Based on almost all available published age data, the protolith ages, peak metamorphic ages and cooling rate of the Sambagawa metamorphic belt have been discussed and the latest constraints on the ages of the Sambagaw...Based on almost all available published age data, the protolith ages, peak metamorphic ages and cooling rate of the Sambagawa metamorphic belt have been discussed and the latest constraints on the ages of the Sambagawa metamorphism and subduction-related accretionary evolutions were summarized. Peak metamorphic conditions attained within the Kuma nappe complex at ca. 145～185 Ma, and uplift through ca. 500℃ at ca. 150 Ma and 350～400℃ at ca. 110～ 115 Ma. The protolith sediments of the Besshi nappe complex were accumulated and subsequently progressively subducted and suffered high P-T prograde metamorphism during the Kuma nappe complex uplifting. The Besshi nappe complex arrived maximum metamorphic conditions at ca. 110 ～ 120 Ma and subsequently started rapid uplift with the cooling rate of ca. 14.2℃/Ma at ca. 75 ～85 Ma, followed with the cooling rate of ca. 6.0 ～8.9℃/Ma. The Oboke nappe complex started subduction later than other tectonic units and arrived the peak metamorphic conditions at ca. 75 Ma, which followed by the uplift with a cooling rate of ca. 8℃/Ma.展开更多
The Yoshii Group of the Akiyoshi terrane is distributed over the Otakeyama area, Okayama Prefecture, southwest Japan. The Yoshii Group is composed of siliceous rocks and clastics, comprising chert-clastic sequences. T...The Yoshii Group of the Akiyoshi terrane is distributed over the Otakeyama area, Okayama Prefecture, southwest Japan. The Yoshii Group is composed of siliceous rocks and clastics, comprising chert-clastic sequences. The Yoshii Group is divided into four structural units （Units Ot 1, Ot 2, Ot 3, and Ot 4） in structurally ascending order. Lithological similarities and radiolarian age suggest that these units exhibit almost identical ocean plate stratigraphy. These units are structurally-repeated to form an imbricate structure. This structure was possibly formed by off-scrape accretion at a toe of an accretionary complex. Additionally, a review of previous studies indicates that an imbricate structure is common in the Akiyoshi terrane. The presence of an imbricate structure in the Akiyoshi terrane, Permian accretionary complex, indicates the possibility that a specific horizon of Permian pelagic sequences acted as a drcollement zone.展开更多
Ordovician diorite-quartz diorite mylonite(Saganoseki quartz diorite) was discovered in Sambagawa metamorphic terrane at the northern margin of Saganoseki Peninsula,Kyushu Island,Japan.The LA-ICP-MS zircon U-Pb geochr...Ordovician diorite-quartz diorite mylonite(Saganoseki quartz diorite) was discovered in Sambagawa metamorphic terrane at the northern margin of Saganoseki Peninsula,Kyushu Island,Japan.The LA-ICP-MS zircon U-Pb geochronology revealed that the intrusion age of Saganoseki quartz diorite was 473.3±3.6 Ma.These rocks show the volcanic arc affinity based on the trace element composition.On the basis of geochronological and geochemical results,Saganoseki quartz diorite is considered to be a member of the Early Paleozoic igneous rocks of the Kurosegawa tectonic zone.Saganoseki quartz diorite is located just south of the Median Tectonic Line(MTL)and is in close contact with the pelitic and psammitic schist without any brittle shear zone.U-Pb ages of detrital zircon grains from two psammitic schists show the estimated sedimentation age of early Late Cretaceous,indicate that these psammitic schists are the member of Sambagawa metamorphic rocks.Together with these results and the mode of occurrence in the field,we argue that the Early Paleozoic igneous rocks of the Kurosegawa tectonic zone existed as an upper structural unit of the Sambagawa terrane.This relationship is the key to reconstruct the Mesozoic tectonics of Japan as a part of East Asia,and its evolution through time.展开更多
