Jadeite- and dolomite-bearing coesite eclogite from western Tianshan, NW China

Abstract

Jadeite associated with omphacite has been reported from high-pressure, low-temperature (H P -L T ) rocks worldwide. However, the upper pressure stability of this association is not clear in nature. This study describes an immiscible texture of clinopyroxenes at ultrahigh-pressure (UH P )-L T conditions from western Tianshan, NW China. The host eclogite mainly consists of quartz, garnet, carbonate, albite, omphacite, white mica and Na-Ca amphibole. The modal contents of constituent minerals vary from place to place, and quartz-rich and carbonate-rich domains are developed. Jadeite (Jd 70–91 Aeg 2–16 ) only occurs as inclusion in garnet, mostly accompanied by patchy omphacite. Micro-Raman analyses show that jadeite and omphacite have different diagnostic peaks, consistent with the quantitative microprobe results. Jadeite is locally replaced by secondary sodic phases like paragonite and albite. A composite inclusion occurs in garnet and consists of lath-like paragonite, quartz, omphacite and titanite, with or without glaucophane. Matrix porphyroblastic albite usually encloses paragonite, titanite and barroisite, with or without omphacite, constituting a multi-phase aggregate. These two types of sodic phase-rich aggregates within and outside garnet are interpreted to be pseudomorphic replacements after jadeite at different conditions during retrogression. In addition to coesite in garnet, inclusions of coesite and its pseudomorphs without radial fractures were found in ankeritic dolomite. This first reported paragenesis of dolomite + coesite at low temperatures suggests that dolomite can be stabilized at depths of up to 90 km in a cold subduction zone. This contribution not only reports a new occurrence of Tianshan UH P rocks, but also suggests that UH P -L T metamorphism has impacted a variety of lithologies in addition to paraschist and basaltic eclogite, thus supporting the internal coherence of the UH P terrain of the Chinese western Tianshan.