Progress of actinolite-forming reactions in mafic schists during retrograde metamorphism: an example from the Sanbagawa metamorphic belt in central Shikoku, Japan

Hydration reactions are direct evidence of fluid–rock interaction during regional metamorphism. In this study, hydration reactions to produce retrograde actinolite in mafic schists are investigated to evaluate the controlling factors on the reaction progress. Mafic schists in the Sanbagawa belt contain amphibole coexisting with epidote, chlorite, plagioclase and quartz. Amphibole typically shows two types of compositional zoning from core to rim: barroisite → hornblende → actinolite in the high‐grade zone, and winchite → actinolite in the low‐grade zone. Both types indicate that amphibole grew during the exhumation stage of the metamorphic belt. Microstructures of amphibole zoning and mass‐balance relations suggest that: (1) the actinolite‐forming reactions proceeded at the expense of the preexisting amphibole; and (2) the breakdown reaction of hornblende consumed more H₂O fluid than that of winchite, when one mole of preexisting amphibole was reacted. Reaction progress is indicated by the volume fraction of actinolite to total amphibole, Y_act, with the following details: (1) reaction proceeded homogeneously in each mafic layer; (2) the extent of the hornblende breakdown reaction is commonly low (Y_act < 0.5), but it increases drastically in the high‐grade part of the garnet zone (Y_act > 0.7); and (3) the extent of the winchite breakdown reaction is commonly high (Y_act > 0.7). Many microcracks are observed within hornblende, and the extent of hornblende breakdown reaction is correlated with the size reduction of the hornblende core. Brittle fracturing of hornblende may have enhanced retrograde reaction progress by increasing of influx of H₂O and the surface area of hornblende. In contrast to high‐grade rocks, the winchite breakdown reaction is well advanced in the low‐grade rocks, where reaction progress is not associated with brittle fracturing of winchite. The high extent of the reaction in the low‐grade rocks may be due to small size of winchite before the reaction.