Jadeite-quartz-K-feldspar rocks in the Kamuikotan zone, Japan

抄録

Several tens of float blocks of jadeite-quartz-K-feldspar rocks were recently found in and around serpentinite masses in the Kamuikotan Gorge area of the central Kamuikotan zone, Japan. They consist mainly of jadeite (Jd_95-100), quartz, K-feldspar, phengite, and lawsonite. Two stages of metamorphism are recognized. The metamorphic conditions during the first and the second stages are estimated at about 250-350 °C and more than 1.1 GPa, and 200-300 °C and 0.4-0.7 GPa, respectively. The rocks are relatively fine-grained, acidic in composition, and characteristically have many jadeitized pseudomorphs of probable rapakivi feldspar, suggesting a hypabyssal fine-grained granite or granite porphyry origin. K-feldspar-bearing granitic rocks have never been reported from the Kamuikotan zone. The discovery of jadeite-quartz-K-feldspar rocks in the zone provides important information about the source of the metamorphic protoliths. The inference is that these materials formed by the activity of felsic magma, which existed near the trench during Late Jurassic to Cretaceous time.