K-Ar age-chemistry-fabric relations of phengite from the Sanbagawa high-pressure schists, Japan

The Sanbagawa high-pressure schists from central Shikoku in Southwest Japan have experienced high-strain ductile deformation during exhumation and coo ling. This study examines the effects of high-strain ductile deformation on K-Ar ages of phengites on the basis of fabric, chemistry and K-Ar ages of phengites from the pelitic, psammitic and quartzose (or albitic) schists co llected from the same outcrop in the albite-biotite zone. Phengites in the pelitic and psammitic schists generally occur forming aggregates consisting of fine-grained phengite crystals and are extremely fine-grained in domain s close to relatively rigid garnet and albite porphyroblasts, indicating th at deformation-induced grain-size reduction had taken place in phengite dur ing the ductile deformation accompanying the exhumation of host schists. We suggest that the grain-size reduction of phengite is due to strain-induced recrystallization or dynamic recrystallization. The matrix phengites in sc hists are chemically heterogeneous on the thin-section scale but the phengi tes from pelitic and psammitic schists from the same outcrop have similar c hemical range. Phengite included in garnet has a high Si value and its Nai( Na + Ii) and Mg/(Mg + Fe) ratios are significantly low in comparison with t hose in matrix. The phengite included in garnet records the chemistry in eq uilibrium with other major silicate phases during the higher pressure stage of the P-T-t history of the schists. In contrast, the matrix phengites hav ing low Si values are likely to have been formed during retrograde meta mor phism in extremely restricted equilibrium domains. The two or three differe nt types of schists from the same outcrop, which have a similar grain size of phengite, have similar Ii-Ar ages, suggesting that the closure temperatu re does not depend on chemistry. However the hematite-rich quartzose schist with strong grain-size reduction of both phengite and quartz has a signifi cantly younger Ii-Ar phengite age than the pelitic and quartzose schists in the same outcrop that do not show grain-size reduction. We suggest that th e exhumation tectonics of the schists, which have experienced strong ductil e deformation at temperatures less than similar to 350 degrees C, played an important role resulting in the observed variation in age.