Geochemistry and Isotopic Age of Zircons from Rocks of Ultramafic Massifs in the Southern Folded Framing of the São Francisco Craton (Southeastern Brazil)

Abstract

Abstract —We present the first data on the geochemistry and isotopic age of zircons from rocks of the Roseta (lherzolite and olivine orthopyroxenite) and Córrego da Areia (olivine orthopyroxenite) ultramafic massifs localized among Archean–Proterozoic metamorphic strata in the southern folded framing of the São Francisco Craton. The examined zircons differ in crystal morphology, the intensity and type of cathodoluminescence, oscillatory zoning, the distribution of trace elements, and U–Pb isotopic age. Zircon crystals from the Roseta lherzolite have an age of 800-728 Ma and show negligible cathodoluminescence. Some of them have reaction rims with an age of 663–619 Ma. The crystal cores are enriched in rare earth elements (REE) but are often depleted in U as compared with the reaction rims. The age of zircons from the Roseta olivine orthopyroxenite showing moderate cathodoluminescence is within 622.6–596.9 Ma. Zircons from the Córrego da Areia olivine orthopyroxenite show moderate to intense cathodoluminescence and often have a rhythmic oscillatory zoning. They are characterized by an uneven distribution of REE and other trace elements and have high contents of U and Th. Most of these zircons have an ancient isotopic age (2558–2100 Ma). The examined zircons are of several types: relict, xenogenic, syngenetic, and epigenetic. The Roseta and Córrego da Areia massifs are considered to be protrusive outliers of a deep-seated Precambrian mafic–ultramafic complex, which were tectonically transferred along faults into the intensely metamorphosed Archean–Proterozoic strata in the southern folded framing of the São Francisco Craton.