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Sub-solidus reactions and corona structures in the Niquelândia layered complex (Central Goiás, Brazil)

Subsolidus-Reaktionen und Coronastrukturen in dem geschichteten Komplex von Niquelândia (Central Goiás, Brasilien)

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Summary

Corona structures occur in the upper units (UGAZ and UA) of the Niquelândia mafic-ultramafic complex (Central Goiás, Brazil), where olivine gabbros, gabbros, anorthosites and “amphibolites” are interlayered. In olivine gabbros, where the corona structures are most complex, primary igneous minerals are olivine, plagioclase ± clinopyroxene and ilmenite. The instability between olivine and plagioclase results in three types of coronas:

  1. 1)

    ol + pl → opx + (cpx + sp)sympl

  2. 2)

    ol + pl → opx + (amph + sp)sympl

  3. 3)

    ol + pl → opx + amph + gar

In the second corona type the amphibole is pargasite; in the third type the amphibole approaches Mg-hornblende in composition. The occurrence of secondary amphibole indicates the presence of a fluid phase during re-equilibration.

Reaction calculations, obtained by means of a “MIXING” program, show that the mineralogical changes in the three corona types may reasonably have occurred in a closed system (with the exception of the fluid phase).

The chemical composition of the reacting igneous phases (especially the Mg/Fe2+ ratio of the mafic phases) constrains the composition of the products, but not the type of secondary assemblage. The nature of these products depends mainly on the variations ofaH2O, in relation to temperature decrease. In gabbros, anorthosites and “amphibolites”, where orthopyroxene and/or amphibole may be intercumulus phases, the following reactions occur between pyroxenes and plagioclase:

  1. 4)

    opx + cpx + pl → hbl

  2. 5)

    opx + cpx + pl → hbl + gar ± qz

  3. 6)

    opx + pl → cpx + gar + qz

In these rocks, garnet formation as a product in reaction (5) depends on the partial pressure of a vapour phase. Reaction (6) develops only when the orthopyroxene is very iron-rich.

Estimates of the re-equilibration pressure are of 5–8 kb; re-equilibration temperatures vary from about 800°C (anhydrous corona in olivine gabbros) to 560°C (plag + hbl + gar + qz secondary assemblages). Temperatures obtained from the hbl - gar geothermometer are directly correlated with the pargasitic component in secondary amphiboles.

The absence of deformation suggests that the coronas formed during slow cooling and not during a metamorphic event. The present data do not provide an answer to the problem of whether H2O has been introduced from the country rocks or is of igneous origin.

Zusammenfassung

Coronastrukturen kommen in den oberen Einheiten (UGAZ und UA) des mafisch-ultramafischen Komplexes von Niqueländia (Central Goiâs, Brasilien) dort vor, wo Olivin-Gabbros, Gabbros, Anorthosite und “Amphiboliten” in Wechsellagerung auftreten.

In den Olivin-Gabbros sind die Coronastrukturen besonders komplex. Die primären magmatischen Minerale sind hier Olivin, Plagioklas ± Klinopyroxen und Ilmenit. Die Instabilität zwischen Olivin und Plagioklas produziert drei verschiedene Arten von Coronen:

  1. 1)

    Ol + Pl → Opx + (Cpx + Sp)Sympl

  2. 2)

    Ol + Pl → Opx + (Amph + Sp)Sympl

  3. 3)

    Ol + Pl → Opx + Amph + Gr

Im Coronatyp (2) ist der Amphibol ein Pargasit, während seine Zusammensetzung im Typ (3) eher einer Mg-Hornblende entspricht. Das Vorkommen von sekundärem Amphibol deutet darauf hin, daß bei der Reequilibrierung eine fluid Phase mitgewirkt hat. Die mittels eines “MIXING”-Programmes erhaltenen Reaktionen sind mit der Vorstellung verträglich, daß sich die mineralogischen Umwandlungen in den drei Coronatypen in einem geschlossenen System abliefen (abgesehen von der fluiden Phase).

Die chemische Zusammensetzung der reagierenden Primärphasen (insbesondere das Mg/Fe2+-Verhältnis der mafischen Phasen) schränkt die Möglichkeiten für die Zusammensetzung der Reaktionsprodukte ein, bestimmt aber nicht ihren Typ. Der Typ ist in erster Linie durch Variationen vonaH2O in Relation zur Abkühlung bestimmt.

In Gabbros, Anorthositen und “Amphiboliten”, in denen es sich bei Orthopyroxen und/oder Amphibol möglicherweise um Interkumulus Phasen handelt, gibt es zwischen Pyroxen und Plagioklas die folgenden Reaktionen:

  1. 4)

    Opx + Cpx + Pl → Hbl

  2. 5)

    Opx + Cpx + Hbl + Gr ± Qz

  3. 6)

    Opx + Pl → Cpx + Gr + Qz

In der Reaktion (5) ist die Granatbildung durch den Partialdruck der Dampfphase bestimmt. Die Reaktion (6) spielt nur bei sehr eisenreichen Orthopyroxenen eine Rolle.

Eine Abschätzung des Reequilibrierungsdrucks ergibt 5–8 kb; Reequilibrierungs-temperaturen variieren zwischen 800°C (für wasserfreie Coronen in Olivingabbros) und 560°C (für sekundäre Paragenesen aus PI + Hbl + Gr + Qz). Die aus der Hbl - Gr Geothermometrie abgeleiteten Temperaturen sind mit der Pargasitkomponente im sekundären Amphibol korreliert.

Das Fehlen von Deformationsstrukturen zeigt, daß sich die Coronen während langsamer Abkühlung, nicht aber im Verlauf eines Metamorphose-Ereignisses ausgebildet haben. Die vorliegenden Daten liefern keine Antwort auf die Frage, ob das H2O aus den Nebengesteinen stammt oder magmatischen Ursprungs ist.

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Authors and Affiliations

  1. Instituto de Geociências, Universidade de São Paulo, Caixa Postal 20899, 01498, Sâo Paulo, Brazil

    Maria Angela Fornoni Candia

  2. Instituto di Mineralogsa e Petrologia, Universitá di Modena, Via S. Eufemia, 19, I-41100, Modena, Italy

    Maurizio Mazzucchelli

  3. Istituto di Scienze della Terra, Universitá di Catania, C. so Italia, 55, 1-95129, Catania, Italy

    Franca Siena

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  1. Maria Angela Fornoni Candia
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  2. Maurizio Mazzucchelli
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  3. Franca Siena
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Fornoni Candia, M.A., Mazzucchelli, M. & Siena, F. Sub-solidus reactions and corona structures in the Niquelândia layered complex (Central Goiás, Brazil). Mineralogy and Petrology 40, 17–37 (1989). https://doi.org/10.1007/BF01162467

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