Intermediate high-pressure exhumation of the northern segment of the Sanbagawa belt, Saruta-gawa area, central Shikoku, Japan

The retrograde chemical zonal structure of amphibole in hematite-bearing basic and quartz schists from the higher grade zone in the Samta-gawa area of the Sanbagawa belt was studied to investigate the relationships between the prograde and retrograde P-T paths of the Sanbagawa metamorphism. This amphibole coexists with chlorite, epidote, muscovite, albite, quartz and hematite, and is composed of Al-rich core and Al-poor mantle. The core is fairly homogeneous and has a barroisitic composition. In the mantle part, Na-[B] increases with decreasing Al-[4] towards the margins, which have winchite-magnesioriebeckite compositions. The barroisite-winchite-magnesioriebeckite composite crystal is sometimes rimmed by actinolite and/or winchite with low Al-[4] and Na-[B]. The Al-rich core and Al-poor mantle are regarded as prograde and retrograde products, respectively. The retrograde mantle in the Saruta-gawa area: (1) is systematically richer in Na-[B] [0.40-1.73 per formula unit (pfu; for O = 23)] than that from the same grade zone in the Asemi-gawa area (0.19-0.78 pfu), about 8 km south of the studied area; (2) tends to be Na-[B]-poorer (less than 1.73 pfu) than prograde sodic amphibole (up to 1.93 Na-[B] pfu) produced in the peak temperature stage from the lower grade zone in the same and other areas; and (3) extends its compositional range towards higher Na-[B] and lower Al-[4] than prograde-formed amphibole from the same grade zone in the same area. These zonal characteristics imply that (1) the Saruta-gawa samples experienced retrograde metamorphism under higher P/T conditions than the Asemi-gawa samples, (2) the retrograde P-T path of the Saruta-gawa area passes on the lower pressure side of the metamorphic field gradient, and (3) the Saruta-gawa samples underwent retrograde metamorphism under higher P/T conditions than the prograde metamorphism. The higher P/T conditions of the retrograde metamorphism suggests an increasing dP/dT of the geotherm during exhumation. Retrograde P-T conditions during the formation of magnesioriebeckite can be roughly estimated at 7-8 kbar, 400-450 degrees C based on semi-quantitative phase relations of actinolite-winchite-magnesioriebeckite-barrosite series associated with chlorite, epidote, muscovite, albite, quartz and hematite. (C) 2000 Elsevier Science B.V. All rights reserved.