Determination of Pb/Pb ages on zircon and monazite by laser-ablation ICPMS and application to a study of sedimentary provenance and metamorphism in southeastern Brazil

The recently developed method of laser ablation-inductively coupled plasma mass spectrometry (LA-ICPMS) allows the determination of ²⁰⁷Pb/²⁰⁶Pb 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 of zircon and monazite by LA-ICPMS and its application to the Ribeira Belt of the Brazilian Orogen in southeastern Brazil. The ²⁰⁷Pb/²⁰⁶Pb ages of one hundred and thirty-seven detrital zircons from amphibolite facies quartzites from three lithotectonic domains in the central Ribeira Belt indicate that they are derived mainly from Paleoproterozoic crust of Transamazonian age (2.0?2.3 Ga). A small number of zircons originated 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 gneisses. 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 amphibolite facies metamorphism. In contrast, monazite from a basement migmatite 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 ²⁰⁷Pb/²⁰⁶Pb ages are coherent and agree with expected results based on previous U---Pb geochronology, and show that the method has immediate applicability. At present, the most significant limitations of the method are the inability to yield reliable U/Pb values, analytical precision in the 1–10% range, and the requirement of grains larger than 80 gmm The method may be advantageous for provenance studies of Precambrian detrital sequences.