白亜紀後期の海嶺衝突現像に伴う鉄・マンガンに富む堆積物:現世の海嶺熱水性堆積物との地球化学的類似性

抄録

Major, trace and rare earth element concentrations of sedimentary rocks including umber, red shale and black shale from the Ryujin Formation (late Campanian-Maastrichtian) in the Shimanto belt were determined by X-ray fluorescence (XRF) and inductively-coupled plasma mass spectrometry (ICP-MS). These sedimentary rocks are closely associated with in-situ greenstones in the accretionary complex of the late Cretaceous age. Geochemical characteristics of sedimentary rocks are as follows; (1) the bulk-rock chemical compositions of black shale are generally similar to those of Post Archean Australian Shale (PAAS), (2) red shale is enriched in Fe, Co, Ni, Zn, Y, Mo and REEs compared to black shale, (3) umber exhibits a remarkable enrichment of Fe, Mn, P, V, Co, Cu, Ni, Zn, Y, Mo and REEs, (4) these enriched elements in umber and red shale show a good positive correlation with Fe, (5) these elements/iron ratios of umber are very similar to those of modern hydrothermal metallifer-ous sediments at mid-ocean ridge (MOR), (6) PAAS-normalized REE patterns of umber are characterized by a conspicuous negative Ce anomaly. These geochemical features suggest that a series of sedimentary rocks were deposited near the trench where temgenous materials from lands and hydrothermal Fe-rich suspended particulates from the MOR were supplied simultaneously. This conclusion supports that ridge-forearc collisions occurred near the late Campanian Japan.