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

Abstract

AbstractThe 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.