研究業績

Many strata-bound Fe−Mn deposits (umbers) occur in accretionary complexes and ophiolites. Previous investigations have been limited to low-grade metamorphic umber deposits, however, and no umber counterparts have been identified in high-P/T metamorphic belts in accretionary complexes. Here, we report new geochemical data from a hematite-quartz schist in the Nagatoro area, central Japan, part of the Sanbagawa high-P/T metamorphic belt. The Nagatoro hematite-quartz schist consists mainly of an Fe- and Mn-rich component, silica, and calcium carbonate. In Fe-rich layers, Mn, Ca, P, Co, Ni, Zn, As, Sr, Pb, and rare-earth elements (REEs) except Ce are enriched relative to post-Archean average Australian shale (PAAS), and bulk concentrations of these elements are positively correlated with Fe2O3. PAAS-normalized REE patterns of the schist have clear negative Ce anomalies indicative of oxic depositional environments and similar to those of Japanese umbers and present-day mid-ocean ridge (MOR) metalliferous non‑carbonate sediments. The schist is enriched in light and middle REEs relative to present-day MOR metalliferous carbonates, implying a continuous uptake of REEs from pore water by Fe-oxyhydroxides via diagenetic reactions after burial. We conclude that the Nagatoro hematite-quartz schist was originally deposited as hydrothermal metalliferous sediments near a MOR, equivalent to umber; the sediment was subsequently subjected to accretion and metamorphism. This study provides a novel report of umber in a high-P/T metamorphic zone in an accretionary complex, demonstrating that the essential geochemical features of the umber were retained even after metamorphism.