1:45 PM - 2:00 PM
[SCG50-01] Crustal evolution of plutono-metamorphic complex from the Kurosegawa tectonic belt in Kyushu
★Invited Papers
Keywords:Kurosegawa tectonic belt, H/P metamorphic rock, H/T metamorphic rock, Ordovician granite, Permian collision metamorphism, Tectonics of Asian continental growth
Kurosegawa tectonic belt in central Kyushu is containing wide spreading serpentinite mélange and is situated at the southern part of E-W trending Usuki - Yatsushiro tectonic line. The serpentinite mélange includes various metamorphic and mylonitic granitic rocks as blocks and lenses surrounded by deformed serpentinite (e.g., Matsumoto & Kanmera, 1964; Karakida, 1977; Saito et al., 2005). Main rock types of metamorphic rocks are Phg-Gln schist, Lws-Gln schist, and Ab-Omp rock and Jd-Gln-bearing metagabbro for high-pressure greenschist- to lawsonite-glaucophane schist-facies metamorphic rock and Hbl gneiss, Grt-Bt gneiss, and H/T-metagabbros (Grt amphibolite, Cpx amphibolite, Grt-Cpx granulite) as high-temperature amphibolite- to high-pressure granulite-facies metamorphic rocks (e.g., Karakida, 1977; Osanai et al., 2000; Saito et al., 2005; Miyazoe et al., 2009). Estimated peak P-T conditions of Grt-Cpx granulite indicate 900–1050 °C and 10-12 kbar. A part of this type of granulite contains Grt-Cpx-Pl leucocratic pod, in which Grt surrounded by characteristic “Chl-Ab corona” could be derived from Opx-Pl symplectite, would be formed during the nearly isothermal decompression.
Geochemical and geochronological analyses for H/P- and H/T- metagabbros show the precursor of H/P-metagabbro was derived from MORB-type magmatic activity of ca. 500 Ma, on the other hands that of H/T-metagabbro was derived from volcanic arc magmatic activity of active continental margin around ca. 450 Ma. (Osanai et al., 2014a, 2014b). Blocks and lenses of mylonitic granites in the serpentinite mélange also indicate ca. 450 Ma volcanic arc-type magmatic activity origin (Yoshimoto et al., 2013; Osanai et al., 2014a, 2014b). Metamorphic age for the H/P-metamorphic rocks has been estimated as 300-270 Ma by K-Ar and Rb-Sr isochron methods (e.g., Yoshimoto et al., 2011; Kamimura et al., 2012), while the metamorphic age for H/T-metamorphic rocks is ca. 250 Ma using LA-ICP-MS U-Pb method, which is detected from the melt-derived euhedral Zrn in leucocratic pod from Grt-Cpx granulite. Anhedral Zrn inclusions in Grt and Cpx from the leucocratic pod show ca. 450 Ma. Therefore, Ordovician magmatic activity (Ca. 450 Ma) at the active continental margin, gabbros and granites formed simultaneously at the lower and shallower crusts, respectively. And then, the shallower granite changed into low-grade metamorphosed mylonitic granite, while the lower gabbro was experienced H/T-metamorphism up to high-pressure granulite-facies with partial melting under the peak metamorphic conditions during Late Permian (ca. 250 Ma).
There are many tectonic models for understanding the Kurosegawa tectonic belt formation as follows; 1) huge allochthonous blocks included in Jurassic accretional prism derived from micro-continent (Ichikawa, 1987; Yoshikura, 1990; Ehiro & Kanisawa, 1999; Kato & Saka, 2006), 2) deep crustal strike-slip movement at Eastern margin of Asian continental crust (Tazawa, 2000; Kato & Saka, 2006), 3) klippe (related with tectonic erosion) derived from inner zone of SW-Japan arc (Isozaki & Maruyama, 1991; Maruyama et al., 1997; Isozaki et al., 2010). As described above, very complicated rock constitution for the Kurosegawa tectonic belt in Kyushu is summerized that Cambrian MORB-type magmatic activity and Early Permian metamorphism for H/P-metamorphic rocks, Ordovician volcanic-arc type magmatic activity and Late Permian collision zone metamorphism with partial melting for H/T-metamorphic rocks, and those metamorphic rocks mixed up with Ordovician granitic rocks in the serpentinite mélange. To realizing the rock-forming processes and mélange formation tectonics of the Kurosegawa tectonic belt, it is clear that the belt had a close relation with Asian continental growth, and therefore it should be re-considered the formation process of N- and S-China cratons and metamorphic process during the collision between N- and S-China cratons.
Geochemical and geochronological analyses for H/P- and H/T- metagabbros show the precursor of H/P-metagabbro was derived from MORB-type magmatic activity of ca. 500 Ma, on the other hands that of H/T-metagabbro was derived from volcanic arc magmatic activity of active continental margin around ca. 450 Ma. (Osanai et al., 2014a, 2014b). Blocks and lenses of mylonitic granites in the serpentinite mélange also indicate ca. 450 Ma volcanic arc-type magmatic activity origin (Yoshimoto et al., 2013; Osanai et al., 2014a, 2014b). Metamorphic age for the H/P-metamorphic rocks has been estimated as 300-270 Ma by K-Ar and Rb-Sr isochron methods (e.g., Yoshimoto et al., 2011; Kamimura et al., 2012), while the metamorphic age for H/T-metamorphic rocks is ca. 250 Ma using LA-ICP-MS U-Pb method, which is detected from the melt-derived euhedral Zrn in leucocratic pod from Grt-Cpx granulite. Anhedral Zrn inclusions in Grt and Cpx from the leucocratic pod show ca. 450 Ma. Therefore, Ordovician magmatic activity (Ca. 450 Ma) at the active continental margin, gabbros and granites formed simultaneously at the lower and shallower crusts, respectively. And then, the shallower granite changed into low-grade metamorphosed mylonitic granite, while the lower gabbro was experienced H/T-metamorphism up to high-pressure granulite-facies with partial melting under the peak metamorphic conditions during Late Permian (ca. 250 Ma).
There are many tectonic models for understanding the Kurosegawa tectonic belt formation as follows; 1) huge allochthonous blocks included in Jurassic accretional prism derived from micro-continent (Ichikawa, 1987; Yoshikura, 1990; Ehiro & Kanisawa, 1999; Kato & Saka, 2006), 2) deep crustal strike-slip movement at Eastern margin of Asian continental crust (Tazawa, 2000; Kato & Saka, 2006), 3) klippe (related with tectonic erosion) derived from inner zone of SW-Japan arc (Isozaki & Maruyama, 1991; Maruyama et al., 1997; Isozaki et al., 2010). As described above, very complicated rock constitution for the Kurosegawa tectonic belt in Kyushu is summerized that Cambrian MORB-type magmatic activity and Early Permian metamorphism for H/P-metamorphic rocks, Ordovician volcanic-arc type magmatic activity and Late Permian collision zone metamorphism with partial melting for H/T-metamorphic rocks, and those metamorphic rocks mixed up with Ordovician granitic rocks in the serpentinite mélange. To realizing the rock-forming processes and mélange formation tectonics of the Kurosegawa tectonic belt, it is clear that the belt had a close relation with Asian continental growth, and therefore it should be re-considered the formation process of N- and S-China cratons and metamorphic process during the collision between N- and S-China cratons.