DETERMINATION OF PB-207 PB-206 AGES ON ZIRCON AND MONAZITE BY LASER-ABLATION ICPMS AND APPLICATION TO A STUDY OF SEDIMENTARY PROVENANCE ANDMETAMORPHISM IN SOUTHEASTERN BRAZIL/

The recently developed method of laser ablation-inductively coupled pl asma mass spectrometry (LA-ICPMS) allows the determination of Pb-207/P b-206 ages of single zircon grains. The main advantages of the method are minimal sample preparation, low cost, and high throughput. In this work we present an analytical routine for geochronological analyses o f zircon and monazite by LA-ICPMS and its application to the Ribeira B elt of the Brazilian Orogen in southeastern Brazil. The Pb-207/Pb-206 ages of one hundred and thirty-seven detrital zircons from amphibolite facies quartzites from three lithotectonic domains in the central Rib eira Belt indicate that they are derived mainly from Paleoproterozoic crust of Transamazonian age (2.0-2.3 Ga). A small number of zircons or iginated in 2.6-2.9 Ga Archean crust. These results are coherent with 2.1-2.2 Ga and 2.6-3.0 Ga U-Pb ages previously obtained for basement g neisses. The viability of the method to date monazite is also assessed . Monazites from the same quartzite samples yield ages between 2.1 Ga and 0.57 Ga indicating variable resetting of the U-Pb system during am phibolite facies metamorphism. In contrast, monazite from a basement m igmatite and syn-metamorphic granitoids yields ages in the 500-700 Ma range, in general agreement with U-Pb ages of 590-565 Ma for the main metamorphic event. The LA-ICPMS Pb-207/Pb-206 ages are coherent and ag ree with expected results based on previous U-Pb geochronology, and sh ow that the method has immediate applicability. At present, the most s ignificant limitations of the method are the inability to yield reliab le U/Pb values, analytical precision in the 1-10% range, and the requi rement of grains larger than 80 mu m. The method may be advantageous f or provenance studies of Precambrian detrital sequences.