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

Autor(es)

Alves, Adriana ^[1] ; Pereira, Giovanna de Souza ^[1] ; Janasi, Valdecir de Assis ^[1] ; Higgins, Michael ^[2] ; Polo, Liza Angelica ^[1] ; Juriaans, Orlando Stanley ^[3] ; Ribeiro, Bruno Vieira ^[1]

Afiliação do(s) autor(es)

^[1] Univ Sao Paulo, Dept Mineral & Geotecton, Inst Geociencias, BR-05508080 Sao Paulo - Brazil

^[2] Univ Quebec Chicoutimi, Sci Terre, Chicoutimi, PQ G7H 2B1 - Canada

^[3] Univ Sao Paulo, Inst Matemat & Estat, Dept Matemat, BR-05508090 Sao Paulo - Brazil

• Tipo de documento: Artigo Científico
• Fonte: LITHOS; v. 239, p. 33-44, DEC 15 2015.
• Citações Web of Science: 3

Resumo

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 from the Salto pluton, Itu Granitic Province, show that the cm-sized MMEs 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 contacts with 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, both MME and FME samples have increased FeO, MgO, TiO2, P2O5 and Zr contents, but their Sr and Nd isotope signatures are identical: FME Sr-87/Sr-86(i) = 0.7088-0.7063, epsilon Nd-i = -10.0 to -10.2; MME Sr-87/Sr-86(i) = 0.7070, epsilon Nd-i = -10.5; host granite Sr-87/Sr-86(i) 0.7056-0.7060, epsilon Nd-i = -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. (C) 2015 Elsevier B.V. All rights reserved. (AU)

• Processo FAPESP: 07/08683-9 - O significado dos enclaves microgranulares félsicos na evolução de magmas graníticos: petrologia de enclaves dos granitos Itapeti e Salto, SP
• Beneficiário: Giovanna de Souza Pereira
• Modalidade de apoio: Bolsas no Brasil - Iniciação Científica
• Processo FAPESP: 11/07074-4 - Uso combinado de técnicas de análise textural quantitativa e análises químico-isotópicas in situ no estudo de enclaves microgranulares félsicos: implicações para evolução de câmaras magmáticas
• Beneficiário: Adriana Alves
• Modalidade de apoio: Auxílio à Pesquisa - Regular