First Data on the Isotopic Age of Zircons from Rocks of the Roseta Ultramafic Massif, Southern Margin of the São Francisco Craton (Brazil)

Resumo

The results of isotope U–Pb dating of zircons from lherzolite and vein olivine orthopyroxenite composing the Roseta ultramafic massif are presented. The zircons differ in their morphological, optical, and geochemical properties and in their age. Lherzolite samples that are almost deprived of cathodoluminescence are represented by two age clusters 788 ± 15 and 638.2 ± 5.4 Ma. The zircons from the first cluster are relict. A later event recorded in the zircons from the second cluster was manifested in an intense metamict mineral transformation. The age of the zircons from vein olivine orthopyroxenite conforms to 608.9 ± 3.4 Ma. These zircons are considered to be syngenetic; therefore, their age is determined by the time of crystallization of olivine orthopyroxenites.