Geology and metamorphic evolution of the Sanbagawa metamorphic belt, Japan

Abstract

Summary The Cretaceous (pre-Japan Sea) Sanbagawa metamorphism affected the Japanese Jurassic complex south of the Median Tectonic Line in the regions now recognized as the Sanbagawa, Mikabu and Chichibu belts. The metamorphic peak (116 Ma) was reached and passed during the tectonic ‘D 1 ’ deformation, corresponding to sinistral shear N30°E along the eastern margin of the Asian continent. This was followed by ‘D 2 ’ (c. 85 Ma) fold and thrust deformation, the vergence of which is normal to the ‘D 1 ’ trend. These deformational events established the present thermal structure. The final regional deformation formed upright ‘D 3 ’ folds. The four metamorphic zones based on pelitic assemblages can be enhanced by using basic schists to subdivide the pelitic chlorite zone. Apparent Fe-Mg partition coefficients between chlorite and garnet show an essential regional continuity of metamorphism and that thrust-offsets do not juxtapose elements from different mineral zones. Peak conditions of metamorphism ranging from 250°C and 6 kbars to 600°C and 10 kbars are consistent with simple P-T-t loops which progress at higher pressures and return at lower pressures to the surface.

Viscous flow and deformation of regional metamorphic belts at convergent plate boundaries

“…The protoliths of the metamorphic rocks consist of accretionary materials, and the metamorphism and deformation are thought to have occurred within the accretionary forearc system [e.g., Toriumi, 1985;Banno and Sakai, 1989;Takasu and Dallmeyer, 1990;Maruyama et al, 1996], which can be compared with the models in this study.…”

Ca-Al hydrous silicates in the chlorite-grade pelitic schists in Sanbagawa metamorphic belt and a petrogenetic analysis in the model mixed-volatile system

“…Detailed petrologic investigations of the Sanbagawa schists in central Shikoku and inferred metamorphic pressure and temperature evolutions are given in a series of articles (Banno et al, 1986;Banno and Sakai, 1989;Higashino, 1990;Otsuki and Banno, 1990;Enami et al, 1994;Aoya, 2001;Wallis et al, 2001;Inui and Toriumi, 2002). In central Shikoku region, the Sanbagawa belt is divided into the chlorite, garnet, albite -biotite and oligoclase -biotite zones on the basis of the mineral assemblages of the pelitic schists in ascending order of peak metamorphic temperature (Enami, 1983;Higashino, 1990).…”

Tectono-metamorphic history of the ophiolitic Lento unit (northern Corsica): evidences for the complexity of accretion-exhumation processes in a fossil subduction system

“…The Na-amphibole is a typical mineral related to the blueschist facies and is indicative of HP/LT gradient metamorphism [e.g. 44,45,46,47]. The minimum pressure (P) conditions of this mineral assemblage, can be estimated using the P-T grid for the epidote-blueschist facies of Evans [46], calculated in the NCMASH (Na 2 O + CaO + MgO + Al 2 O 3 + SiO 2 + H 2 O) system assuming a H2O =0.9.…”

“…48]. According to the semi-quantitative phase diagram of amphiboles proposed by Banno and Sakai [45] and Otsuki and Banno [47], as no hornblende or barroisite are present in the studied metamorphic assemblages, temperatures are estimated to have been below 450°C. For temperatures lower than 450 °C, the minimum pressure estimated from the P-T grid for the epidote-blueschist facies [46] is about 0.60-0.80 GPa (Fig.…”