Origin of detrital garnet and chromian spinel from the Middle to Upper (?) Jurassic chert breccia in the Mino terrane, central Japan

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Provenance analysis of sand-matrices of chert breccias in the Kamiaso unit, Mino terrane, was carried out on the basis of the chemical composition of detrital garnets and chromian spinels.Chert breccia is a clast-supported breccia composed mainly of radiolarian chert clasts, and includes clasts of siliceous shale, claystone and sandstone. The matrix of chert breccia is quartzo-feldsparthic sandstone and contains mica minerals, garnet, zircon, tourmaline, chromian spinel and opaque minerals.Detrital garnets, which commonly occur in the sand-matrices, consist mainly of almandine with a higher pyrope component, and these Mg-rich garnets are likely to have been derived from the upper amphibolite to granulite facies metamorphic rocks. Detrital chromian spinels are characterized by lower TiO2 content, lower Cr/(Cr+Al) atomic ratio, and a negative correlation of Cr/(Cr+Al) and Mg/(Mg+Fe2+) atomic ratios, and were probably derived from mantle peridotites.The heavy mineral analysis suggests that the provenance of the sand-matrices of chert breccias was composed mainly of a continental basement comprising high-grade metamorphic rocks. The absence of clasts derived from continental basements in chert breccias suggests that the clasts of chert breccias had fallen from the surface of the accretionary prism to the trench and been mixed with trench-fill sandy sediments derived from continental basements.
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