Detrital Garnets in Permian to Cretaceous Sandstones of the Kurosegawa Terrane and its Geological Significance

Detrital garnets in Permian to Cretaceous sandstones of the Kurosegawa Terrane in the western part of Kyushu (the Kashinoki— Pass area of Kumamoto Prefecture) have been studied from the sedimentary petrological viewpoint. EPMA analysis of detrital garnets (a total of 1166 grains) have revealed that:
There are significant differences in chemical composition between detrital garnets of the sandstones from the Upper Permian Kuma Formation and the Mesozoic formations. Detrital garnets of the Kuma Formation consist mostly of grandite associated with some almandine garnet and rare spessartine being derived from calcareous metamorphic rocks, probably skarn. On the other hand, detrital garnets of the Upper Triassic formation consist mainly of almandine with minor amount of Mg content, some Mn—Fe garnet, and minor grandite. Those of the Middle to Upper Jurassic formation consist mainly of almandine garnet with minor amount of Mg and Mn content. These Triassic to Jurassic detrital garnets are considered to be derived from high—grade, partly granulite facies, metamorphic rocks. And, detrital garnets of the Lower Cretaceous Yatsushiro Formation consist of almandine with minor amount of Mg and Mn content and grandite derived from calcareous metamorphic rocks (probably skarn) and high—grade metapelites. Based on such mineralogical characters of detrital garnets it can be pointed out that there are significant differences in provenance (namely, rock component in the source area) between the Upper Permian Kuma Formation and the Mesozoic formations in the Kurosegawa Terrane.