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[SMP25-01] Prolonged subduction metamorphism and deep forearc accumulation of Sanbagawa Complex: an example from Okawara district, southern Japanese Alps
Keywords:Sanbagawa, Subduction zone, Metamorphism
Sanbagawa and Chichibu complexes are distributed between the north-south trending MTL and BTL in the Okawara district, southern Japanese Alps. Both complexes were fragmented by some north-south trending vertical faults. Except for near MTL , dips of main schistosities ara gentle. Nearly horizontal structures are preserved within fault-bounded blocks. The Okawara district possesses about 2000 m difference of elevation, therefore this area is adequate for revealing geological architecture of the Sanbagawa Complex . The Sanbagawa Complex consists of Kurokawasawa (mainly pelitic schist), Kamasawa (mainly pelitic schist with small amount of marble), Mikabu (mainly ultramafic to mafic metamorphic rocks), and Terasawa units (mainly pelitic phyllite and metachert), in order from bottom to top with referring to the gentle-dipping main schistosity. The Mibugawa unit (mainly mixed rock, chert and basalt) of the Chichibu complex tectonically overlaps the Sanbagawa Complex.
Boundaries between Mikabu and Kurokamasawa units and between Kamaswa and Terasawa units are almost identical to 325 °C and 315 °C isotherms, restpectively. These planes were used as references. Using the references, lithological distribution, main schistosity, and RSCM isotherms, many geological cross sections generated by QGIS plug-in, mineral assemblages, solid solution composition of minerals, , bulk compositions, Gibbs free energy minimum modeling, and MCMC maximum likelihood estimation and information criterion of the youngest age component with detrital zircon U-Pb ages, we reconstruct geological architecture of the Sanbagawa Complex. The results show that thickness measured in the direction perpendicular to the main schistosity, peak temperatures and pressures, the youngest age components of detrital zircon are as follows; the Kurokawasawa unit: 1400 m, 325-400 °C, 6-8 kbar, 72.3 ± 0.7 Ma (lower part ), 96.3 ± 2.8 Ma (middle part), 104.0 ± 1.1 Ma (upper part); Kamasawa unit: 1200 m, 285-325 °C, 5.0-6.5 kbar, 105.7 ± 1.2 Ma and 109.8 ± 1.9 Ma; Mikabu unit: 285-325 °C, 5.0-6.5 kbar; Terasawa unit: 800 m, 135.8 ± 1.8 Ma; Mibugawa unit: 1000 m, 280-305 °C, 4-5 kbar, 171.3 ± 2.2 Ma. Using these values, we also estimate thermal gradient perpendicular to the main schistosity as follows; Kurokawasawa unit: 55 °C/km; Kamasawa unit: 40 °C/km; Mibugawa unit: 33 °C/km. These high values are possible in inverted thermal structure in subduction channel with descending cold slab. However, downward increases of temperatures in our case are opposite to what one expected for the inverted thermal structure. Assuming crustal density of 2.9 g/cm3, our geological estimation of total thickness from the Kurokawasawa to Mibugawa units is about one third of the petrological estimation of pressure differences. This large discrepancy suggest large amount of crustal thinning took place with formation of main schistosity during exhumation.
P-T conditions from the Kurokawasawa to lower part of the Mibugawa units show linear array on P-T diagram, and are resemblant to those along subduction channel with shear heating (e.g. Ishii and Wallis, 2020). Metamorphic phengite K-Ar ages from bottom and top of the Kurokawasawa unit yield ca. 65 Ma (Shibata and Takagi, 1988) and 105 Ma (Watanabe et al., 1982), respectively. These metamorphic ages suggest at least 40 Myr duration of the Sanbagawa metamorphism. Our geological, petrological and geochronological results lead to the following formational picture for the Sanbagawa Complex; 1) steady-state P-T conditions observed by the Sanbagawa Complex was maintained along subduction channel for at least 40 Myr duration, 2) protolith of the Sanbagawa Complex was continuously subducted to the subduction channel and suffered the Sanbagawa metamorphism, 3) metamorphic rocks were successively exhumed with thinning and formation of main schistosity, and 4) the exhumed metamorphic rocks accumulated from bottom at deeper depths of forearc in arc-trench system.
Boundaries between Mikabu and Kurokamasawa units and between Kamaswa and Terasawa units are almost identical to 325 °C and 315 °C isotherms, restpectively. These planes were used as references. Using the references, lithological distribution, main schistosity, and RSCM isotherms, many geological cross sections generated by QGIS plug-in, mineral assemblages, solid solution composition of minerals, , bulk compositions, Gibbs free energy minimum modeling, and MCMC maximum likelihood estimation and information criterion of the youngest age component with detrital zircon U-Pb ages, we reconstruct geological architecture of the Sanbagawa Complex. The results show that thickness measured in the direction perpendicular to the main schistosity, peak temperatures and pressures, the youngest age components of detrital zircon are as follows; the Kurokawasawa unit: 1400 m, 325-400 °C, 6-8 kbar, 72.3 ± 0.7 Ma (lower part ), 96.3 ± 2.8 Ma (middle part), 104.0 ± 1.1 Ma (upper part); Kamasawa unit: 1200 m, 285-325 °C, 5.0-6.5 kbar, 105.7 ± 1.2 Ma and 109.8 ± 1.9 Ma; Mikabu unit: 285-325 °C, 5.0-6.5 kbar; Terasawa unit: 800 m, 135.8 ± 1.8 Ma; Mibugawa unit: 1000 m, 280-305 °C, 4-5 kbar, 171.3 ± 2.2 Ma. Using these values, we also estimate thermal gradient perpendicular to the main schistosity as follows; Kurokawasawa unit: 55 °C/km; Kamasawa unit: 40 °C/km; Mibugawa unit: 33 °C/km. These high values are possible in inverted thermal structure in subduction channel with descending cold slab. However, downward increases of temperatures in our case are opposite to what one expected for the inverted thermal structure. Assuming crustal density of 2.9 g/cm3, our geological estimation of total thickness from the Kurokawasawa to Mibugawa units is about one third of the petrological estimation of pressure differences. This large discrepancy suggest large amount of crustal thinning took place with formation of main schistosity during exhumation.
P-T conditions from the Kurokawasawa to lower part of the Mibugawa units show linear array on P-T diagram, and are resemblant to those along subduction channel with shear heating (e.g. Ishii and Wallis, 2020). Metamorphic phengite K-Ar ages from bottom and top of the Kurokawasawa unit yield ca. 65 Ma (Shibata and Takagi, 1988) and 105 Ma (Watanabe et al., 1982), respectively. These metamorphic ages suggest at least 40 Myr duration of the Sanbagawa metamorphism. Our geological, petrological and geochronological results lead to the following formational picture for the Sanbagawa Complex; 1) steady-state P-T conditions observed by the Sanbagawa Complex was maintained along subduction channel for at least 40 Myr duration, 2) protolith of the Sanbagawa Complex was continuously subducted to the subduction channel and suffered the Sanbagawa metamorphism, 3) metamorphic rocks were successively exhumed with thinning and formation of main schistosity, and 4) the exhumed metamorphic rocks accumulated from bottom at deeper depths of forearc in arc-trench system.