Phase equilibria modelling using major and trace element compositions of zoned garnet and clinopyroxene from southwestern Tianshan eclogites, China

Abstract

We present major and trace element zoning in clinopyroxene and garnet in the low-temperature eclogites from southwestern Tianshan ultrahigh-pressure metamorphic belt. The clinopyroxene crystals in the matrix exhibit three distinct growth zones, including the aegirine-rich core, a transition mantle and the omphacite rim. The aegirine core has weak Ce anomaly, flat light rare earth element pattern, and slightly enriched heavy rare earth element contents. The omphacite rim has lower trace element contents, and is depleted in large-ion lithospheric elements, light and heavy rare earth elements. The garnet zonation is featured by the core-to-rim increase in pyrope and grossular contents, and distinct rare earth element patterns: the core has left-leaning rare earth element patterns and the rim is depleted in both light and heavy rare earth elements. Phase equilibria modelling based on the phase assemblage and mineral zoning patterns reveal that the aegirine eclogite was heated and buried along the ca. 8°C/km geotherm in the cold subduction zone, and reached the peak metamorphic condition of 23–25kbar, 540–560°C. The jadeite content in the clinopyroxene increases along the prograde-to-exhumation P-T path (Jd: from 0.25 to 0.45), with the transitional mantle corresponds to the peak conditions. The omphacite rim with the highest jadeite content (Jd∼45) formed during decompression. For a Fe3+-rich clinopyroxene, the maximum in Jd component does not necessarily represent the peak pressure, so the conventional thermobarometers should be used with caution for mineral assemblages with such complicated zonation patterns. The variation of oxygen fugacity correlates with the changes in mineral assemblage, P-T condition, and the bulk-rock Fe3+/FeT ratio. The oxygen fugacity is not always positively correlated with the Fe3+/(Fe3++Al) or Fe3+/FeT ratio in clinopyroxene. Rare earth element zoning garnet and clinopyroxene are modelled based on mass balance. The modelled growth zonation closely resembles the observed compositional profiles, suggesting that rare earth elements are passively distributed among phases near chemical equilibrium during the mineral growth.