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

JOURNAL OF Journal of Petrology, 2018, Vol. 59, No. 7, 1253–1280 PETROLOGY Advance Access Publication Date: 11 June 2018 Original Article doi: 10.1093/petrology/egy060 Phase Equilibrium Modeling of MT–UHP Eclogite: a Case Study of Coesite Eclogite at Yangkou Bay, Sulu Belt, Eastern China Bin Xia1,2*, Michael Brown2,3, Lu Wang1,3, Song-Jie Wang1,2,3,4 and Philip Piccoli2 1 School of Earth Sciences, State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Wuhan 430074, China; 2Department of Geology, Laboratory for Crustal Petrology, University of Maryland, College Park, MD 20742, USA; 3Center for Global Tectonics, China University of Geosciences, Wuhan 430074, China; 4College of Earth Science and Engineering, Shandong University of Science and Technology, Qingdao 266590, China *Corresponding author. Present address: College of Earth Sciences, State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Wuhan 430074, China. E-mail: Received December 3, 2016; Accepted May 28, 2018 ABSTRACT 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 > 55 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 P ¼ 40–34 GPa at T ¼ 850–800 C and P ¼ 24–17 GPa at T ¼ 800–750 C, before reaching crustal levels at P ¼ 13–09 GPa at T ¼ 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 hornblende þ 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 eclogite- to 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. C The Author(s) 2018. Published by Oxford University Press. All rights reserved. For permissions, please e-mail: V 1253 1254 Journal of Petrology, 2018, Vol. 59, No. 7 Key words: intergranular coesite; MT–UHP eclogite; phase equilibrium modeling; P–T path; Yangkou, Sulu Belt INTRODUCTION Ultrahigh-pressure (UHP) metamorphic rocks, particularly eclogite and associated country rock gneisses in orogenic belts, demonstrate that continental crust can be subducted to and returned from mantle depths (Chopin, 2003; Liou et al., 2004; Brown & Johnson, 2018). Pressure–temperature–time (P–T–t) paths tracing the deep subduction and exhumation of these UHP rocks provide insight into geological processes during continental collision at convergent plate boundaries and form the basis for geodynamic modeling of these processes (Gerya & Stockhert, 2006; Warren et al., 2008; Roda et al., 2012; Sizova et al., 2012). Thus, robust quantification of P–T–t paths from natural samples of UHP metamorphic rocks is important if our geodynamic modeling is to provide deeper understanding of processes during continental collision at convergent plate boundaries. Although the P–T conditions of UHP metamorphism can be qualitatively constrained by the presence of indicative UHP minerals such as coesite and diamond, the quantitative estimation of these conditions is undertaken using conventional thermobarometry (e.g. Krogh Ravna & Terry, 2004) and/or phase equilibrium modeling (e.g. Wei & Clarke, 2011; Wei et al., 2013). Forward modeling involves the calculation of phase equilibria for a given rock composition using an internally consistent thermodynamic dataset and appropriate activity– composition models for the phases of interest (Holland & Powell, 1998; Powell et al., 1998), which may then be related to the observed mineral assemblages, mineral proportions and mineral compositions for that particular sample. In addition, we may calculate phase equilibria for a representative composition (e.g. mid-ocean ridge basalt; MORB) to investigate how variables such as H2O content and oxidation state affect these equilibria (Rebay et al., 2010). During the last decade, phase equilibrium modeling has become the preferred thermobarometric method in many studies because it utilizes the maximum information available from the sample being studied, and, in many cases, allows the evolution of mineral assemblages to be quantified to determine a robust P–T path (Powell & Holland, 2008). This method has proven useful in the study of UHP eclogites, in part because some minerals, such as garnet and phengite, may retain prograde or peak stage compositional information that has allowed quantification of these P–T conditions as well as those recorded during exhumation (e.g. Wei et al., 2009, 2013; Massonne, 2011, 2012; Li et al., 2016b). One particular challenge in modeling UHP eclogites is the large P–T stability field of high-variance mineral assemblages at peak conditions [e.g. Grt þ Omp þ Coe 6 Ph 6 Ky; mineral abbrevi (...truncated)