João Batista de Matos

The 1590-1520 Ma Cachoeirinha magmatic arc and its tectonic implications for the Mesoproterozoic SW Amazonian craton crustal evolution

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 TDM 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 (TDM 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) TDM and arc-related chemical affinity support the 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.

Petrografia, geoquímica e geocronologia das rochas do orógeno Rio Alegre, Mato Grosso: um registro de Crosta oceânica mesoproterozóica no SW do Cráton Amazônico

The volcano-sedimentary rocks of the Rio Alegre terrane occur in the SW part of the Amazonian Craton and comprise mafic and ultramafic volcanic rocks, chemical sedimentary rocks, and mafic to felsic intrusive rocks metamorphosed at greenschist to low amphibolite facies. Petrographic, chemical and isotopic studies allowed the subdivision of the complex into three subunits: the basal Minouro formation comprises abundant basic to ultrabasic volcanic rocks and subvolcanic intrusive rocks associated with cherts and banded iron formations. Chemical data indicate an ocean-floor tectonic setting for these rocks; the middle Santa Isabel formation comprises intermediate and acid lavas and pyroclastic rocks. Chemical results indicate an island arc setting for them, which is corroborated by the U-Pb zircon ages of 1509 - 1503 Ma and T DM ages of 1.54 Ga and 1.48 Ga and µNd(t) values from +3.7 to +4.1 for the intermediate rocks, suggesting a mantle origin for the magmas. The uppermost Sao Fabiano formation is composed of metasedimentary rocks with a chemical signature similar to that of the volcanic rocks, suggesting that the latter are the source rocks. The basic to ultrabasic intrusive rocks are differentiated gabbroic rocks and serpentinites. Chemical data indicate the influence of fractional crystallization in the evolution of these rocks. U-Pb zircon dating of these rocks yielded ages from 1509 Ma to 1494 Ma, T DM of 1.67 to 1.48 Ga, and µNd(t) values of +4.5 to +2.5 suggesting mantle-derivation. U-Pb zircon analyses carried out on associated intrusive basic and felsic rocks yielded ages of 1.48 - 1.46 Ga. T DM ages vary from 1.53 Ga to 1.50 Ga and µNd(t) values from +3.7 to +4.1 suggesting that they were generated in a magmatic arc environment. The lithologic association, chemical and isotopic evidence suggest that volcanic and subvolcanic basic and ultrabasic rocks originated in a back-arc basin or ocean ridge; and that the basic to intermediate rocks were formed in an island arc setting at 1.51 - 1.50 Ga. These rocks were later intruded at 1.48 - 1.46 Ga by basic and felsic rocks formed in a magmatic arc setting. Metamorphism under green-schist to lower amphibolite facies (biotite to garnet-kyanite zone), and mylonitization producing a N20W foliation were associated with collision of the arc against the Amazonian Craton during Mesoproterozoic time.

Geology, Geochemistry, and Geochronology (U-Pb) of the Rio Fortuna Gneiss - Serra do Baú Intrusive Suite - Paraguá Terrane - SW Amazonian Craton

O Gnaisse Rio Fortuna aflora na regiao da serra Santa Barbara, nas imediacoes do Destacamento Militar Fortuna, na fronteira Brasil-Bolivia. Do ponto de vista geologico-tectonico, estes gnaisses estao inseridos no Terreno Paragua, em um setor afetado pela Orogenia Sunsas (1.0 a 0.9 Ga). Os dados geologicos e petrograficos obtidos mostram que estas rochas sao ortognaisses de composicao monzo a granodioritica, com registros de tres fases de deformacao. Idade U-Pb em zircao de 1711 ± 13 Ma obtida por MC-ICP-MS com ablacao a laser, deve corresponder aidade de cristalizacao do protolito igneo. Geoquimicamente, essas rochas constituem uma sequencia acida formada por um magmatismo subalcalino, do tipo calcio-alcalino de alto potassio, metaluminoso a peraluminoso. No contexto tectonico do Terreno Paragua, os dados obtidos indicam que o Gnaisse Rio Fortuna foi formado durante a Orogenia LomasManechis. Os resultados obtidos apontam semelhancas entre esta rocha e a que se prolonga na contraparte boliviana.