Staurolite occurrences in the Shizunai river area in the Hidaka metamorphic belt, Hokkaido, Japan.

A new staurolite locality in the Hidaka metamorphic belt (HMB) is reported. Staurolite occurs as inclusions in garnet and plagioclase in alternating garnet-cordierite and garnet-cordierite-gedrite gneisses in the Shizunai river area, central part of the belt. Two types of staurolite-breakdown reactions, Staurolite+Chlorite+Quartz=Garnet+H2O and Staurolite+Gedrite (Na-, Al-poor)+Plagioclase (Ca-poor)=Gedrite (Na-, Al-rich)+Plagioclase (Ca-rich)+Quartz+H2O, are recognized. Subsequently, Gedrite (Na-rich)+Plagioclase (Ca-rich)+Quartz=Gedrite (Na-poor)+Plagioclase (Ca-poor) occurred. Estimated H content of the staurolite is low (2.2-3.2), suggesting low pressure type. Those from the both Menashunbetsu river and Hidakahorobetsu river areas, locating in the south of the Shizunai river area, show similar values. Fe-Mg partitioning coefficients between included staurolite and host garnet are plotted on and around the line of KD =1. Despite of large uncertainty (±60-70°C) of the Koch-Müller (1997)'s staurolite-garnet geothermometry, the application to the pair from the Shizunai river area shows a consistent result with other methods. The minimum temperature estimate for staurolite-garnet pairs is almost constant through the whole HMB. The closure temperature of the Fe-Mg exchange reaction between staurolite and garnet in the HMB is ca. 700°C. The similarities of H contents of staurolite and Fe/Mg distribution and re-equilibration processes of staurolite-garnet pairs may indicate that at least high-grade part of the central area of HMB has been suffered similar thermal histories at similar depths.