Compositions of detrital spinels from Lower Cretaceous Sasayama Group, Hyogo Prefecture, Japan: Constraints on source lithology and tectonic setting.

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  • 兵庫県，下部白亜系篠山層群から得られた砕屑性スピネルの化学組成: 供給源の岩相とテクトニックな環境に対する制約
  • Compositions of detrital spinels from L

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Sandstones from the Lower Cretaceous Sasayama Group contain chromian spinel and chromian magnetite as accessory minerals. Microprobe analyses show that the chromian spinel has Cr/(Cr+Al) ratios between 0.21 and 0.59, Mg/(Mg+Fe2+) ratios between 0.39 and 0.72, Fe3+/(Fe3+Al+Cr) ratios less than 0.10, and TiO2 contents less than 0.4 wt%. Comparison with spinels from literature suggests that they were derived from Alpine-type peridotite. The lithology of the Alpine-type peridotite was mostly clinopyroxene-bearing harzburgite and/or refractory lherzolite with subordinate cumulates. Chromitite did not contribute detritus to the Sasayama basin.<br>     The chemistry of the chromian spinel further suggests that the Alpine-type peridotite source was most probably formed under an oceanic back-arc basin 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. In comparison with ophiolitic rocks widely distributed north of the study area, no specific provenance is constrained although the ultramafic rocks distributed in the Chugoku Zone are the most likely candidates for provenance.

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