Major and trace element analyses of Miocene rocks from the Shinji-Matsue district and Shimane peninsula, SW Japan

Major and trace element analyses of mainly sedimentary rocks from the Kawai-Kuri，Omoh，Ushikiri，and Fujina Formations were made to add to a growing database for Miocene backarc sediments in Shimane，SW Japan．Samples were collected in the Shinji-Matsue district and in Shimane Peninsula．Kawai-Kuri sandstones and mudrocks are silica-rich，and most elements are depleted relative to Upper Continental Crust（UCC），suggesting derivation from a felsic granitoid source．Overlying Omori Formation volcanogenic sandstones have considerably lower SiO_2 contents and abundances of other elements lie closer to UCC，reflecting derivation from coeval Omori andesites and dacites．Deeper water turbiditic sediments in the correlative Ushikiri Formation have almost identical compositions．Mudrocks and very fine-grained sandstones from the Fujina Formation are moderately SiO_2-rich，and have elemental abundances similar to or less than UCC．This and immobile element ratios imply a mixed source comprising felsic granitoids and subordinate Omori volcanic and sedimentary rocks．The succession thus records initial exposure and erosion of basement granitoids，superimposed volcanism，and subsequent stripping of the volcanic cover．