R esumen Con el objetivo de ampliar el conjunto de materiales de referencia del Laboratorio de Geocronología del Servicio Geológico Colombiano (LG-SGC) se colectaron circones provenientes de la sienita nefelínica de San José del Guaviare, localidad del Jordán (Colombia). La muestra SNG-1 se compuso de 24 circones seleccionados de la fracción situada entre 2 y 5 milímetros de largo, montados y pulidos en un bloque de resina epóxica. La datación se realizó mediante análisis de relaciones isotópicas de uranio y plomo, utilizando el método LA-ICPMS. En el LG-SGC se seleccionaron 92 puntos sobre los circones y en la Universidad de Rennes, en Francia (LG-URF), se analizaron 24 puntos en áreas cercanas de los mismos cristales mediante estudio previo de catodoluminiscencia. Las edades concordantes obtenidas de los circones corresponden, en ambos laboratorios, a la edad de cristalización de 608 ± 1,2 Ma y 602 ± 3 Ma, respectivamente, que difieren en menos del 1 %, pese a la diferencia en instrumentación y las rutinas analíticas. Para corrección de errores sistemáticos y control de calidad se emplearon materiales de referencia internacionales, como circones de Plešovice, GJ-1, FC-5 Duluth y Mount Dromedary. Las incertidumbres producidas durante los

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U-Th-Pb geochronology of REE-bearing minerals such as monazite and other phosphates has become a precise tool to date mineralization events through geological time. In the E-Colombian Guainía department, from 3 colluvial sites over western Guiana Shield Mesoproterozoic rocks Nb-Ta, U-Th and REE bearing minerals probably of pegmatitic origin were sampled including up-to-0.5 cm long monazite crystals. The monazite pan concentrate samples from sites near to the indigenous communities of Barranquilla, San Jose and Chorro Bocón (Espina hills) along the Cuyari, Guainía, and Inírida rivers respectively, were analyzed geochemically and geochronologically using LA-ICP-MS and EPMA. The monazites obtained from the 3 sites are rather similar among each another and of approximate composition Ce0.44La0.16Nd0.16Sm004Y0.03Gd0.02Dy0.01Pr0.05Th0.08Pb0.01Ca0.05Si0.05P0.91O3.94.Their EPMA and LA-ICP-MS ages suggest at least two mineralization events, a first one between 1528 and 1523 Ma in the Barranquilla and San Jose sites, related to rapakivi granites outcropping along the Cuyarí River and the Tabaquen rapakivi granite near the Guainía River, both emplaced ~1545 Ma ago. The second mineralization event from 1390-1343 Ma recorded in the Espina-hills is in the age range of the Matraca and Danta rapakivi granites that crystalized between 1400 and 1340 Ma ago. These different anorogenic rapakivi granites were emplaced between 1550 and 1300 Ma into Mesoproterozoic basement rocks and were probably caused by intra-plate processes associated with mantle plume activity within the Columbia (Nuna) supercontinent.