The Kurosegawa belt forms a relatively narrow terrane that can be traced continuously throughout southwest and central Japan.The major constituent continental fragments of the Kurosegawa belt include Early Paleozoic g...The Kurosegawa belt forms a relatively narrow terrane that can be traced continuously throughout southwest and central Japan.The major constituent continental fragments of the Kurosegawa belt include Early Paleozoic granitic rocks,high-grade展开更多
文摘Based on almost all available published age data, the protolith ages, peak metamorphic ages and cooling rate of the Sambagawa metamorphic belt have been discussed and the latest constraints on the ages of the Sambagawa metamorphism and subduction-related accretionary evolutions were summarized. Peak metamorphic conditions attained within the Kuma nappe complex at ca. 145～185 Ma, and uplift through ca. 500℃ at ca. 150 Ma and 350～400℃ at ca. 110～ 115 Ma. The protolith sediments of the Besshi nappe complex were accumulated and subsequently progressively subducted and suffered high P-T prograde metamorphism during the Kuma nappe complex uplifting. The Besshi nappe complex arrived maximum metamorphic conditions at ca. 110 ～ 120 Ma and subsequently started rapid uplift with the cooling rate of ca. 14.2℃/Ma at ca. 75 ～85 Ma, followed with the cooling rate of ca. 6.0 ～8.9℃/Ma. The Oboke nappe complex started subduction later than other tectonic units and arrived the peak metamorphic conditions at ca. 75 Ma, which followed by the uplift with a cooling rate of ca. 8℃/Ma.
文摘The Yoshii Group of the Akiyoshi terrane is distributed over the Otakeyama area, Okayama Prefecture, southwest Japan. The Yoshii Group is composed of siliceous rocks and clastics, comprising chert-clastic sequences. The Yoshii Group is divided into four structural units （Units Ot 1, Ot 2, Ot 3, and Ot 4） in structurally ascending order. Lithological similarities and radiolarian age suggest that these units exhibit almost identical ocean plate stratigraphy. These units are structurally-repeated to form an imbricate structure. This structure was possibly formed by off-scrape accretion at a toe of an accretionary complex. Additionally, a review of previous studies indicates that an imbricate structure is common in the Akiyoshi terrane. The presence of an imbricate structure in the Akiyoshi terrane, Permian accretionary complex, indicates the possibility that a specific horizon of Permian pelagic sequences acted as a drcollement zone.
基金This study was supported by the Fukada Geological Institute[Fukada Grant-in-Aid].
文摘Ordovician diorite-quartz diorite mylonite(Saganoseki quartz diorite) was discovered in Sambagawa metamorphic terrane at the northern margin of Saganoseki Peninsula,Kyushu Island,Japan.The LA-ICP-MS zircon U-Pb geochronology revealed that the intrusion age of Saganoseki quartz diorite was 473.3±3.6 Ma.These rocks show the volcanic arc affinity based on the trace element composition.On the basis of geochronological and geochemical results,Saganoseki quartz diorite is considered to be a member of the Early Paleozoic igneous rocks of the Kurosegawa tectonic zone.Saganoseki quartz diorite is located just south of the Median Tectonic Line(MTL)and is in close contact with the pelitic and psammitic schist without any brittle shear zone.U-Pb ages of detrital zircon grains from two psammitic schists show the estimated sedimentation age of early Late Cretaceous,indicate that these psammitic schists are the member of Sambagawa metamorphic rocks.Together with these results and the mode of occurrence in the field,we argue that the Early Paleozoic igneous rocks of the Kurosegawa tectonic zone existed as an upper structural unit of the Sambagawa terrane.This relationship is the key to reconstruct the Mesozoic tectonics of Japan as a part of East Asia,and its evolution through time.
文摘The Kurosegawa belt forms a relatively narrow terrane that can be traced continuously throughout southwest and central Japan.The major constituent continental fragments of the Kurosegawa belt include Early Paleozoic granitic rocks,high-grade
