Detrital chromian spinels from the Ishido Formation of the Sanchu Cretaceous Formations, Kanto Mountains, central Japan

Abstract

In the Sanchu belt, detrital chromian spinels are found in the Cretaceous sandstone and conglomerate of the Ishido Formation near the southern marginal faults, along which small serpentinite bodies are emplaced. The detrital chromian spinels can be classified into two, low-Ti and high-Ti, groups in terms of chemistry. The low-Ti spinels (TiO₂ < 0.4 wt%; Cr # (=Cr/(Cr+Al) atomic ratio), 0.5-0.9) occur as coarse discrete grains. They are relatively low in Mg # (=Mg/(Mg+Fe²⁺) atomic ratio) and were probably derived from low-temperature peridotites. The high-Ti spinels (TiO₂, ca.1 wt%; Cr #, 0.5 to 0.6) sometimes occur as small euhedra in serpentine or chlorite aggregates, which are alteration products either of olivine phenocryst or of volcanic glass. They are relatively high in Mg #. The low-Ti spinels in the Cretaceous sediments are slightly different from those in the currently exposed serpentinite: the former spinels are similar in TiO₂ level and Cr # range to, but are slightly higher both in average Cr # and in Fe³⁺ contents than, the latter ones. Spinels in metavolcanics associated with the serpentinite bodies are definitely higher both in TiO₂ and in Fe³⁺ contents and are lower in Mg # than the detrital high-Ti spinels.
Based on the spinel chemistry, the serpentinite-volcanics complex which had supplied detritus to the Sanchu Cretaceous sediments are proved to be slightly but clearly different in lithology from that currently exposed along the southern margin of the Sanchu belt. The Sanchu serpentinites were most probably derived from arc to fore-arc upper mantle. The spinel chemistry and preliminary bulk rock chemistry indicate that the volcanics associated with the serpentinite, on the other hand, have characteristics of within-plate basalt or E-type MORB. A transcurrent fault (plate boundary) formed in a fore-arc region was possibly responsible for the protrusion of the serpentinite-volcanics complex, which had dammed up the terrigenous sediments as the ocean-ward border of the Sanchu sedimentary basin.