Analysis of detrital chromian spinel to unravel petrologic character of serpentinite mélange: Example from the Mineoka belt, central Japan

We propose a new method for petrologic analysis of serpentinite mélange using modern detrital chromian spinel. Serpentinite mélange or strongly sheared serpentinite complexes are composed of hard blocks and softer matrix, where the samples for petrologic examination tend to be taken from the former. The matrix composed of comminuted serpentinite has therefore far less thoroughly sampled than the blocks for petrologic analysis. To test our method, we analyzed detrital chromian spinel from placer-like concentrations on the beach near Mineoka serpentinite mélange of the Miocene, central Japan. The Mineoka serpentinite mélange consists of sheared or comminuted serpentinite which encloses hard serpentinized peridotite blocks of various sizes, which are similar to abyssal peridotites with relatively low Cr# (= Cr/(Cr + Al) atomic ratio; < 0.6) of chromian spinel. In the analyzed detrital spinel grains, high-Cr# (>0.6) spinels are appreciably present, almost equal in amount to the low-Cr# (< 0.6) ones. This means the protolith of the sheared matrix of the mélange, which has been unsampled, was mainly the depleted arc-type peridotites that contain high-Cr# (up to 0.8) spinel. The source peridotite complex for the Mineoka serpentinite mélange was possibly composed of abyssal peridotites and depleted arc-type peridotites before the strong shearing. The latter have been obliged to the shearing because of their preferential serpentinization and mechanical weakness. The serpentinite mélange composed of abyssal peridotite blocks and arc-type peridotite-derived matrix can be therefore produced from a single coherent mantle fragment. Detrital chromian spinel should be used more not only for sedimentological work but also for petrologic work on peridotite or serpentinite complexes.