Abstract
Large zircon porphyroblasts (up to 3 mm) occur in jadeitite of the Osayama serpentinite melange (OSM), central Chugoku Mountains. Zircons coexist with jadeite, grossular and rutile, and they contain tiny inclusions of jadeite and rutile. Zircons analysed by SHRIMP-RG are characterized by low Th- and U-abundances (Th = 1-81 ppm; U = 6-149 ppm). Textural and geochemical features define three zones: normal growth zone with Th/U ratio of 0.2-0.5 (Type I), texturally inherited cores with Th/U ratio of 0.7-0.8 (Type II), and replacement zone (Type III). The ratio of common Pb to radiogenetic Pb is clearly high (10-26 %) in Type III zones, whereas other zones are less than 5.4 % (mostly < 0.5 %). The Type I and Type II zones yield a homogeneous age population with a weighted mean of 206Pb/238U spot ages of 476 ± 10 Ma (n = 34). Since zircon porphyroblasts contain high-P mineral inclusions, the U-Pb age represents the time of jadeitite formation, i.e., the timing of interaction between alkaline fluid and ultramafic rocks in a subduction-zone, and constraints the minimum age for serpentinization. The jadeitite formation is significantly older than phengite K-Ar ages of the OSM blueschists (273-327 Ma). This relationship suggests the jadeitite formation is not related to regional Late Paleozoic blueschist metamorphism. The jadeitite formation and serpentinization may instead be related to Early Paleozoic high-P epidote-amphibolites with hornblende K-Ar age of 403-443 Ma in a serpentinized peridotite body in the eastern Chugoku Mountains, about 180 km to the east of the OSM.
