pinel-group minerals as a record of magmatic and metamorphic processes: evidence from the highly altered Morro do Onca ultramafic suite, Sao Francisco Craton (Brazil

Author(s)

Guice, George L. ^[1] ; Magalhaes, Joana Reis ^[2] ; Piacentini Pinheiro, Marco Aurelio ^[2] ; Ramos Ferreira, Raianny Carolini ^[2] ; Meira, Vinicius Tieppo ^[3] ; Melo-Silva, Paola ^[4] ; Ackerson, Michael R. ^[1]

Affiliation

^[1] Smithsonian Inst, Dept Mineral Sci, Natl Museum Nat Hist, 10th St & Constitut Ave NW, Washington, DC 20560 - USA

^[2] Geol Survey Brazil CPRM, Av Brazil 1731, BR-30140002 Belo Horizonte, MG - Brazil

^[3] Univ Estadual Campinas, Dept Geol & Nat Resources, Inst Geosci, BR-13083855 Campinas, SP - Brazil

^[4] Univ Sao Paulo, Inst Geosci, Rua Lago 562, BR-05508080 Butanta, SP - Brazil

• Document type: Journal article
• Source: CONTRIBUTIONS TO MINERALOGY AND PETROLOGY; v. 177, n. 3 MAR 2022.
• Web of Science Citations: 0

Abstract

The Morro do Onca suite is a package of ultramafic rocks in the southern Sao Francisco Craton, Brazil. Owing to the complete alteration of primary silicate minerals, the Morro do Onca suite has been the subject of diverging interpretations, including: part of a retrogressed eclogite-facies Paleoproterozoic ophiolite/accretionary wedge or an Archean komatiite flow(s). In this paper, we utilize spinel-group mineral chemistry-alongside field mapping, petrography, silicate mineral chemistry and bulk-rock geochemistry-to decipher the magmatic and metamorphic evolution of the Morro do Onca suite. A komatiite origin is supported by the identification of spinifex-textured layers, as well as primary spinel-group mineral compositions (mean Cr-number = 79; mean TiO2 = 0.3 wt%) and chondrite-normalized bulk-rock rare-earth element values ({[}Sm-Lu](N) = 1.2-5.1). Metamorphism reached mid/upper amphibolite-facies and was likely responsible for Al mobility on mineral and bulk-rock scales, demonstrating that the bulk-rock Al/Ti proxy commonly used to classify komatiites is susceptible to alteration. Thus, the Morro do Onca komatiites record variable bulk-rock Al/Ti values (17-47) that overlap with both Munro- (Al/Ti = 22) and Weltevreden-type (Al/Ti = 30) komatiites. Based on immobile trace-element ratios ({[}Gd/Lu](N) = 0.6-2.0) and primary spinel-group mineral compositions, we classify the Morro do Onca komatiites as Weltevreden-type. Mesoarchean komatiites related to mantle plume-magmatism are now recognized throughout the southern Sao Francisco Craton, potentially suggesting that its precursor lithosphere was continuous during this Era. (AU)

• FAPESP's process: 21/00967-5 - Shining a light on ancient mountains with petrochronology and synchrotron
• Grantee: Vinícius Tieppo Meira
• Support Opportunities: Regular Research Grants