Megacircones ediacáricos de la sienita nefelínica de San José del Guaviare y su potencial como material de referencia para datación U/Pb mediante LA-ICP-MS

Abstract

With the aim of expanding the reference materials of the Geochronology Laboratory of the Colombian Geological Survey (LG-SGC, in Spanish), zircons were collected from the nepheline syenite of San José del Guaviare, Jordán locality (Colombia). The SNG-1 sample was composed of 24 zircons selected from the fraction between 2 and 5 millimeters long, which were mounted and polished in an epoxy resin block. The zircons were dated by analyzing the isotopic ratios of uranium and lead using the LA-ICPMS method. In the LG-SGC, 92 points on the zircons were selected for analysis, and at the University of Rennes in France (LG-URF), 24 points from crystals found nearby were selected for analysis from a previous cathodoluminescence study. Concordant ages for the zircons for both laboratories corresponded to a crystallization age of 608 ± 1.2 Ma and 602 ± 3 Ma, respectively, which differ by less than 1% despite the difference in instrumentation and analytical procedures. For systematic error correction and quality control, international reference material, such as Plešovice, GJ-1, FC-5 Duluth and Mount Dromedary zircon, was used. The uncertainties generated during the analyses are similar (1.2 and 3.2 Ma), and the mean square-weighted deviation (MSWD) of the concordia diagrams was < 1 in both cases, indicating low dispersion of the results. Some discordant ages show that the U/Pb ratios are not uniform, as expected for a primary reference material. However, we believe that the zircons of the Jordán syenite have potential use as a secondary reference material for the LG-SGC provided they are carefully selected and prepared, their geochemical content (including rare earth elements, REE) is characterized, and quality monitoring of U/Pb dating is performed, as these zircons are large, reaching up to 4 cm, abundant, and possible to collect in the Colombian territory.