Chemical Composition of Detrital Garnets in Permian to Jurassic Sandstones of the Kurosegawa Terrane in the Kayaba: Nishinoiwa Area, Western Kyushu, Japan

In the Kayaba—Nishinoiwa area, Kumamoto Prefecture, the Miyama Formation, the Kakisako Formation, the Kuma Formation, the Upper Triassic formation, the Lower to Middle Jurassic formation and the Hashirimizu Formation are distributed from north to south. The stratigraphy is described in this paper in some detail, and besides detrital garnets in Permian to Jurassic sandstones in the well-organized stratigraphic units of the Kurosegawa Terrane in this area have been studied from the sedimentary petrological viewpoint. EPMA analysis of detrital garnets (a total of 637 grains) have revealed that:
There are significant differences in chemical and mineral composition between detrital garnets of the sandstones from the Upper Permian Kuma Formation and the Triassic to Jurassic formations. Detrital garnets of the Kuma Formation consist mostly of grandite (grossular plus andradite) associated with some almandine and some Mn- and Mg-rich almandine, being chiefly derived from calcareous metamorphic rocks, probably skarn. Detrital garnets of the sandstones of the Upper Triassic formation consist mainly of almandine with some amount of Mg content, some grandite and minor spessartine. And, those of the Jurassic formation consist mainly of Mg-rich almandine, some almandine and minor spessartine without grandite garnets. These Triassic to Jurassic detrital garnets are considered to be chiefly derived from high-grade, partly granulite facies, metamorphic rocks.
Based on such mineralogical characters of detrital garnets, it can be pointed out that there are significant differences in provenance between the Upper Permian Kuma Formation and the Mesozoic formations in the Kurosegawa Terrane.