Metamorphic Ages of the Jurassic Accretionary Complexes in the Kanto Mountains, Central Japan, Determined by K–Ar Dating of Illite: Implications for the Tectonic Relationship between the Chichibu and Sanbagawa Belts

To determine the metamorphic ages of accretionary complexes in Northern Chichibu Belt SW Japan, K–Ar dating was conducted using weakly metamorphosed sedimentary rocks collected from Kanto Mountains, Central Japan. Whole-rock were obtained for chert and red shale samples, mineral fine-grained illite with a grain size less than 4 μm chert, shale, mudstone, acidic tuff, basic tuff. The presented large variations, systematically older compared to those mudstone tuff same strata. influence submarine hydrothermal activities on before subduction is possible cause this deviation. which fractionated into four grain-size classes suspension method, yielded higher crystallinity (i.e., smaller values Kübler’s index) larger classes. peak determined 3–4 class Late Jurassic Earliest Cretaceous complex lowest structural unit (Kashiwagi Unit) dated within small range between 117–110 Ma, distinctly white mica reported Sanbagawa Belt. age (113 Ma) at type locality Mikabu Greenstones indicates that subducted seamount constituent Kashiwagi Unit. Manba Kamiyoshida Units as 132–107 Ma 163–144 respectively. Major discontinuity suggested Middle complexes.