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dc.contributor.authorSantos, Caio A.
dc.contributor.authorWhite, Richard W.
dc.contributor.authorMoraes, Renato
dc.contributor.authorSzabó, Gergely A.J.
dc.identifier.citationSantos , C A , White , R W , Moraes , R & Szabó , G A J 2020 , ' The gabbro to amphibolite transition along a hydration front ' , Journal of Metamorphic Geology , vol. Early View .
dc.identifier.otherPURE: 271572282
dc.identifier.otherPURE UUID: 02e9d579-9c1b-4a5b-a50c-70c9a3e63403
dc.identifier.otherRIS: urn:917432F7E45C6478A9FF023D57377340
dc.identifier.otherORCID: /0000-0002-5270-3985/work/86987231
dc.identifier.otherWOS: 000598654200001
dc.identifier.otherScopus: 85104558856
dc.descriptionThis study was supported by FAPESP (processes nº 13/04007-0 and 16/22627-3). C. A. Santos received a PhD scholarship from CNPq.en
dc.description.abstractIn most metamorphic terrains the transition to the preserved mineral assemblage can be established to have occurred during progressive metamorphism through lower-grade facies involving fluid saturated conditions. However, in some circumstances metamorphism can occur in a non-progressive way as a distinct overprint of either igneous protoliths such as gabbro, or of earlier higher-grade metamorphic rocks such as granulites. In this contribution we use thermodynamic modelling, together with petrographic and mineral chemistry analysis, to study a suite of amphibolite facies metamafic rocks that were variably affected by metamorphism, with the objective of explaining the textures and mineral assemblages observed. The study samples form a continuum in terms of textural relationships and mineral compositions, but they can be divided into three main types: metagabbro, lineated metagabbro and amphibolite. Metagabbro samples preserve both relict igneous and metamorphic textures and a combination of igneous and metamorphic mineralogy. They preserve a variety of reaction textures, such as coronas of garnet around ilmenite, double coronas of low-Al (actinolitic) and high-Al (pargasitic) hornblende around clinopyroxene and limited degrees of recrystallization of igneous plagioclase. Lineated metagabbro samples preserve similar reaction textures as the metagabbro but show a strong lineation and have higher proportions of pargasitic hornblende and polygonal plagioclase combined with lower proportions of clinopyroxene, igneous plagioclase and actinolitic hornblende. Amphibolite samples are composed of pargasitic hornblende and polygonal plagioclase, presenting little igneous plagioclase and practically no garnet and clinopyroxene, and they preserve none of the reaction textures of the other two types. These samples are interpreted to be near chemically equilibrated on a hand sample scale, though with some mineral zoning preserved. P-XH2O and T-XH2O pseudosections show that the main mineral assemblage differences between the amphibolites and the other two types are primarily controlled by the amount of H2O that equilibrated with the rocks. While all rocks experienced an influx of fluid, only the amphibolites represent thermodynamically fluid-saturated conditions, the metagabbro and lineated metagabbro having equilibrated under H2O undersaturated conditions, allowing the presence of garnet. In parallel with the variation in fluid influx, the variation in microstructure appears to be the result of varying length scales of diffusion. The complex corona structures in the metagabbros are interpreted to reflect the pre-existing igneous texture and the apparent low mobility of Al during metamorphism. Here, igneous clinopyroxene is largely pseudomorphed by the actinolitic hornblende whereas the higher-Al pargasitic hornblende and garnet occur adjacent to plagioclase or replacing it. Zoning patterns in amphibole crystals in the amphibolite are similar to those in metagabbro samples, which is consistent with earlier stages of the evolution of the amphibolite actually being similar to those preserved in the metagabbro.
dc.relation.ispartofJournal of Metamorphic Geologyen
dc.rightsCopyright © 2020 John Wiley & Sons Ltd. This work has been made available online in accordance with publisher policies or with permission. Permission for further reuse of this content should be sought from the publisher or the rights holder. This is the author created accepted manuscript following peer review and may differ slightly from the final published version. The final published version of this work is available at
dc.subjectReaction texturesen
dc.subjectQE Geologyen
dc.titleThe gabbro to amphibolite transition along a hydration fronten
dc.typeJournal articleen
dc.contributor.institutionUniversity of St Andrews.School of Earth & Environmental Sciencesen
dc.description.statusPeer revieweden

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