Gabrielle Aparecida de Lima

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.

Geology and petrology of the Salto do Céu Suite: tectonic and stratigraphic implications on the SW Amazonian Craton

The basic rocks of the Salto do Ceu Suite outcrop as sills and flows in the southwestern Amazon Craton, Rondonian-San Ignacio Province, with an age of about 1.44 Ga. Sills are 2 to 30 m thick being hosted by pelites of the Aguapei Group. The flows are up to 5 m thick and cover this unit. Typical textures of magma mingling are observed near the contact with granites of Rio Branco Suite. The Rio Branco Intrusive Suite is composed of basic and acid rocks, as well as hybrid rocks that indicate mixing processes between basic and acids magmas. U-Pb (TIMS zircon) results indicate ages around 1.4 Ga for both terms. The Salto do Ceu Suite rocks have tholeiites affinity classified as subalkaline and iron-rich tholeiitic basalts, with mg# values between 0.30 and 0.51. They can be separated into two groups, based on La N ; one is richer in ETR with La N greater than 100, while the other one has La N less than 100. Rocks of the Salto do Ceu Suite and Rio Branco Suite are interpreted as a bimodal suites showing magma mingling features such as those developed in continental intraplate settings, extensional regime associated to the Columbia/Nuna breakup.