Origin of basalts from Sambosan accretionary complex, Shikoku and Kyushu

Abstract

Sambosan accretionary complex comprises a Late Jurassic to Early Cretaceous subduction-generated accretionary complex of southwest Japan. In order to understand the origin of basalts of the Sambosan accretionary complex, this paper describes their modes of occurrence, petrographic features, and geochemical properties. Sambosan basaltic rocks crop out as,（1）exotic blocks in a mudstone matrix of melange units;（2）displaced blocks in basaltic volcaniclastic rocks;（3）pillow fragment breccias intercalated in Middle Triassic rib- bon chert. In the former two cases, the blocks of basaltic rocks, rang- ing in size from a few meters to several tens of meters, are mainly com- posed of vesiculated pillow lavas with interpillow limestone and subor- dinate massive lavas, pillow fragment breccias, and hyaloclastites. The basalts are described mostly as aphyric basalt, clinopyroxene basalt, and olivine basalt. Dolerite occurs locally. Major and trace elements of the Sambosan basalts were analyzed by using the X-ray fluorescence spectrometry. Interpretation of geochemi- cal data focuses on the relatively alteration-insensitive elements（Ti, Zr, Nb, Y）. On the basis of the discrimination diagrams and the N-MORB normalized patterns, it is revealed that most of the Sambosan basalts have a geochemical affinity with an oceanic island basalt（OIB）enriched in Ti and Nb. Field and petrographic properties indicate that the Sambosan basalts were formed at shallower depth than the carbonate compensation depth in mid-oceanic realm. The predominance of OIB-type basalt means that the Sambosan basalts were erupted to form seamounts by an intraplate volcanism