Geochemical characteristics of carboniferous greenstones in the inner zoneof Southwest Japan

Greenstones, representing remnants of paleo-oceanic crust, occur in Permian and Jurassic accretionary complexes of the Inner Zone in the Southwestern Japan arc. The formation age of most of the greenstones is early Carbonifer ous, based on fossil ages for overlying limestones and Sm-Nd isotope ages o f the greenstones themselves. The geochemistry of such greenstones is simil ar to those of present-day oceanic islands. Greenstones of the Permian accr etionary complex (Akiyoshi belt) are alkalic and tholeiitic in composition. Some alkali basalts show peculiar feature from an EM-1 mantle source, such as the Gough Island and Tristan da Chunha basalts in the South Atlantic. G reenstones of the Jurassic accretionary complex (Tamba belt) are also alkal i and tholeiitic basalts with both basalt types in the northern part of the Tamba belt coming from strongly depleted characters similar to a mid-ocean ridge basalt source mantle. The variable geochemistry of the oceanic basal ts is explained by hypothesis on existence of a Carboniferous mantle plume below the spreading ridge which divides the Farallon and Izanagi plates. Th e Akiyoshi belt seamounts and/or oceanic islands of the Farallon plate and Tamba belt seamounts and/or oceanic islands of the Izanagi plate formed sim ultaneously by the upwelling of the thermal plume. Some part of the Akiyosh i belt basalts originated locally from an EM-1 mantle source, while basalts from the northern parts of the Tamba belt have a normal-type mid-ocean rid ge basalt (N-MORB) source component. Existence of an N-MORB signature is co nsistent with the presence of a spreading center in a Carboniferous 'Pacifi c Ocean' that caused separation of the Farallon and Izanagi plates. Dispari ty in accretion ages of the basaltic rocks in the Permian and Jurassic may have been caused by differences in the relative motion of the two plates.