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In this study, we present an example of phase equilibrium modeling of medium-temperature–ultrahigh-pressure (MT–UHP) eclogites that includes consideration of the influence of ferric iron (O) and H2O on the phase equilibria. As a case study, we focus on the intergranular coesite-bearing eclogites at Yangkou in the Sulu Belt. Based on phase equilibrium modeling of four eclogites, we monitor changes in phase relations during deep subduction and exhumation, and investigate fluid behavior during decompression. To determine the appropriate O and H2O contents to use in calculating P–T pseudosections for these eclogites, we use an iterative process in which calculated temperature/pressure (T/P)–O/ H2O phase diagrams are combined with constraints from petrological observations. P–T pseudosections were calculated for each of the four eclogites to constrain the P–T conditions. The highest P–T conditions retrieved were P> 5 5 GPa at T>850 C, although variation in mineral compositions suggests that the maximum P–T conditions could have been higher. A P–T path was reconstructed based on microstructural evidence, mineral compositions that constrain P–T conditions within phase assemblage fields, average P calculations and mineral thermobarometry. During exhumation, the retrograde P–T path passed through metamorphic conditions of P1⁄4 4 0–3 4 GPa at T1⁄4 850–800 C and P1⁄42 4–1 7 GPa at T1⁄4 800–750 C, before reaching crustal levels at P1⁄4 1 3–0 9 GPa at T1⁄4 730–710 C. The prograde evolution is suggested to have followed a high dT/dP path during the early stage of subduction, followed by a low dT/dP segment to the metamorphic peak. During exhumation, the eclogites at Yangkou became domainal, made up of host-rock with low a(H2O) in which garnet and omphacite have partially reequilibrated and intergranular coesite has been preserved, cut by veins and veinlets where a(H2O) was higher and new mineral assemblages have developed. In the veins, the new assemblage comprises coarse phengite and quartz with symplectites of K-feldspar þ plagioclase þ biotite þ quartz around the phengite. By contrast, the veinlets comprise symplectites of hornblendeþ plagioclase 6 quartz 6 clinopyroxene after omphacite; similar symplectites occur at the edges of the phengite–quartz veins against host eclogite. We interpret the coarse phengite and quartz, which previously could have been coesite, to have formed by precipitation of solutes from fluid migrating under UHP conditions, whereas we interpret the symplectites around the phengite to have formed by local melting and crystallization during exhumation from HP eclogiteto HP amphibolite-facies conditions. The symplectites in the veinlets and along the edges of the phengite–quartz veins are interpreted to have formed by reaction of local grainboundary fluid with the host under HP amphibolite-facies conditions. VC The Author(s) 2018. Published by Oxford University Press. All rights reserved. For permissions, please e-mail: journals.permissions@oup.com 1253 J O U R N A L O F P E T R O L O G Y Journal of Petrology, 2018, Vol. 59, No. 7, 1253–1280 doi: 10.1093/petrology/egy060 Advance Access Publication Date: 11 June 2018

Phase Equilibrium Modeling of MT–UHP Eclogite: a Case Study of Coesite Eclogite at Yangkou Bay, Sulu Belt, Eastern China