PROGRADE AMPHIBOLES IN HEMATITE-BEARING BASIC AND QUARTZ SCHISTS IN THE SAMBAGAWA BELT, CENTRAL SHIKOKU - RELATIONSHIP BETWEEN METAMORPHIC FIELD GRADIENT AND P-T PATHS OF INDIVIDUAL ROCKS

The prograde amphibole that coexists with chlorite, epidote, muscovite , albite, quartz and hematite in Sanbagawa schists was examined to inv estigate the relationship between the prograde P-T paths of individual rocks and the metamorphic field gradient in the Sanbagawa metamorphic belt, central Shikoku. The amphibole changes from actinolite, through ferri-winchite and crossite, to barroisite and hornblende with increa sing grade along the metamorphic field gradient. However, the sequence of prograde amphibole compositions in each sample varies in different mineral zones. The general scheme can be summarized as: magnesioriebe ckite-riebeckite-->crossite in the upper chlorite zone of lower-grade rocks; crossite or glaucophane-->barroisite in the garnet zone of medi um-grade rocks; and actinolite or winchite-->barroisite--> hornblende in the albite-biotite zone of higher-grade rocks. Changes of amphibole composition indicate that the prograde P-T path recorded in the highe r-grade rocks was situated on the higher-temperature side of that of t he lower-grade rocks and on the lower-pressure side of the metamorphic field gradient. The systematic change of P-T paths implies an increas ing dP/dT during continuous subduction. These features can be interpre ted as documenting prograde metamorphism within a young subduction zon e that has a non-steady-state geotherm.