Abstract
Isotopic and chemical data of rocks from the Cachoeirinha suite provide new insights on the Proterozoic evolution of the Rio Negro/Juruena Province in SW Amazonian craton. Six U-Pb and Sm-Nd analyses in granitoid rocks of the Cachoeirinha suite yielded ages of 1587-1522 Ma and T DM model ages of 1.88-1.75 Ga (EpsilonNd values of -0.8 to +1.0). In addition, three post-tectonic plutonic rocks yielded U-Pb ages from 1485-1389 Ma (T DM of 1.77-1.74 Ga and EpsilonNd values from -1.3 to +1.7). Variations in major and trace elements of the Cachoeirinha suite rocks indicate fractional crystallization process and magmatic arc geologic setting. These results suggest the following interpretations: (1) The interval of 1590-1520 Ma represents an important magmatic activity in SW Amazonian craton. (2) T DM and arc-related chemical affinity supportthe hypothesis that the rocks are genetically associated with an east-dipping subduction zone under the older (1.79-1.74 Ga) continental margin. (3) The 1590-1520 Ma age of intrusive rocks adjacent to an older crust represents similar geological framework along the southern margin of Baltica, corroborating the hypothesis of tectonic relationship at that time.
Highlights
The Amazonian craton represents a key region for reconstruction of Meso- and Paleoproterozoic continents
In order to improve the knowledge of the SW portion of the Amazonian craton, this paper presents new U-Pb ages, Sm-Nd isotopic results and chemical data on granitoids constraining the nature of the Mesoproterozoic plutonic rocks that crop out in SW Mato Grosso State (Brazil)
U-Pb and Sm-Nd major, trace and REE data provide time and tectonic setting constraints for the rocks here studied and allow to suggest tectonic implications on the Mesoproterozoic crustal evolution of the SW Amazonian craton, as the following: 1) Tonalites, granodiorites and granites ascribed to the Cachoeirinha suite represent juvenile magmatic activity that occurred between 1590 and 1520 Ma
Summary
The Amazonian craton represents a key region for reconstruction of Meso- and Paleoproterozoic continents. Three major geochronological and tectonic provinces (Teixeira et al 1989, Tassinari and Macambira 1999) are traditionally considered and represented in the western part of the Amazonian craton in Mato Grosso state: Rio Negro-Juruena Province, Rondonian-San Ignacio Province, and Sunsás Province. The Proterozoic basement in SW Mato Grosso (Figure 1) consists of igneous and metamorphic rocks interpreted as belonging to the RNJP This province includes distinct rock associations, such as the oldest Alto Jauru orogenic rocks (Pinho et al 1997) encompassing acid metavolcanics associated to BIF’s and metasedimentary rocks and tonalitic to granitic gneisses. The 1.42-1.36 Ga San Ignacio orogen (Geraldes et al 2002) and the 1.501.48 Ga Rio Alegre orogen (Matos et al 2004) may be interpreted as accretionary arcs related of the SW Amazonian craton crust formation during the Mesoproterozoic
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