Abstract
Multi-approach studies combining modal, heavy mineral and garnet analyses of sandstones have been carried out on the Late Cretaceous Terasoma Formation and the Palaeogene Otonashigawa Accretionary Sequence (AS) of the Shimanto Belt in the Kii Peninsula, Southwest Japan. Modal compositions of the Terasoma Formation change upward from feldspathic wacke to lithic wacke. Heavy mineral assemblages also change upward from the one rich in zircon and garnet with epidote, titanite and allanite to the one rich in euhedral zircon. The garnets from the Lower Member are mostly almandine, low and intermediate P/T types with minor high P/T type and grandite. In the Middle Member, the proportion of low P/T type increases at the expense of high P/T type and grandite garnets. These trends suggest that Coniacian violent felsic magmatism took place and their products thickly roofed the basement rocks. Modal compositions of the Haroku formation in the Otonashigawa AS change upward from lithic wacke to feldspathic arenite. The heavy mineral assemblages of the lower members are rich in zircon and garnet accompanied by allanite and greenish-brown hornblende. Sandstones of the uppermost member contain abundant epidote, allanite and titanite. The chemical compositions of detrital garnets also show a decreasing trend of the pyrope-rich almandine (intermediate P/T type garnet) and an increasing trend of spessartine-rich almandine (low P/T type garnet) from the lower to upper members. A few grandites were extracted from the uppermost horizon. These data suggest that felsic volcanic products and intermediate to high grade metamorphic rocks are the probable sources of the Otonashigawa AS. Due to the successive erosion, granitic, low P/T metamorphic rocks and calcareous metamorphic rocks were exposed to the surface. Roofing and unroofing processes are especially suggested by changes of the proportion of euhedral zircon, allanite, spessartine-rich almandine garnet (L), high P/T garnet and grandite garnet.
