Características isotópicas (Nd e Sr) do plutonismo intrusivo no extremo NW do Cráton Amazônico, Venezuela, e implicações para a evolução paleoproterozóica

Abstract

Nd and Sr analyses were performed on selected granitoid plutons that intrude Archean and Paleoproterozoic domains of the Guyana shield (Venezuela). The isotopic signatures of these plutons together with the geochronologic background of the country rocks are used to constrain their magma genesis and tectonic setting within the Paleoproterozoic evolution of mobile belts (Maroni-Itacaiunas and Ventuari-Tapajós provinces) of the Amazonian Craton. The Encrucijada Suite (2187 ± 94 Ma), which intrudes Archean rocks of the Imataca Complex, originated predominantly from partial melt of this crust, as supported by negative µNd(2, 1Ga) values (-2.2 to -4.9) and T DM ages between 2.82 and 2.49 Ga. Conversely, the plutons from the Supamo Complex (2230 - 2050 Ma) and Cuchivero Group (1980-1830 Ma), occurring within the adjoining Paleoproterozoic provinces, are juvenile in nature (derived from roughly contemporary protoliths). These bodies display T DM ages between 2.13 and 2.22 Ga, as well as positive µNd(2, 1Ga) values (+0.74 to +3.05). Isotopic correlation diagrams (143Nd/144Nd vs. 147Sm/144Nd and 143Nd/144Nd vs. time) plotted together with the plutonic rocks and Imataca Complex rocks were evaluated taking into account the geologic background of the NW part of the Amazonian Craton. Interpretation of these isotopic data supports the idea of tectonic juxtaposition between the Imataca Complex and the Maroni-Itacaiúnas province during the Transamazonian orogeny (2.25 - 2.05 Ga). On the other hand, the Cuchivero Group plutons have a constrasting isotopic signature compared to the other Paleoproterozoic plutonic rocks. This is consistent with the existence of a tectonic boundary between the Maroni-Itacaiúnas and the Ventuari-Tapajós province in the late Paleoproterozoic.