Chapter 4

Abstract

The crystalline basement of eastern Colombia, east of the frontal deformation zone of the north Andean Eastern Cordillera, is comprised by Precambrian igneous, metamorphic, and sedimentary rocks of the western Guiana Shield. Designated in the late seventies with the all-embracing stratigraphic name of 'Mitú Migmatitic Complex', the age, petrology, and tectonic history of the Precambrian basement in eastern Colombia has remained one of the least explored issues in South American geology. This chapter aims to present a brief overview of recent advances made to improve our general understanding of the geology of this wide region, using a compilation of the available U-Pb, Sm-Nd, Lu-Hf, and δ 18 O isotopic data obtained using modern methods. Using all the available U-Pb geochronologic data we show that, in general: (i) The Precambrian basement of the western Guiana Shield exhibits magmatic crystallization ages in the range from ca. 1.99 to ca. 1.38 Ga, and (ii) that four broad periods of magmatic activity, two in the mid-to late-Paleoproterozoic (ca. 1.99 and ca. 1.81-1.72 Ga), one in the early Mesoproterozoic (ca. 1.59-1.50 Ga), and one in the mid Mesoproterozoic (ca. 1.41-1.39 Ga) dominate the geology of the area. The (whole-rock) Nd and combined (zircon) Hf-O datasets indicate a general lack of 'depleted mantle' like mid-Paleoproterozoic or Mesoproterozoic crust, thus indicating that either the Proterozoic sub-continental mantle in the region was not as radiogenic as global mantle evolution models would suggest, or that reworking of older crust might have played an important role in the geological and geochemical evolution of the western Guiana Shield. Therefore, although the geochronologic results confirm that most of the exposed basement in eastern Colombia can be broadly considered to be of Rio Negro-Juruena-like affinity, this belt exhibits some distinct isotopic characteristics relative to similar age domains exposed south of the Amazon Basin. Furthermore, we note that the geochronologic data obtained to this date has failed to clearly identify an early-to mid-Mesoproterozoic terrane boundary in the Colombian basement, thus opening the possibility that a Rondonian-San Ignacio-like province is not represented in the Guiana Shield. Based on these recent field, geochemical, and geochronological observations, we consider the long and extensively used term 'Mitú Migmatitic Complex' to be now inadequate and obsolete, and argue that the current state of the knowledge of the Colombian Precambrian basement is such that the community should move towards adopting Proterozoic

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

“…Moreover, data presented by Arango et al (2012) show that stepwise-extracted Ar fractions do not plateau well, which may result from excessive recoil during the 39 K(n, p) 39 Ar reaction produced by irradiation, a complex thermal history, or the combination of both factors. The two possible events that may have generated a complex thermal history and that may have disturbed the K-Ar/ 40 Ar-39 Ar systems are: 1) the Cambro-Ordovician sedimentary cover in the area may have been quite thick, which would have resulted in a complex cooling history of the syenites or allowed the syenites to reheat during tectonic burial; and 2) an important thermal event may have affected the western margin of the Amazonian Craton during the Cretaceous, as suggested by the presence of basaltic dikes dated at 102 Ma in the Araracuara region (Ibáñez-Mejía and Cordani, 2020). Although these rocks have not been reported in the Guaviare region, currently available data prevent us from ruling out the presence of a recent thermal event (for example, Cretaceous) in the area.…”

Presentation

“…In chapter 4, using isotopic data (U-Pb, Sm-Nd, Lu-Hf, and δ18O), Ibañez-Mejia & Cordani (2020) show that the western part of the Guiana Shield, which includes the Proterozoic basement of eastern Colombia, has crystallization ages ranging from ca. 1.99 to ca.…”