A revisit to the Yorii jadeite–quartz rock, the Kanto Mountains, central Japan: Implications for petrogenesis

Abstract

The jadeite–quartz rock (or jadeitite) from the Yorii area, the Kanto Mountains, central Japan occurs in greenstone mélange subjected to metamorphism related to the Sanbagawa event. This rock was previously suggested to have formed through “vein precipitation” process contemporaneous with the Jurassic accretionary event. Mineral inclusions in zircons from this jadeite–quartz rock were re-examined in this study. The results clearly showed that these zircons contain mineral inclusions of both primary (igneous) and secondary (metasomatic) origin. The former are not present in matrix of the jadeite–quartz rock and the latter are pseudo-inclusions. These zircons are thus inherited or incompletely recrystallized ones. The geochemical characteristics of these zircons, including Th/U ratio and Ce anomaly, are also in accord with this conclusion. The jadeite–quartz rock should therefore have formed through “wholesale metasomatic replacement” process at an age younger than 141Ma from a protolith of probable igneous origin aged at 162.2±0.6Ma.Based on the available data, a few criteria, including texture, mineral inclusion, Th/U ratio and Ce anomaly of zircon, were proposed to categorize zircons in jadeitite into inherited, incompletely recrystallized and metasomatic ones. The results may be used to infer the formation mechanism of the hosting jadeitite. Some possible ambiguities in applying the proposed criteria were discussed. Following the proposition, it was shown that previous genetic conclusions for some jadeitites from various occurrences around the world would be subjected to question and should be re-examined by careful investigation of mineral inclusions in zircons, similar to this study, in the future.