Geochemistry of komatiites and basalts from the Rio das Velhas and Pitangui greenstone belts, Sao Francisco Craton, Brazil: Implications for the origin, evolution, and tectonic setting

Author(s)

Verma, Sanjeet K. ; Oliveira, Elson P. ; Silva, Paola M. ; Moreno, Juan A. ; Amaral, Wagner S.

• Document type: Journal article
• Source: LITHOS; v. 284, p. 560-577, JUL 2017.
• Web of Science Citations: 4

Abstract

The Neoarchean Rio das Velhas and Pitangui greenstone belts are situated in the southern Sao Francisco Craton, Minas Gerais, Brazil. These greenstone belts were formed between ca. 2.79-2.73 Ga, and consist mostly of mafic to ultramafic volcanics and clastic sediments, with minor chemical sediments and felsic volcanics that were metamorphosed under greenschist fades. Komatiites are found only in the Rio das Velhas greenstone belt, which is composed of high-MgO volcanic rocks that have been identified as komatiites and high-Mg basalts, based on their distinctive geochemical characteristics. The Rio das Velhas komatiites are composed of tremolite + actinolite + serpentine + albite with a relict spinifex-texture. The Rio das Velhas komatiites have a high magnesium content ((Mg0)(adj) >= 28 wt.%), an Al-undepleted Munro-type {[}(Al2O3/TiO2)adj and (CaO/Al2O3)(adj)] ratio ranging from 27 to 47 and 0.48 to 0.89, relatively low abundances of incompatible elements, a depletion of light rare earth elements (LREE), a pattern of non-fractionated heavy rare- earth elements (HREE), and a low (Gd/Yb)pm ratio (<= 1.0). Negative Ce anomalies suggest that alteration occurred during greenschist facies metamorphism for the komatiites and high-Mg basalts. The low {[}(Gd/Yb)(pm) <1.0] and {[}(CaO/Al2O3)(adj) <0.9)], high {[}Al2O3/TiO2)(adj)>18] and high HREE, Y, and Zr content suggest that the Rio das Velhas komatiites were derived from the shallow upper mantle without garnet involvement in the residue. The chemical compositions {[}(Al2O3/TiO2)(adj), (FeO)(adj), (Mg0)adj, (CaO/Al2O3)adj, Na, Th, Ta, Ni, Cr, Zr, Y, Hf, and REE] indicate that the formation of the komatiites, high-Mg basalts and basalts occurred at different depths and temperatures in a heterogeneous mantle. The komatiites and high -Mg basalts melted at liquidus temperatures of similar to 1450-1550 degrees C. The Pitangui basalts are enriched in the highly incompatible LILE (large-ion lithophile elements) relative to the moderately incompatible HFS (high field strength) elements. The Zr/Th ratio ranging from 76 to 213 and the relationship between the Nb/Th and Th/Yb ratios indicate that there is no crustal contamination in the Pitangui greenstone basalts. New multi-dimensional discrimination diagrams and conventional normalized multi-element diagrams indicate an island arc (IA) setting for the komatiites and high -Mg basalts from the Rio das Velhas and a mid ocean-ridge (MOR) to IA setting for the basalts from the Pitangui greenstone belts. (C) 2017 Elsevier B.V. All rights reserved. (AU)

• FAPESP's process: 12/15824-6 - Evolution of archaean terranes of the São Francisco Craton and the Borborema Province, Brazil: global environmental and geodynamic implications
• Grantee: Elson Paiva de Oliveira
• Support Opportunities: Research Projects - Thematic Grants
• FAPESP's process: 14/04920-0 - Geological and geochemical investigations on archaen granite-gneiss terranes of São Francisco Craton
• Grantee: Juan Antonio Moreno Moreno
• Support Opportunities: Scholarships in Brazil - Post-Doctoral
• FAPESP's process: 12/07243-3 - Application of new multi-dimensional discrimination diagrams and geochemical modeling to Archaean igneous rocks of Brazilian cratons
• Grantee: Sanjeet Kumar Verma
• Support Opportunities: Scholarships in Brazil - Post-Doctoral