10:45 AM - 12:15 PM
[SGL23-P01] Stratigraphic relation of aphyric basalt and picrite in lower Sorachi Group, Mt. Ashibetsu area, Hokkaido, Japa
Keywords:picrite
The Sorachi Groupe has been formed by the deposition of clastic supplied from the island arc on the basalts accreted on the eastern margin of the Oshima Belt. Recently, the focus has shifted to picrites discovered from Mt. Ashibetsu. However, the stratigraphic relationship between the aphyric basalts that comprise most of the Sorachi Group, including the schematic area and the picrites that are locally distributed on the western slope of Mt. Ashibetsu, has not been confirmed, leaving room for doubt as to whether the picrites can be discussed as the Sorachi Group. In this study, we review the relationship between aphyric basalts and picrites in the lower part of the schematic Sorachi Group based on field studies and whole-rock compositions and discuss the origin and igneous activity of the two rock types.
The Sorachi Group on the east side of Mt. Ashibetsu is subdivided into the Ashibetsudake Formation, consisting of aphyric basalt, and the Main Yubarigawa Formation, composed of volcaniclastic sandstone and siliceous mudstone. The basal conglomerate consisting of red mudstone matrix, basalt, and dolerite breccia at the base of the Main Yufure River Formation is newly designated as the Yufure River Conglomerate Member. The flow plane of the pillow lava of the Ashibetsudake Formation is inclined 20° to 50° to the west and is facing west. The lower part of the Ashibetsudake Formation is intruded by a dolerite sill. The Yufure River Conglomerate Member and the upper stratigraphy of the Yufure River are steeply dipping and east-facing, suggesting that the Main Yubari River Formation overlies the Ashibetsudake Formation in an inclined unconformity. Picrite and basalt of the Gokurakudaira Formation are distributed on the western slope of Ashibetsudake and near the northern ridge line. As mentioned above, the Ashibetsudake Formation is west facing. Therefore, the Gokurakudaira Formation on the west side of Mt. Ashibetsu is the apparent upper part of the Ashibetsudake Formation.
The Ashibetsudake Formation basalts are non-foaming, while the Gokurakudaira Formation picrites are strongly effervescent. Picritic and picritic basalts of the Gokurakudaira Formation are dominated by clinopyroxene without plagioclase and are more mafic than the Ashibetsudake Formation rocks. The thin sections of the Ashibetsudake Formation show that the basalts contain zeolites and prehnite. In contrast, the picrites and basalts of the Gokurakudaira Formation contain metamorphic minerals such as pumpellyite and actinolite. This suggests that the Gokurakudaira Formation has undergone metamorphism corresponding to the phase of pumpellyite-actinolite and has been metamorphosed at higher pressures than the zeolite facies of the Ashibetsudake Formation.
Picrites of the Gokurakudaira Formation are very rich in MgO (20~25 wt%). The Ashibetudake Formation basalts and the Gokurakudaira Formation picrites and basalts are depleted in incompatible elements and show trace element compositions like N-MORB. However, the rocks of the Gokurakudaira Formation differ from those of the Ashibetsudake Formation, which are not depleted in HREE by showing slightly depleted HREE characteristics in contrast to MREE.
The difference in metamorphic grade and total rock composition makes it difficult to consider that the rocks of the Gokurakudaira and Ashibetsudake Formations are the same, especially since the Gokurakudaira is subjected to high-pressure metamorphism comparable to that of the Kamii Kohtan metamorphism. If the Gokurakudaira is included in the Kamuikotan metamorphic rocks, the Gokurakudaira is subducted into the Sorachi Group. This suggests that the Ashibetsudake and Gokurakudaira Formations belong to different plates and are unlikely to have been formed in the same igneous field. Further studies on the origin of these two Formations are expected to contribute to the reconstruction of the plate configuration of the western Pacific at that time.
The Sorachi Group on the east side of Mt. Ashibetsu is subdivided into the Ashibetsudake Formation, consisting of aphyric basalt, and the Main Yubarigawa Formation, composed of volcaniclastic sandstone and siliceous mudstone. The basal conglomerate consisting of red mudstone matrix, basalt, and dolerite breccia at the base of the Main Yufure River Formation is newly designated as the Yufure River Conglomerate Member. The flow plane of the pillow lava of the Ashibetsudake Formation is inclined 20° to 50° to the west and is facing west. The lower part of the Ashibetsudake Formation is intruded by a dolerite sill. The Yufure River Conglomerate Member and the upper stratigraphy of the Yufure River are steeply dipping and east-facing, suggesting that the Main Yubari River Formation overlies the Ashibetsudake Formation in an inclined unconformity. Picrite and basalt of the Gokurakudaira Formation are distributed on the western slope of Ashibetsudake and near the northern ridge line. As mentioned above, the Ashibetsudake Formation is west facing. Therefore, the Gokurakudaira Formation on the west side of Mt. Ashibetsu is the apparent upper part of the Ashibetsudake Formation.
The Ashibetsudake Formation basalts are non-foaming, while the Gokurakudaira Formation picrites are strongly effervescent. Picritic and picritic basalts of the Gokurakudaira Formation are dominated by clinopyroxene without plagioclase and are more mafic than the Ashibetsudake Formation rocks. The thin sections of the Ashibetsudake Formation show that the basalts contain zeolites and prehnite. In contrast, the picrites and basalts of the Gokurakudaira Formation contain metamorphic minerals such as pumpellyite and actinolite. This suggests that the Gokurakudaira Formation has undergone metamorphism corresponding to the phase of pumpellyite-actinolite and has been metamorphosed at higher pressures than the zeolite facies of the Ashibetsudake Formation.
Picrites of the Gokurakudaira Formation are very rich in MgO (20~25 wt%). The Ashibetudake Formation basalts and the Gokurakudaira Formation picrites and basalts are depleted in incompatible elements and show trace element compositions like N-MORB. However, the rocks of the Gokurakudaira Formation differ from those of the Ashibetsudake Formation, which are not depleted in HREE by showing slightly depleted HREE characteristics in contrast to MREE.
The difference in metamorphic grade and total rock composition makes it difficult to consider that the rocks of the Gokurakudaira and Ashibetsudake Formations are the same, especially since the Gokurakudaira is subjected to high-pressure metamorphism comparable to that of the Kamii Kohtan metamorphism. If the Gokurakudaira is included in the Kamuikotan metamorphic rocks, the Gokurakudaira is subducted into the Sorachi Group. This suggests that the Ashibetsudake and Gokurakudaira Formations belong to different plates and are unlikely to have been formed in the same igneous field. Further studies on the origin of these two Formations are expected to contribute to the reconstruction of the plate configuration of the western Pacific at that time.