2023 Volume 129 Issue 1 Pages 89-104
This study investigated the mode of occurrence, petrography and geochemistry of basaltic rocks of the Upper Cretaceous accretionary complex (Matoya Group) in the Shimanto Belt in the Kii Naga-shima-Taiki area, eastern part of the Kii Peninsula (Mie Prefecture) to understand their genesis. The Matoya Group in this study area is classified into several stratigraphic units that are distinguished by lithologic assemblage and age. Basaltic rocks included in two of these units; most of which are stratigraphically and closely associated with chert and/or siliceous mudstone, but the remainder occurs as an isolated block in fault contact with the host clastic rocks. Major and trace elements of the basaltic rocks (one sample from the lower unit and five from the upper unit) were analyzed by X-ray fluorescence spectrometry. On the basis of geochemical discrimination diagrams and primitive mantle-normalized incompatible element plots, it is inferred that most of the basaltic rocks have a geochemical affinity to mid-oceanic ridge basalt (MORB) and one sample possibly to oceanic island basalt (OIB). The field observations and petrogenetic pro-perties indicate that the basaltic rocks in the study area formed at depths greater than the carbonate compensation depth in a pelagic environment.
