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The origin of felsic microgranitoid enclaves: Insights from plagioclase crystal size distributions and thermodynamic models

Alves Adriana, de Souza Pereira Giovanna, de Assis Janasi Valdecir, Higgins Michael D., Polo Liza Angelica, Stanley Juriaans Orlando et Vieira Ribeiro Bruno. (2015). The origin of felsic microgranitoid enclaves: Insights from plagioclase crystal size distributions and thermodynamic models. Lithos, 239, p. 33-44.

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Magma mixing is widely recognized in contemporary petrology as one of the primary igneous processes. Microgranitoid enclaves (MEs) are considered to be remnants of such mixing processes, and the term has a well-established genetic implication. However, microgranitoid enclaves span a wide range of compositions, and felsic varieties are also frequently reported. Nd–Sr isotope and textural data from felsic microgranitoid enclaves (FMEs), mafic microgranitoid enclaves (MMEs) and host granites fromthe Salto pluton, Itu Granitic Province, show that the cm-sizedMMEs are dioritic, have medium-grained igneous textures and xenocrysts of alkali feldspar and quartz. The FMEs are cm- to metersized, have spheric shapes, show corrugated contactswith the host granites, and have resorbed feldspars and deformed quartz crystals interpreted as xenocrysts set in a fine-grained groundmass. Compared to the host granites, bothMME and FMEsamples have increased FeO,MgO, TiO2, P2O5 and Zr contents, but their Sr and Nd isotope signatures are identical: FME 87Sr/86Sri =0.7088–0.7063, εNdi = −10.0 to −10.2; MME 87Sr/86Sri = 0.7070, εNdi = −10.5; host granite 87Sr/86Sri 0.7056–0.7060, εNdi = −10.2 to −10.3. These indicate that the enclaves derive from a similar source, although the melts from which they formed were probably hotter and chemically more primitive than their host granites. Crystal size distributions (CSDs) of plagioclase in samples drilled from rinds and cores of three FMEs show that the rind samples are systematically finer-grained than the samples from the cores, which indicates that the FMEs cooled inwards and contradict interpretations that the FMEs are autoliths. Thermal modeling suggests that a slightly more primitive, hotter magma would be thermally equilibrated with an evolved resident melt within weeks after mixing/mingling. Upon thermal equilibrium, the FMEs would have an increased crystal cargo, and the resulting touching framework would impart a solid-like behavior to the FME-forming magma, which would lead to a contrast in rheology, fragmentation, dragging and preservation of felsic replenishment batches as distinct enclaves.

Type de document:Article publié dans une revue avec comité d'évaluation
Pages:p. 33-44
Version évaluée par les pairs:Oui
Date:21 Octobre 2015
Sujets:Sciences naturelles et génie > Sciences naturelles > Sciences de la terre (géologie, géographie)
Département, module, service et unité de recherche:Départements et modules > Département des sciences appliquées > Unité d'enseignement en sciences de la Terre
Mots-clés:Magma mixing, felsic microgranular enclaves, chilled margins, plagioclase crystal size distribution, mélange de magma, enclaves microgranulaires felsiques, marges refroidies, distribution de la taille des cristaux de plagioclase
Déposé le:18 juin 2018 14:08
Dernière modification:18 juin 2018 14:08
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