Provenance of sandstones from the Lower Cretaceous Sasayama Group, Inner Zone of Southwest Japan

Abstract

Modal analysis and phase chemistry of sandstones of the Lower Cretaceous Sasayama Group, Southwest Japan, indicate that the detritus were derived from heterogeneous sources that include medium- to high-grade metamorphic, mafic-ultramafic, and acidic magmatic rocks. Garnet geochemistry suggests their derivation from epidote-amphibolite to granulite facies metamorphic rocks. Sphene and chlorite geochemistry suggests their derivation from mafic rocks. Chemical compositions of chromian spinel indicate input from Alpine-type peridotite. The chemistry of the chromian spinel further suggests that the Alpine-type peridotite source was most probably formed under an oceanic back-arc basin setting rather than mid-oceanic ridge setting. Chemical compositions of chromian magnetite indicate that the Alpine-type peridotite provenance was subjected to metamorphism of up to the lower amphibolite facies prior to erosion and transport. The metamorphic and granitic rocks of the Hida-Oki Terrane, ophiolitic rocks of the Sangun-Renge Terrane, and rhyolitic rocks from uncertain terrane (Akiyoshi?) are most probably the major source of detritus to the Sasayama basin. The basin is most likely a strike-slip basin that formed along the eastern margin of the Asian continent. (C) 2000 Elsevier Science B.V. All rights reserved.