Major and trace element compositions of Cretaceous-Paleocene sandstones and mudrocks from the Idonnapu Zone and the Yezo Supergroup, Urakawa, Hokkaido

The Idonnappu Zone，a Cretaceous-Paleocene accretionary complex，occurs between the Hidaka and Sorachi-Yezo Belts in central Hokkaido．This report contains whole-rock X-ray fluorescence analyses of 75 Cretaceous-Paleocene sandstones and mudrocks from the accretionary complex Idonnappu Zone and the Yezo Supergroup forearc sequence in the Urakawa district，southern central Hokkaido．The Idonnappu Zone suite comprises 45 samples from the MN-Unit of the Naizawa Complex and the PT-，MH- and T-Units of the Horobetsugawa Complex．The remaining 30 samples are evenly divided between the Upper and Lower Yezo Groups．The suites generally exhibit linear or curvilinear trends on oxide- or element-Al_2O_3 variation diagrams，typical of sorting fractionation．Elemental abundances and trends in the three suites generally overlap，suggestive of a common source．However，contrasts in abundances for some elements between suites，and trends in Basicity Index and K_2O/Na_2O ratios suggest some change in provenance over time，with shift from a mature or dissected arc source in the Early Cretaceous to a more primitive（but still relatively evolved）arc source in the Late Cretaceous and Paleocene．This change is recorded in both the accretionary complex and the forearc sequence．