New LA-ICP-MS U-Pb zircon dating, Ar-Ar and Sm-Nd model ages: Evidence of the Grenvillian Event in the basement of the Falcón and Maracaibo Basins, northwestern Venezuela

Marvin Baquero^{1, 2}, Sebastián Grande³, Urbani Franco^{3, 4}, Umberto Cordani⁵, Kei Sato⁵, Peter Shaaf⁶, Chris Hall⁷, David Mendi³, Manuel Azancot³

¹ PDVSA Exploración, Gerencia de Proyectos Exploratorios y Delineación, Puerto La Cruz, Anzoátegui, Venezuela;

²  INTEVEP,  Gerencia  de  Investigación  Estratégica de  Exploración,  Los  Teques,    Miranda,   Venezuela; 

³ Escuela de Geología, Minas y Geofísica, Ciudad Universitaria, UCV, Caracas, Venezuela; 

⁴ Fundación Venezolana de Investigaciones Sismológicas, calle Mara, El Llanito, Miranda, Venezuela;  

⁵ Centro de Pesquisas Geocronológicas, Instituto de Geociências, Ciudade Universitária, Universidade de São Paulo, Brazil

⁶ Laboratorio Universitario de Geoquímica Isotópica, Instituto de Geofísica, Ciudad Universitaria, UNAM, D.F., México; 

⁷ Argon Geochronology Laboratory, University of Michigan, Ann Arbor,  MI,  USA.

Corresponding author: baqueroms@pdvsa.com / mbaquero75@gmail.com

Abstract

U-Pb zircon ages of high-grade metamorphic rocks described in basement cores from the Ensenada de la Vela, Falcón Basin, Venezuela, revealed the presence of Grenvillian rocks. This new finding represents a breakthrough not only in terms of crustal growth processes, but also in providing new lines of evidence to initiate a review of the geodynamic models for the evolution of northern South America. This allochthonous block named hereby as the Falconia Terrane (FT) and consist of a sequence of high-grade metamorphic rocks of dolomitic marble, calc-silicate rock, meta-anorthosite, felsic, mafic and ultramafic granulite, metapelite, garnet amphibolite and amphibolite. New LA-ICP-MS U-Pb zircon dating in metapelites and granulites from the FT showed upper-intercept ages at about 1.3 - 1.2 Ga. interpreted as the crystallization age of inherited zircons and a concordant age ca. 0.92 Ga which may document the younger granulitic metamorphic event. In addition, ⁴⁰Ar-³⁹Ar ages from hornblende crystals in amphibolites, phlogopite crystals present in the dolomitic marble, and biotite crystals in the felsic granulite shows variable behavior. The hornblende preserves older apparent ⁴⁰Ar-³⁹Ar ages around 0.93 Ga, which can be related to younger metamorphic event, whereas phlogopite and biotite spectra do not define plateau ages. The apparent ages of the incremental steps vary between 837 Ma and 785 Ma, respectively. Igneous rock samples collected from well cores and outcrops around the Maracaibo Basin yielded Grenvillian-type Sm-Nd depleted mantle model ages (T_DM) of 1.3 – 1.2 Ga. They probably represent juvenile crustal material rather than evolved continental crustal fragments. This terrane exhibits a complex crustal history, and the radiometric ages here presented suggest that they probably formed part of a single Western South America terrane located initially in the northwestern margin of the Amazonian Craton during the Neoproterozoic. The geodynamic evolution envisioned for the FT involved an accordion-type of tectonics in which this terrain separated the South America craton during the break-up Gondwana and later docked against the South America margin during the Late Cretaceous – Middle Eocene collision of the Caribbean plate.

Keywords: Grenvillian orogeny, U-Pb zircon, Falcón basin, Maracaibo basin, Venezuela