P-t Path of Sediment Subduction-Underplating-Exhumation Process Related to the Formation of the Sambagawa Schists

The Valanginian accretionary complex and the Barremian accretionary complex of the Chichibu megaunit I of the Southern Chichibu belt in east Shikoku, which consist of prehnite-pumpellyite facies rocks and overlie the Albian accretionary complex of the Chichibu megaunit I and the Cenomanian-Turonian accretionary complex of the Shimanto megaunit, have been clarified by Hara et al. (1992) to be of the same age with reference to the subduction beginning age (youngest fossil age) as the Saruta nappe (I+II) schists and the Fuyunose nappe schists of the Sambagawa megaunit as high P/T type metamorphic rocks respectively. K-Ar ages of muscovites from the former two accretionary complexes, which are considered to have been roughly comparable with the exhumation beginning age, were determined in this paper to be 114 ± 6Ma and 108 ± 5Ma respectively. The exhumation beginning age appears to have been different by ca. 20Ma between the Chichibu megaunit I of subcretion depth of a few kilobars (less than 4kb) and the Sambagawa megaunit of subcretion depth of ca. 10kb with the same subduction beginning age. It would said that this is a rough estimate of P-t path of sediment subduction-underplating-exhumation process related to the formation of the Sambagawa megaunit.