Accretion and tectonic erosion processes revealed by the mode of occurrence and geochemistry of greenstones in the Cretaceous accretionary complexes of the Idonnappu Zone, southern central Hokkaido, Japan

The Cretaceous accretionary complexes of the Idonnappu Zone in the Urakawa area are divided into five lithological units, four of which contain greenstone bodies. The Lower Cretaceous Naizawa Complex consists of two lithologic units. The Basaltic Unit (B-Unit) is a large-scale tectonic slab of greenstone, consisting of depleted tholeiite similar to that of the Lower Sorachi Ophiolite (basal forearc basin ophiolite) in the Sorachi-Yezo Belt. The Mixed Unit of Naizawa Complex (MN-Unit) contains oceanic island-type alkaline greenstones which occur as slab-like bodies and faulted blocks with tectonically dismembered trench-fill sediments. Repeated alternations of the two units in the Naizawa Complex may have been formed by the collision of seamounts with forearc ophiolitic body (Lower Sorachi Ophiolite) in the trench. The Upper Cretaceous Horobetsugawa Complex structurally underlies the Naizawa Complex in its original configuration, and it also contains greenstone bodies. Greenstones in the MH-Unit occur as blocks and sedimentary clasts in a clastic matrix, and exhibit depleted tholeiite and oceanic-island alkaline basalt/tholeiite chemistry. This unit is interpreted as submarine slide and debris flow deposits. Greenstones in the PT-Unit occur at the base of several chert-clastic successions. Most of the greenstones are severely sheared and show normal-type mid-ocean ridge basalt composition. The PT-Unit greenstones are considered to have been derived from abyssal basement peeled off during accretion. The different accretion mechanism of the greenstones in the Naizawa and Horobetsugawa complexes reflects temporal changes in subduction zone conditions. Seamount accretion and tectonic erosion were dominant in the Early Cretaceous, due to highly oblique subduction of the old oceanic crust and minimal sediment supply. Whereas, thick sediments with minor mid-ocean ridge basalt and olistostrome accreted in the Late Cretaceous, due to near-orthogonal subduction of young oceanic crust with voluminous sediment supply.