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

Author(s)

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]

Affiliation

^[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

• Document type: Journal article
• Source: LITHOS; v. 239, p. 33-44, DEC 15 2015.
• Web of Science Citations: 3

Abstract

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)

• FAPESP's process: 07/08683-9 - The meaning of felsic microgranular enclaves in the evolution of granitic magmas: petrology of enclaves in the Itapeti and Salto granites, State of São Paulo
• Grantee: Giovanna de Souza Pereira
• Support Opportunities: Scholarships in Brazil - Scientific Initiation
• FAPESP's process: 11/07074-4 - Combination of quantitative textural analyses and in situ chemical-isotopic techniques in the study of felsic microgranular enclaves: implications to the evolution of magmatic chambers
• Grantee: Adriana Alves
• Support Opportunities: Regular Research Grants