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

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

Apatite LA-ICP-MS U–Pb and fission-track geochronology of the Caño Viejita gabbro in E-Colombia: Evidence for Grenvillian intraplate rifting and Jurassic exhumation in the NW Amazonian Craton

“…About 200km to the East of our study area, the youngest known intracratonic magmatic rocks in the NW Amazonian Craton, the Nepheline Syenite of San José del Guaviare, of Neoproterozoic-Ediacaran to Cambrian ages of ~578 Ma (Mejia et al, 2012) and biotite K/Ar and Rb/Sr cooling ages of 445 -495 Ma (Pinson et al, 1962) outcrops over > 10 square kilometers. Recently obtained own LA-ICP-MS U-Pb zircon ages of ~609 Ma in the southern extension near Jordan (Franco et al, 2018;Muñoz Rocha et al, 2019) suggest a long emplacement and cooling history for this large body. Further west, no intracratonic intrusions are reported until now.…”

“…The youngest known intracratonic magmatic episode in the NW Amazonian Craton is a riftrelated alkaline subsiliceous event (Nepheline Syenite of San José del Guaviare) with Neoproterozoic-Ediacaran to Cambrian ages (U-Pb zircon emplacement ages) of ~578 Ma (Mejia et al, 2012) and biotite K-Ar and Rb-Sr cooling ages of 445 -495 Ma (Pinson et al, 1962). Recent own LA-ICP-MS U-Pb zircon ages of ~ 609 Ma (Franco et al, 2018;Muñoz Rocha et al, 2019) near Jordan to the SE of the intrusion, suggest a long emplacement and cooling history for this large body that is probably associated with the Pan-African -Braziliano orogeny. Further west, no intracratonic intrusions are reported.…”

San José de Guaviare Syenite, Colombia: Repeated Ediacaran intrusions in the northwestern Amazonian Craton

“…Geochronological analyses were performed using the LA ICP-MS U-Pb zircon method in two syenite samples (Annex 3), one located in the El Capricho body (JDB-0001R) and the other located in the Cerrito body (OPP-0074R) (Figure 14). Authors such as Pinson et al (1962), Arango et al (2011Arango et al ( , 2012, Franco et al (2018), andMuñoz Rocha et al (2019) have reported ages ranging from the Ordovician to the Neoproterozoic for syenite rocks, calculated using different methods ( Ar/ 39 Ar biotite method, was reported by Arango et al (2011Arango et al ( , 2012. This age corresponds to the late Cambrian (Furongian) and was interpreted as a cooling age.…”

“…northern SJGS bodies. Other studies, such as those of Franco et al (2018) and Muñoz Rocha et al (2019), have complemented the age records of syenite rocks and their geochemistry.…”

“…Different studies have reported a total of four plutonic bodies (B) in the SJGS, of which the two northernmost bodies (La Pizarra B and Las Delicias B), located in La Lindosa mountain range, were identified and studied by Trumpy (1943Trumpy ( , 1944, Gansser (1954), Pinson et al (1962), Vesga and Castillo (1972), Galvis et al (1979), and Arango et al (2011Arango et al ( , 2012. The two southernmost bodies (El Capricho B and Cerritos B), located near El Capricho corregimiento, have been explored in more recent studies such as those of Maya et al (2018), Franco et al (2018), and Muñoz Rocha et al (2019).…”