials are most likely restites (garnetite and/or garnet-pyroxenite) that could descend in the wedge mantle and passively induce the asthenospheric upwelling that compensates for the removal of the lower crust. Furthermore, the delaminated lower crust may descend even lower than the subducting slab and the fragmented subducting slab (Poroshiri ophiolite) attached to the HMB as part of the passive asthenospheric upwelling.