Isochemical metamorphism origin of the newly discovered Baqing jadeitoid, eastern-central Tibet, China

Abstract

The formation of most jadeitites and other jadeite-rich rocks (jadeitoids) during subduction is thought to occur by precipitation (P-type) or metasomatism (R-type) by infiltration of Na-Al-Si-rich aqueous fluids because of the compositional similarity of the rocks to inferred subduction fluids. Whether these rocks can form by isochemical metamorphism (I-type) during subduction is still hotly debated. A characteristic of I-type jadeitoid is that it exhibits a similar prograde metamorphic record as associated eclogite, in contrast to P- and R-type jadeitite and jadeitoids that are typically enclosed in serpentinite derived from the mantle wedge and either lack a prograde metamorphic history (R-type and P-type) or probably experience a prograde history (R-type) that is difficult to discern owing to the high variance of the jadeite-dominated assemblages and alteration by subduction fluids. The recently discovered Baqing (eastern-central Tibet) jadeitoid is enclosed by quartzo-feldspathic schist and has a peak metamorphic assemblage of almandine + jadeite/omphacite + phengite/paragonite + rutile + quartz, similar to eclogite. Abundant mineral inclusions in almandine, especially rutile inclusions with increasing Zr contents from the core to rim of almandine, provide an opportunity to further decode the jadeitoid-forming processes. In this study, pseudosections and Zr-in-rutile thermometry, together with conventional geothermobarometers, were employed to decipher the metamorphic history of Baqing jadeitoids. Two analysed Baqing jadeitoids exhibit a similar clockwise P–T path, starting from early metamorphic conditions of 5–7 kbar, 350–440°C, to different peak conditions (27–29 kbar, 730–760°C, or 20–23 kbar, 670–710°C), followed by relatively consistent retrograde metamorphic conditions of 6–7 kbar, 530–600°C. This result indicates a similar subduction history to the Baqing eclogite. In addition, the Baqing jadeitoids show similar geochemical characteristics to some Na-rich, K-depleted and Ca-depleted sedimentary rocks or plagiogranite. Therefore, we propose an isochemical genesis for the Baqing jadeitoid, rather than a metasomatic origin.