PbPb, RbSr, and KAr systematics of the Lagoa Real uranium province (south-central Bahia, Brazil) and the Espinhaço cycle (ca. 1.5-1.0 Ga)

Abstract

Geochronologic investigations using PbPb, RbSr, and KAr systems have been carried out on the granite-orthogneiss complex of Lagoa Real, Bahia — one of Brazil's most important uranium provinces. RbSr whole rock data on the basement complex yielded an isochron age of about 2700 Ma. The age corresponds to the peak of the Jequié orogenic cycle, when the regional crustal province was probably consolidated. PbPb data for five undeformed examples of the São Timóteo granite and seven gneissic samples defined a well-fitted isochron with an age of 1710 ± 100 Ma. The same undeformed samples furnished a similar RbSr whole rock isochron age of 1710 ± 45 Ma. These ages agree with published UPb (zircon) dates, confirming that the intrusion age was around 1700–1720 Ma. The magmas were probably formed largely by melting of crustal components, as shown by ( 87Sr/ 86Sr) i and μ 1 values. In complete contrast to the PbPb system, the RbSr system was seriously affected by later events in the orthogneisses. A RbSr isochron apparent age of about 1500 Ma, obtained in one locality, is repeated in other isolated RbSr ages, and also by a UPb (zircon) age obtained in albitites within the region. Other RbSr apparent ages of ca. 1200 and ca. 1000 Ma were obtained, but their real geologic significance remains uncertain. KAr ages in separated minerals fall within the interval 570-500 Ma and show the influence of the Brasiliano tectonothermal cycle.