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 Volume 1 Chapter 4

Chapter 4


Zircon U–Pb Geochronology and Hf–Nd–O Isotope Geochemistry of the Paleoproterozoic to Mesoproterozoic Basement in the Westernmost Guyana Shield

Mauricio IBANEZ–MEJIA and Umberto G. CORDANI

https://doi.org/10.32685/pub.esp.35.2019.04


Citation is suggested as: 

Ibanez–Mejia, M. & Cordani, U.G. 2019. Zircon U–Pb geochronology and Hf–Nd–O isotope geochemistry of the Paleoproterozoic to Mesoproterozoic basement in the westernmost Guiana Shield. In: Gómez, J. & Mateus–Zabala, D. (editors), The Geology of Colombia, Volume 1 Proterozoic – Paleozoic. Servicio Geológico Colombiano, Publicaciones Geológicas Especiales 35, p. 97–125. Bogotá. https://doi.org/10.32685/pub.esp.35.2019.04


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 δ18O 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 Ga 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 “depleted” and 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 suture 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 knowledge of the Colombian Precambrian basement is such that the community should move towards adopting a more accurate and modern tectonic–stratigraphic nomenclature. Lastly, we note that the recent discovery of Cretaceous magmatism affecting the Colombian continental interior in the Araracuara basement high highlights the importance of Mesozoic tectonic reactivation in controlling the structural and landscape evolution of the Colombian Amazon. This observation indicates that future geochronologic studies aimed at better understanding the temporal history of mafic magmatism in this region will be crucial for understanding its structural and tectonic evolution.


Keywords: Amazonian Craton, Proterozoic tectonics, U–Pb geochronology, Lu–Hf isotopes, Sm–Nd isotopes.