Metamorphic evolution of the Kamuikotan high-pressure and low-temperature metamorphic rocks in central Hokkaido, Japan

The Kamuikotan complex in the Sorachi-Yezo belt, located in central Hokkaido, Japan, has been extensively subjected to subduction-related high-pressure and low-temperature metamorphism. On the basis of geological, petrological, and geochronological data, the coherent metamorphic rocks are divided into six units as follows: Susunai, Horokanai, Harushinai, Biei, Pankehoronai, and Shizunai. Based on their metamorphic mineral sequences, the six units can be classified into the following three baric types: high-pressure 1 (HP1), high-pressure 2 (HP2), and high-pressure intermediate (HI). The Horokanai and Shizunai units belong to the HP1 type, the Harushinai and Biei units belong to the HP2 type, and the Susunai and Pankehoronai units belong to the HI type. Metabasites of the HP1 type are defined by the association of lawsonite + Na amphibole or epidote + Na amphibole (glaucophane or crossite). The HP2 type is characterized by the assemblage of pumpellyite + Na pyroxene + chlorite, and the representative low-variance asssemblages are lawsonite + pumpellyite + Na pyroxene + chlorite and pumpellyite + Na pyroxene + Na amphibole + chlorite. In the HI type, the most common low-variance assemblage is pumpellyite + epidote + actinolite + chlorite. These results, together with the previously published K-Ar and ⁴⁰Ar-³⁹Ar ages, suggest that the Kamuikotan metamorphic rocks gradually decreased in pressure during the Cretaceous. This tendency indicates an increase in the Kamuikotan subduction geotherm during Early Cretaceous to early Eocene time. The increase in the paleogeotherm may result from the change of subduction rates and subducting slab ages.