Abstract
Geochemical data including 389 XRF analyses, 203 ICP-MS trace element analyses, 188 Sr-Nd, 120 Pb, and 83 Hf isotope compositions were examined to reveal the origin of the late Cenozoic magmatic activity (~15 mys) in the SW Japan arc. This activity includes OIB-type alkali basalts, IAB-type sub-alkali basalts, HMA, and adakites (ADK). Activity in the SW Japan volcanic arc commenced at ~15 Ma when the back arc basin (BAB) of the Japan Sea opened. Initiation of subduction of the young Shikoku Basin Philippine Sea plate (PHS) occurred at the same time. At ~15 Ma, OIB activity occurred in the rear arc due to upwelling of the asthenosphere by BAB opening, whereas IAB and Setouchi HMA were produced in the forearc due to dehydration and melting of the young slab. ADK and IAB activity between these two areas began at about 8-9 Ma This ADK-IAB zone migrated to the rear arc by 2 Ma, and has remained active to the present. At the same time, OIB activity ceased in the area where ADK-IAB activity began. OIB activity does not require slab inputs and thus originates from asthenospheric mantle. In contrast, IAB, HMA, and ADK obviously require slab fluids and melts. The temporal rear arc-ward migration of the slab-related magmas indicates progressive penetration of the PHS into the anomalously hot mantle asthenosphere that was emplaced during BAB opening. Interaction of BAB opening tectonics and initiation of young PHS slab subduction combined to form the complex volcanism present in the SW Japan arc.
