K-Ar ages of pelagic claystones near P-T boundary from Jurassic-earliest Cretaceous accretionary complexes in Gifu and Okinawa areas, Japan

Abstract

K-Ar ages of illite rich fraction have been determined for pelagic claystone near the Permian-Triassic boundary from the Jurassic-earliest Cretaceous accretionary complexes in Gifu and Okinawa areas, Japan. The Early-Middle Triassic pelagic claystones give 181.5±3.8 Ma, 184.4±3.9 Ma, and 190.2±4.1 Ma (Early Jurassic), whereas the latest Permian pelagic claystone, 233.4±4.9 Ma (Middle Triassic). These ages are younger than the depositional ages of the protoliths and older than the accretion ages of the complexes. Time-lags between the depositional age and K-Ar age are ca. 15 m.y. for the latest Permian claystone and ca. 50-60 m.y. for the Early-Middle Triassic claystones. These results suggest that K-Ar ages represent average ages of K-bearing clay minerals such as illite and/or illite-smectite mixed layer which formed during illitization in the process of diagenesis on an oceanic crust and subsequent regional metamorphism within a subduction zone. The age relation between depositional age and K-Ar age depends on a mixing of older and younger illite which formed during illitization, and a partial Ar depletion from illite during regional metamorphism.