The parentage of low-grade metasediments in the Sanbagawa Metamorphic Belt, Shikoku, southwest Japan, based on whole-rock geochemistry

Abstract

In order to identify the parentage of low-grade metasediments in the Besshi unit of the Sanbagawa Metamorphic Belt in Shikoku, southwest Japan, we have studied the bulk chemistry of psammitic and pelitic rocks in the Sanbagawa Belt and in related nonmetamorphosed Middle/Late Jurassic–Cretaceous accretionary complexes (Southern Chichibu and Northern Shimanto Belts). Tectonostratigraphic relationships and paleontological evidence suggest a post-Middle Jurassic depositional age for the protoliths of the Sanbagawa psammitic/pelitic schists. Zr/TiO 2 ratios of metasandstones in the Besshi unit differ from those of the Middle/Late Jurassic sandstones in the Southern Chichibu Belt. Besshi metasandstones from the Iya–Inouchidani area have chemical affinities with sandstones from the KSII unit (Coniacian–Campanian) of the Northern Shimanto Belt. The Besshi unit in the Asemi–Tachikawa area can be subdivided into northern and southern parts based on the geochemistry of the metasandstones and metapelites. The geochemistry of the metasediments suggests that the parents of the northern and southern parts are the KSII unit and KSIc unit (Late Albian–Early Coniacian), respectively. The depositional ages of the protoliths of the Besshi metasandstones and metapelites are thus inferred to be Late Albian–Early Coniacian (southern part of the Asemi–Tachikawa area), and Coniacian–Campanian (Iya–Inouchidani area and the northern part of the Asemi–Tachikawa area). These conclusions are consistent with the radiometric ages of metasediments from Asemi–Tachikawa, and imply that most of the low-grade metasediments in the Sanbagawa Belt are an underplated, deeper facies of the Cretaceous Shimanto accretionary complex. Protoliths of the Besshi unit represent muddy facies with greenstones, whereas the KSIc and KSII units in the Shimanto Belt mainly consist of sandy facies of probable trench-fan and trench-wedge origin. The lithologic contrast between the KSIc/KSII and Besshi units may be explained by selective underthrusting and underplating of hemipelagic mud deposited outboard of a trench, and as offscraping of turbidite sequences deposited within a trench.