Detrital chrome spinels in the Cretaceous Sanchu sandstone, central Japan: indicator of serpentinite protrusion into a fore-arc region
Abstract The Sanchu Group is regarded as Lower Cretaceous fore-arc basin-fill sediments underlain unconformably by the Jurassic accretionary wedge. This group is accompanied with the Sanchu serpentinite which is probably correlated to the Kurosegawa Tectonic Zone or serpentinite melange of SW Japan. The Sanchu Group is divided into the newly defined Untis 1–3. Sandstones of Unit 1 contain numerous detrital chrome spinel grains which were probably derived from ultramafic and mafic rocks. These chrome spinels are classified into high-Ti and low-Ti groups based on microprobe analysis. The low-Ti group chrome spinels were supplied ultramafic rocks similar to the Sanchu serpentinite, while the high-Ti group ones seem to have been derived from volcanics which are geochemically different from the presently exposed greenstone (120±6 Ma) of the Otchizawa Formation. The spinel chemistry suggests that the source rocks of the low-Ti and the high-Ti groups were originated from the arc to fore-arc upper mantle and the intraplate basalt or E-type MORB, respectively. The serpentinite may have formed a complex together with the volcanics. The sedimentary environment of Unit 1 is presumed to have been the foreshore to nearshore shelf. The graded accumulation of chrome spinels in sandstone suggest that sediment gravity flow such as a storm-generated turbidity current was the transportation mode. The serpentinite-volcanics complex emerged subaerially as a ridge during the sedimentation of Unit 1 (late Hauterivian to early Aptian). This ridge referable to a tranch slope break was produced by the protrusion of serpentinized peridotite (the Sanchu serpentinite and its precursor) associated with E-type MORB or intraplate basalt along the plate boundary in teh fore-arc region. Left lateral slip-fault movements may facilitate the protrusion, which can be considered as phenomena in the “periphery” of the Kurosegawa Tectonic Zone or serpentinite melange.