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[SMP24-05] Exhumation process of rocks in the Sangun-Renge Belt exposed at Sasaguri, Fukuoka Prefecture
Keywords:Exhumation, Serpentinization, Antigorite, Quartz, CPO
The high-pressure metamorphic rocks of the Sangun-Renge Belt are exposed around the Narubuchi dam in Sasaguri, Fukuoka Prefecture. Based on microstructural observation and chemical composition analysis, we have considered the exhumation process of the antigorite serpentinite, siliceous schist, and amphibolite of the Sangun-Renge Belt in this area. The strike and dip of distributed rocks are NE-SW to EW and ca. 80°N, respectively.
The antigorite serpentinite is distinguished into two types as coarse-grained and mylonitic antigorite based on the microstructure. The grain size of the coarse-grained antigorite is a few mm, showing undulose extinction and dynamic recrystallization, but no crystallographic preferred orientation (CPO). The mylonitic antigorite is less than 100μm in size, with (001) and [010] oriented parallel to foliation and lineation, respectively. The composition of the accessory mineral Cr-spinel indicates the origin of the antigorite as forearc peridotite. The presence of ferritchromite rim in the Cr-spinel grains suggests that the peridotite was re-equilibrated at greenschist to amphibolite temperatures (400-700°C). A magnetite overgrowth around the ferritchromite rim indicates that the peridotite was serpentinized after ferritchromitization, which suggests that the serpentinization occur at 250-400°C.
The source rock chert upon subduction was metamorphosed to siliceous schist, containing dominantly quartz with a minor amount of phyllosilicate layers comprising garnet and biotite. The quartz grains exhibit a porphyroclastic texture with the c-axis of the recrystallized grains forming a Type-I crossed girdle, which is known to be formed at 400-500°C. This temperature range is consistent with the fact that dynamic recrystallization proceeded by subgrain rotation. The CPO indicates the top-to-north sense of shear, implying that the dynamic recrystallization occurred during exhumation if the oceanic plate was subducted in a northerly direction, as it is now. Subgrain boundary trace analysis of porphyroclasts indicates that Rhomb < a > and Prism < a > are the active slip systems, known to form at 400-650°C. This suggests that the first deformation event of the silicious schist occurred during high-temperature subduction near the peak metamorphic temperatures of 600-800°C (at 5 kbar) determined by garnet-biotite geothermometry.
The hornblende grains in amphibolite are deformed to form the kink band, and within the kink band, it has been metamorphosed to actinolite. This suggests that the amphibolite was affected by retrograde metamorphism in a greenschist facies condition. The formation temperature of hornblende is estimated by the hornblende thermometer. Two different results, 550-650°C and 650-750°C, are obtained from the northern and southern parts of the study area. The temperature difference is consistent with the grain size, which is estimated by grain size distribution analysis using ImageJ software. The average grain sizes of 100-400μm and 900-1100μm corresponds to 550-650°C and 650-750°C, respectively. This suggests that the southern part amphibolite is from much deeper portion than the northern.
To conclude, the chert and basalt of the oceanic plate were subducted and metamorphosed to siliceous schist and amphibolite respectively. During subduction, the siliceous schist has experienced the first deformation event. Then the siliceous schist and amphibolite were incorporated into the continental region, followed by exhumation. During exhumation, the siliceous schist undergoes a second deformation event at greenschist-amphibolite facies, and amphibolite undergoes retrograde metamorphism at greenschist facies. The peridotite from the upper mantle was exhumed and combined with the siliceous schist and amphibolite near the lower granulite-amphibolite facies condition. Upon further exhumation, the peridotite has undergone metamorphism at greenschist to amphibolite facies condition, followed by serpentinization.
The antigorite serpentinite is distinguished into two types as coarse-grained and mylonitic antigorite based on the microstructure. The grain size of the coarse-grained antigorite is a few mm, showing undulose extinction and dynamic recrystallization, but no crystallographic preferred orientation (CPO). The mylonitic antigorite is less than 100μm in size, with (001) and [010] oriented parallel to foliation and lineation, respectively. The composition of the accessory mineral Cr-spinel indicates the origin of the antigorite as forearc peridotite. The presence of ferritchromite rim in the Cr-spinel grains suggests that the peridotite was re-equilibrated at greenschist to amphibolite temperatures (400-700°C). A magnetite overgrowth around the ferritchromite rim indicates that the peridotite was serpentinized after ferritchromitization, which suggests that the serpentinization occur at 250-400°C.
The source rock chert upon subduction was metamorphosed to siliceous schist, containing dominantly quartz with a minor amount of phyllosilicate layers comprising garnet and biotite. The quartz grains exhibit a porphyroclastic texture with the c-axis of the recrystallized grains forming a Type-I crossed girdle, which is known to be formed at 400-500°C. This temperature range is consistent with the fact that dynamic recrystallization proceeded by subgrain rotation. The CPO indicates the top-to-north sense of shear, implying that the dynamic recrystallization occurred during exhumation if the oceanic plate was subducted in a northerly direction, as it is now. Subgrain boundary trace analysis of porphyroclasts indicates that Rhomb < a > and Prism < a > are the active slip systems, known to form at 400-650°C. This suggests that the first deformation event of the silicious schist occurred during high-temperature subduction near the peak metamorphic temperatures of 600-800°C (at 5 kbar) determined by garnet-biotite geothermometry.
The hornblende grains in amphibolite are deformed to form the kink band, and within the kink band, it has been metamorphosed to actinolite. This suggests that the amphibolite was affected by retrograde metamorphism in a greenschist facies condition. The formation temperature of hornblende is estimated by the hornblende thermometer. Two different results, 550-650°C and 650-750°C, are obtained from the northern and southern parts of the study area. The temperature difference is consistent with the grain size, which is estimated by grain size distribution analysis using ImageJ software. The average grain sizes of 100-400μm and 900-1100μm corresponds to 550-650°C and 650-750°C, respectively. This suggests that the southern part amphibolite is from much deeper portion than the northern.
To conclude, the chert and basalt of the oceanic plate were subducted and metamorphosed to siliceous schist and amphibolite respectively. During subduction, the siliceous schist has experienced the first deformation event. Then the siliceous schist and amphibolite were incorporated into the continental region, followed by exhumation. During exhumation, the siliceous schist undergoes a second deformation event at greenschist-amphibolite facies, and amphibolite undergoes retrograde metamorphism at greenschist facies. The peridotite from the upper mantle was exhumed and combined with the siliceous schist and amphibolite near the lower granulite-amphibolite facies condition. Upon further exhumation, the peridotite has undergone metamorphism at greenschist to amphibolite facies condition, followed by serpentinization.