Whole-rock geochemical compositions of Miocene sedimentary and volcanic rocks from the lzumo-Matsue districts and shimane Peninsu1a,SW Japan

抄録・要旨

Miocene backarc clastic sediments crop out in a strip along coastal Shimane prefecture between Izumo and Matsue，and in Shimane Peninsula．New XRF analyses of 147 sandstones，mudrocks and associated volcanic rocks from the Koura，Ushikiri，Omori，Fujina，Jinzai and Matsue Formations show considerable compositional variation occurs over relatively short stratigraphic intervals．Koura Formation sandstones have Upper Continental Crust（UCC）-normalized multielement patterns intemediate between interbedded felsic tuff and coeval andesite．Turbidite sedimentsfrom the Ushikiri Formation at Chikumi also have andesitic patterns，but shallow water equivalents in the Omori Formation near Izumo exhibit dacitic signatures identical to the volcanic rocks on which they rest．Elemental abundances in the overlying Fujina Formation are depleted relative to UCC，suggesting influx of weathered plutonic detritus．Similar signatures occur in the Jinzai Formation，except for localized basaltic contributions around the Jinzai Basalt edifice．Sediments of the Matsue Formation，which is correlative with the Jinzai Formation，also have depleted patterns indicative of plutonic source，but carry signatures suggestive of more intense source weathering and more advanced sorting in a shallower water depositional environmen．The stratigraphic and lateral variations observed reflect relatively proximal deposition，complex source geology，and sediment supply from restricted catchments．