The Sambagawa metamorphic belt exposed in central Shikoku, Japan, is tectonostratigraphically represented by the Oboke and structurally overlying Besshi nappe complexes. Oboke units consist of metamorphic rocks largely derived from clastic protoliths (sandstone and conglomerate) whereas Besshi units are dominated by rocks with oceanic protolith affinities (shale, chert, and greenstone). Whole-rock schist samples from low-grade Oboke units record intermediate temperature, 40Ar/39Ar plateau ages of 70-77 Ma, which are interpreted to closely date attainment of maximum metamorphic conditions. Whole-rock schist samples from Besshi units (chlorite zone) record plateau ages of 85-94 Ma suggesting an earlier metamorphic climax within this structural level. Amphibole within the albite-biotite zone in Besshi records Ar-36/40Ar vs. 39Ar/40Ar plateau isotope correlation ages of 84-87 Ma, which are interpreted as dating the post-metamorphic cooling through temperatures required for intracrystalline retention of argon. A slightly older isotope correlation age (94 Ma) is recorded by amphibole from the higher grade oligioclase-biotite zone. Contrasts in amphibole cooling ages are interpreted as reflecting thrust-related metamorphic inversion and resultant earlier cooling of higher grade structural units. Muscovite from all structural levels of the Besshi nappe complex records 40Ar/39Ar plateau ages of 76-89 Ma. These are interpreted to date post-metamorphic cooling through appropriate closure temperatures following structural assembly of the Besshi nappe complex. Protoliths of the Besshi units originated at approx. 130 Ma and attained maximum metamorphic conditions during entrainment in an accretionary complex at approx. 90-100 Ma. Higher grade sequences developed at deeper levels along the hanging wall of the accretionary complex. During subsequent structural uplift to shallower crustal levels the Besshi nappe complex was tectonically emplaced over Oboke units. The Sambagawa metamorphic belt exposed in central Shikoku, Japan, is tectonostratigraphically represented by the Oboke and structurally overlying Besshi nappe complexes. Oboke units consist of metamorphic rocks largely derived from clastic protoliths (sandstone and conglomerate) whereas Besshi units are dominated by rocks with oceanic protolith affinities (shale, chert, and greenstone). Whole-rock schist samples from low-grade Oboke units record intermediate temperature, 40Ar/39Ar plateau ages of 70-77 Ma, which are interpreted to closely date attainment of maximum metamorphic conditions. Whole-rock schist samples from Besshi units (chlorite zone) record plateau ages of 85-94 Ma suggesting an earlier metamorphic climax within this structural level. Amphibole within the albite-biotite zone in Besshi records Ar-36/40Ar vs. 39Ar/40Ar plateau isotope correlation ages of 84-87 Ma, which are interpreted as dating the post-metamorphic cooling through temperatures required for intracrystalline retention of argon. A slightly older isotope correlation age (94 Ma) is recorded by amphibole from the higher grade oligioclase-biotite zone. Contrasts in amphibole cooling ages are interpreted as reflecting thrust-related metamorphic inversion and resultant earlier cooling of higher grade structural units. Muscovite from all structural levels of the Besshi nappe complex records 40Ar/39Ar plateau ages of 76-89 Ma. These are interpreted to date post-metamorphic cooling through appropriate closure temperatures following structural assembly of the Besshi nappe complex. Protoliths of the Besshi units originated at approx. 130 Ma and attained maximum metamorphic conditions during entrainment in an accretionary complex at approx. 90-100 Ma. Higher grade sequences developed at deeper levels along the hanging wall of the accretionary complex. During subsequent structural uplift to shallower crustal levels the Besshi nappe complex was tectonically emplaced over Oboke units.
