High-T, low-P formation of rare olivine-bearing symplectites invariscan eclogite

Abstract

Extremely rare veinlets and reaction textures composed of symplectites of olivine (Fo43-55)plagioclase±spinel±ilmenite, associated with more common pyroxenepplagioclase and amphibolepplagioclase varieties, are preserved within eclogites and garnet pyroxenites in theMoldanubian Zone of the BohemianMassif.Thermodynamic modelling integrated with conventional geothermometry conducted on an eclogite reveals that the symplectite-forming stage occurred at highT (~850°C) and low P (<6 and >2·5 kbar). The development of the different symplectite types reflects reactions that took place in micro-scale domains.The breakdown of high-P garnet controlled the formation of olivine-bearing and amphibolepplagioclase symplectites, whereas breakdown of high-P omphacite led to formation of pyroxenepplagioclase symplectites. In addition, post-eclogite facies but pre-symplectite stage porphyroblastic amphibole and phlogopite were also replaced by olivine-bearing symplectites. Material transfer calculations and thermodynamic modelling indicate that the formation of different symplectite types was linked despite their different bulk compositions. For example, the olivine-bearing symplectites gained Fe±Mg, whereas adjacent amphibolepplagioclase and pyroxenepplagioclase symplectites show losses in Fe and Mg; Al, Si and Ca were also variably exchanged. The olivine-bearing symplectites were particularly sensitive to Na despite the small concentration of this element. In eclogites where Na was readily available, the plagioclase composition in the olivine-bearing symplectites shifted from pure anorthite to bytownite, with the less calcic feldspar partitioning Si and inhibiting the formation of orthopyroxene. This regional high-T, low-P granulite-facies symplectite overprint may have been caused by advective heat loss from rapidly exhumed high-T, high-P granulitic bodies (Gföhl Unit) that were emplaced into and over the middle crust (Monotonous and Varied Series) during Carboniferous continent-continent collision. © The Author 2013. Published by Oxford University Press. All rights reserved.