Use of fracture filling mineral assemblages for characterizing water-rock interactions during exhumation of an accretionary complex: An example from the Shimanto Belt, southern Kyushu Japan

Various fracture filling minerals and secondary minerals in fracture walls were formed by fluid-rock interaction during the exhumation of the Palaeogene Shimanto Belt of Kyushu, Japan, which is located in an accretionary complex. Each mineral formed under favourable geological conditions and can be used to estimate the conditions of accretion and formation of the related rock sequences. Petrographic observations, mineralogical and geochemical analyses were made on fracture filling minerals and secondary minerals from boreholes of ca. 140 m depth, drilled in the Shimanto Belt. Results reveal that the secondary minerals were formed in three major stages distinguished by the sequential textural relationships of the minerals and the interpreted environment of mineral formation. Filling mineral assemblages show that the studied rock formation has been subducted to a depth of several km and the temperature reached was ca. 200-300 °C. After the subduction, the rock formation was uplifted and surface acidic water penetrated up to 80 m beneath the present ground surface. The acid water dissolved calcite fracture filling minerals to form the present groundwater flow-paths, which allowed recent wall rock alteration to occur. The results shown here imply that filling mineral assemblages can be an effective tool to evaluate the environmental changes during exhumation of an accretionary complex.