Abstract
Multi-approach combining with modal, grain size, heavy mineral and garnet composition analyses of sandstones were carried out for the Paleogene Otonashigawa Group of the Kii Peninsula, Southwest Japan. The Otonashigawa Group consists of two formations: The Uridani Formations is overlain by the Haroku Formation. The Uridani Formation is composed mainly of mudstones. The Haroku Formation is subdivided into the Lower Member, consisting of alternating beds of sandstone and mudstone, and the Upper Member, consisting of thick sandstone beds and conglomerates. The modal composition of the sandstones in the Haroku Formation shows a change from felsic volcanic-rich lithic wacke at its base, to quartz-rich feldspathic arenite toward its top. Heavy minerals in these sandstones consist of abundant opaque minerals, zircon, garnet, and allanite along with minor titanite, epidote, and hornblende. Hornblende is found in the lower Member, whereas the sandstones from the uppermost section are characterized by abundant epidote, allanite, and titanite. From the Lower to the Upper Member, the pyrope-rich almandine (intermediate P/T type) decreases while in the spessartine-rich almandine (low P/T type) increases. A few grossular garnets were extracted from the upper most part of the Haroku Formation. In terms of provenance, our results indicate that the Otonashigawa Group was sourced chiefly from felsic volcanic rocks, pyroclastics, intermediate P/T metamorphic rocks, and granulite-facies metamorphic rocks from the Paleocene to early Eocene time. The existence of pyrope-rich almandine (Ig2 type) and purple zircon suggests that granulite-facies Precambrian units were also part of the sedimentary source. Owing to unroofing, granitic rocks, low P/T metamorphic rocks and minor calcareous metamorphic rocks cropped out in early Eocene time. Though the timing of the unroofing event differs between Kyusyu, Shikoku, Kii Peninsula, and the Akaishi Mountain areas, it took place everywhere in the provenance of the Shimanto Belt.
