Metamorphic evolution of stilpnomelane-bearing felsic schists from the subducted complex of southwestern Tianshan, China

The ultrahigh-pressure (UHP) metamorphic belt in the southwestern Tianshan, NW China is characterized by voluminous pelitic-felsic schists with scattered eclogite blocks. Stilpnomelane is abundant in some of the Tian- shan felsic schists, which is typical of low-temperature metamorphism as either a peak blueschist facies phase or a retrograde phase; its formation conditions and P-T path of the host rocks provide important constraints regarding the internal structure of the Tianshan UHP metamorphic belt. Here we report a detailed petrological study of two types of stilpnomelane-bearing felsic schists classified based on mineral modes and microtextures. Although similar in mineral species, that is, dominated by albite, quartz and chlorite, modes and textures of garnet (variably chloritized), amphibole and white mica are different. Porphyroblastic garnet includes jadeite, chloritoid, glaucophane, and lawsonite (pseudomorphed by paragonite and clinozoisite), a well-constrained HP - LT assemblage formed at-24 kbar and 430 degrees C corresponding to a geotherm of-5 degrees C/km. P-T pseudosection modeling shows that peak metamorphic conditions of 26-31 kbar and 470-560 degrees C by contouring garnet com- positions, consistent with the observed inclusion assemblage in garnet and the presence of coesite in adjacent rocks. The similarity in observed peak mineral assemblage and P-T conditions between the stilpnomelane- bearing felsic schists and reported Tianshan UHP rocks suggests their common metamorphic history and thus supports the internal coherence of the Tianshan UHP metamorphic belt. Stilpnomelane usually intergrown with chlorite occurs along the rim and cleavage of matrix phengite, pseudomorphing porphyroblastic garnet and glaucophane (overgrown by barroisite), or within and between albite porphyroblasts. Such textures of stilpno- melane clearly suggest lower greenschist facies conditions, consistent with the results (3-6 kbar and 280-340 degrees C) by conventional thermobarometer. Various hydration reactions are proposed to interpret domainal-scale pseu- domorphing processes, which involve significant losses or gains of major elements, such as Si, Fe, Mg, Ca, Na and Al. Since their whole-rock compositions are comparable to the stilpnomelane-free fresh felsic schists, we suggest negligible lengthy mass transport during stilpnomelane formation and thus a nearly closed system at least on a hand specimen scale.