[S-MP27] 変形岩・変成岩とテクトニクス

[SMP27-12] Spatial variation of lithosphere-asthenosphere interaction revealed from peridotites and gabbros in the Hidaka metamorphic belt

*小澤 一仁¹、田上 創² (1.東京大学大気海洋研究所、2.JX石油開発)

キーワード：リソスフェアーアセノスフェア相互作用、日高変成帯、カンラン岩、はんれい岩

Peridotite bodies exposed on the Earth’s surface on the scale of a few meters to a few hundred kilometers are brought up from the mantle by large-scale tectonic processes along plate boundaries, such as subduction zones, transform faults, and rifted continental margins. Such exhumation processes are results of lithosphere-asthenospheric interaction driven by either active or passive asthenospheric flow. Therefore, temporal and spatial changes of pressure, temperature, and material state of the uplifted mantle provides useful information to understand dynamics of lithosphere-asthenosphere interaction. Moreover, if contemporaneous gabbro bodies occur in the proximity of the peridotites, they could provide thermal and decompression processes of the asthenosphere. We thus better understand lithosphere-asthenosphere interaction by coupling information from peridotite and gabbro bodies. The Hidaka metamorphic belt is suited for this purpose, where large peridotite and gabbro bodies emplaced at nearly the same time in the western zone of the metamorphic belt. We focus on the spatial variation of ascent processes and magma generation conditions on the scale of 150 km along the metamorphic belt, which may help to understand a large-scale interaction of lithosphere and asthenosphere. We examined pairs of peridotite and gabbro bodies from the two segments in the metamorphic belt far apart with each other: the Horoman-Nikanbetsu peridotite/Nikanbetsu gabbro bodies in the southern end and the Magarisawa-Wenzaru peridotite/Pankenushi gabbro bodies in the northern end of the metamorphic belt. We found that the Wenzaru peridotite body originally resided in the shallow(<30km) lithosphere, contrasting to the original residence depth of >60km for the Horoman body. They both underwent contrasting deformation and heating from the ascending asthenosphere, which may have generated melt solidified as gabbroic rocks. The heating is very strong in Horoman characterized by porphyritic to granular microstructure, but it is weak in Wenzaru with wide development of ultramylonite. Higher potential temperature and deeper melting of a MORB source mantle are estimated for the south (Nikanbetsu gabbro) than for the north (Pankenushi gabbro). Combining these results, we infer that passive asthenospheric upwelling took place in the north whereas active upwelling of deeper asthenosphere took place in the south of the Hidaka metamorphic belt. Possible tectonics responsible for the contrasting lithosphere-asthenosphere interaction may be slab breakoff at different depths: shallow in the north and deep in the south.