The retrograde chemical zonal structure of amphibole in hematite-bearing basic and quartz schists from the higher grade zone in the Saruta-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,
[B]Na increases with decreasing
[4]Al towards the margins, which have winchite–magnesioriebeckite compositions. The barroisite–winchite–magnesioriebeckite composite crystal is sometimes rimmed by actinolite and/or winchite with low
[4]Al and
[B]Na. 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
[B]Na [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
[B]Na-poorer (less than 1.73 pfu) than prograde sodic amphibole (up to 1.93
[B]Na 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
[B]Na and lower
[4]Al 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 d
P/d
T of the geotherm during exhumation. Retrograde
P–
T conditions during the formation of magnesioriebeckite can be roughly estimated at 7–8 kbar, 400–450°C based on semi-quantitative phase relations of actinolite–winchite–magnesioriebeckite–barroisite series associated with chlorite, epidote, muscovite, albite, quartz and hematite.
相似文献