THE JURUENA MAGMATIC ARC AND ITS EVOLUTION DURING THE PROTEROZOIC IN THE SOUTHWEST OF THE AMAZON CRATON.

Leo Adriano de Oliveira-1.2; Natã José de França-1.2; Antonio João Paes de Barros-2, André Campos Rocha Pinto-1, Mauro Cesar Geraldes-1.

1-University of the State of Rio de Janeiro-UERJ. PPGG-Postgraduate Program in Geosciences-UERJ

2-Metamat-Company Mato-grossense de Mining-Geological Service of the state of Mato Grosso.

We report 16 new U-Pb ages of rocks from the Bacaeri-Mogno units outcroping in the South área of Alta Floresta Gold Province, Mato Grosso (Brazil). The results are interpreted with published data from several authors for a better understanding of the geological and magmatic context of the evolution of the Juruena Magmatic (JMA) Arc in order to bring new insights into its evolution during the Paleoproterozoic, and possible implications of the geodynamics of this region wih metalogenetic processes.

The 16 ages reported here clearly define three groups, the first comprising 7 samples with U-Pb ages varying between 1883 ± 27 Ma, 1858 ± 6 Ma, 1848 ± 6 Ma and 1838 ± 34 Ma interpreted as the formation interval of magmatic rocks. The second group has the interval between 1808 ± 8 Ma and 1806 ± 4 Ma, 1801 ± 9 Ma, 1800 ± 9 Ma, 179 ± 14 Ma, 1792 ± 5 Ma, 1775 ± 8 Ma and 1772 ± 40 Ma and can be interpreted as the time of metamorphism or partial melting. A third group is composed of metasedimentary rocks and the results point to a back-arc basin related to the Juruena magmatic arc.

Part of the literature uses age estimates based on relative ages with rocks in the Rio Negro-Juruena Province. The metamorphism was interpreted as being early Statherian -1.65 Ga. The age of partial melting in the Bacaeri-Mogno complex is here constrained between 1,78-1,72 Ga regionally related to partial melting (migmatization).

Although there are numerous previous studies on the evolution of the Paleoproterozoic magmatic arc in the region, there are still doubts about its origin. Two hypotheses emerged, the first is linked to the accretionary processes that form the JMA; the second is related to extensional processes post-orogenic or even anorogenic magmatism related to the cratonization of the Tapajós-Parima Province

The results here reported point out to an accretionary orogens emerged from tectonic processes related to oceanic lithosphere subduction and are the main characters in the production of juvenile material of granitic composition, therefore responsible for the growth of continental crust. The juvenile nature of the material of the Bacaeri-Mogno rocks contrasts with the classical models (Andean, Cordilleran, and Himalayan type, of continental collision, precisely because the participation of the pre-existing continental basement does not occur. At the beginning of the Proterozoic there was a predominance of juvenile granitoids, time when much of the material that currently makes up the continental crust was formed.

As conclusion, the orogenic belt characterized here in the southwest of the Amazon Craton and northwest of the State of Mato Grosso, forms a band of rocks with well-marked contacts by faults and regional shear zones that extend for 500 kilometers in a preferential NW direction.  The JMA is composed of a set of Paleoproterozoic plutonic and volcanic rocks that extend along the boundary between the tectonic provinces defined as Tapajós-Parima (2.03 -1.88 Ga) and Rondônia-Juruena (1.80-1.50 Ga) as a importante limits between terranes with disting geologic records.