Timing and petrogenesis of metamafic-ultramafic rocks in the Southern Brasília orogen: Insights for a Rhyacian multi-system suprasubduction zone in the São Francisco paleocontinent (SE-Brazil)

• Published in: Precambrian research Vol. 321; pp. 328 - 348
• Main Authors: Pinheiro, M.A.P., Suita, M.T.F., Lesnov, F.P., Tedeschi, M., Silva, L.C., Medvedev, N.S., Korolyuk, V.N., Pinto, C.P., Sergeev, S.A
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 01-02-2019
• Subjects: N-MORB amphibolite; Southern Brasília orogen; Suprasubduction zone; São Francisco paleocontinent; U-Pb (SHRIMP) geochronology; São Francisco paleocontinent; U-Pb (SHRIMP) geochronology; N-MORB amphibolite; Southern Brasília orogen; Suprasubduction zone
• ISSN: 0301-9268; 1872-7433
• DOI: 10.1016/j.precamres.2018.12.006

•Rhyacian multi-system SSZ in the southern São Francisco paleocontinent.•Rhyacian metabasalts interleaved in Neoproterozoic and Paleoproterozoic units.•Hybrid fractionated metaultrabasic rock records Rhyacian crystallization in a SSZ.•Metabasalts in the Southern Brasília orogen record N and E-MORB signatures.
A systematic investigation of amphibolites and a metaultramafite granofels was carried out in the southernmost Brasília Orogen, part of the reworked margin of the São Francisco paleocontinent. These rocks occur interleaved within Neoproterozoic metasediments and the Archean-Paleoproterozoic basement. The amphibolites from the Andrelândia, Liberdade and the Carrancas nappe system show an N-MORB signature, enriched to slightly depleted in LILE (Large Ion Lithophile Elements) and LREE (Light Rare Earth Elements) and depletion in the HFSE. They define a flat sub primitive mantle (PM) normalized curve with negative anomalies of Nb and Ce. The Lima Duarte nappe amphibolite has an andesitic basalt with an E-MORB signature and magmatic arc filiation. U-Pb (SHRIMP) zircon dating on the different samples yielded similar crystallization ages of ca. 2.15 Ga. Metamorphic conditions record amphibolitic facies for all the samples and can be linked to lower intercepts aged between 560 and 590 Ma. Whole rock Nd isotopic data suggest a juvenile source with TDM ages between 1.95 and 2.22 Ga. The associated metaultramafite, from Carrancas Klippe, highlights inherited cryptic layering and fractional crystallization processes from mineral and whole rock chemistry. It is enriched in LILE and shows negative anomalies of Th and Nb and positive anomalies of Sr and Zr. The Platinum group elements present a total PGE abundance of 16.5 and 30.8 ppb. The Iridium group defines a steep positive normalized curve while the Palladium group defines a gentle positive one, as expected for differentiated liquids. U-Pb zircon dating yielded three data clusters: i) the ca. 2.5 Ga, interpreted as inheritance, ii) the ca. 2.1 Ga, considered as the crystallization age and, iii) the youngest, being ca. 600 Ma, associated with the amphibolite facies metamorphic overprint. REE analyses on Rhyacian zircons suggest plagioclase crystallization under the influence of free-REE bearing minerals. A magmatic zircon grain containing a quartz inclusion, presents a distinctive REE pattern, with a positive normalized curve and a strong negative Eu anomaly. It suggests a hybridized crystallization under influence of plagioclase-rich and silica-saturated fluids prior or during the igneous crystallization, possibly due to the influence of slab derived materials. These data unravel new insights towards mafic-ultramafic rocks formation in different settings within a Rhyacian suprasubduction system in the southernmost part of the São Francisco paleocontinent, later reworked during the Gondwana amalgamation.