Corona structures occur in the upper units (UGAZ and UA) or the Niquelandia mafic-ultramafic complex (Central Goias, Brazil), where olivine gabbros,mgabbros, 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) ol + pl → opx + (cpx + sp)sympl2) ol + pl → opx + (amph + spsympl3) ol + pl → opx + amph + garIn the second corona type the amphibole is pargasite; in the third type the amphibole approaches Mg-hornblende in composition. The occurrence or 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 (especial1y the Mg/Fe2+ ratio or the mafic phases) constrains the composition or the products, but not the type of secondary assemblage. The nature of these products depends mainly on the variations of aH20, 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:4) opx + cpx + pl_→ hbl5) opx + cpx + pl → hbl + gar ± qz6) opx + pl → cpx + gar + qzIn 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-quilibration 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.

Sub-solidus Reactions and Corona Structures in the Niquelandia Layered Complex (Central Goias, Brazil) / Fornoni Candia, M. A.; Mazzucchelli, Maurizio; Siena, F.. - In: MINERALOGY AND PETROLOGY. - ISSN 0930-0708. - STAMPA. - 40:(1989), pp. 17-37.

Sub-solidus Reactions and Corona Structures in the Niquelandia Layered Complex (Central Goias, Brazil)

MAZZUCCHELLI, Maurizio;
1989

Abstract

Corona structures occur in the upper units (UGAZ and UA) or the Niquelandia mafic-ultramafic complex (Central Goias, Brazil), where olivine gabbros,mgabbros, 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) ol + pl → opx + (cpx + sp)sympl2) ol + pl → opx + (amph + spsympl3) ol + pl → opx + amph + garIn the second corona type the amphibole is pargasite; in the third type the amphibole approaches Mg-hornblende in composition. The occurrence or 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 (especial1y the Mg/Fe2+ ratio or the mafic phases) constrains the composition or the products, but not the type of secondary assemblage. The nature of these products depends mainly on the variations of aH20, 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:4) opx + cpx + pl_→ hbl5) opx + cpx + pl → hbl + gar ± qz6) opx + pl → cpx + gar + qzIn 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-quilibration 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.
1989
40
17
37
Sub-solidus Reactions and Corona Structures in the Niquelandia Layered Complex (Central Goias, Brazil) / Fornoni Candia, M. A.; Mazzucchelli, Maurizio; Siena, F.. - In: MINERALOGY AND PETROLOGY. - ISSN 0930-0708. - STAMPA. - 40:(1989), pp. 17-37.
Fornoni Candia, M. A.; Mazzucchelli, Maurizio; Siena, F.
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