Geocronologia do complexo máfico-ultramáfico alcalino de Santa Fé, Goiás

The Santa Fé mafic-ultramafic alkaline complex, Goiás, is a subcircular body with a rough concentric zoning of rock types, containing dunites at the center and peridotites and pyroxenites at the border. Minor rock types associated with the major units are missourites, malignites and phonolites. An essexitic body occurs as a small intrusion close to the W border of the main massif. Country rocks are leucocratic quartz-feldspatic gneisses with rare amphibolite bands. Lamprophyre dikes occur to the south, cutting Precambrian gneisses and Devonian sandstones. K-Ar determinations yielded the following ages: essexite 74.4 (± 3.4) m.y., missourite 84.7 (± 1.8) m.y., malignite, ranging from 82.6 (± 4.5) to 88.4 (± 2.7) m.y., and a lamprophyre dike 86.6 (± 4.5) m.y. These ages are similar to those determined for other complexes of the Upper-Cretaceous Iporá Group of mafic-ultramafic intrusions. Sorrounding gneisses show a whole-rock. Rb/Sr age of 467 (± 10) m.y., thus corresponding to the Brasiliano Cycle.O complexo máfico-ultramáfico alcalino de Santa Fé é formado principalmente por dunitos, peridotitos e piroxenitos exibindo grosseira disposição concêntrica. Missouritos, malignitos e fonolitas ocorrem em quantidades subordinadas. Essexitos formam corpo menor a W do maciço principal. As rochas encaixantes são gnaisses quartzo-feldspáticos, geralmente pobres em máficos e localmente portadores de bandas anfibolíticas. Ao sul da área ocorrem arenitos devonianos capeando tais rochas, que são por sua vez cortados por diques de lamprófíro (minette). As idades potássio-argônio foram efetuadas em cinco amostras, obtendo-se os seguintes valores: essexito 74,4 (± 3,4) m.a. missourito 84,7 (± 1,8) m.a. lamprófíro 86,6 (± 4,5) m.a. e malignitos 82,6 (± 4,5) m.a. e 88,4 (± 2,7) m.a. Tais dados mostram boa concordância com outros complexos do Grupo Iporá, reafirmando sua idade Cretáceo Superior. As determinações rubídio-estrôncio efetuadas nos gnaisses encaixantes propiciaram a elaboração de isócrona de 467 (± 10) m.a., fato que posiciona tais rochas no Ciclo Brasiliano

Efeitos termais associados aos diabásios mesozóicos da bacia do Paraná

This paper deals with hornfelses produced by sills and dikes related to the basaltic activity of the Paraná Basin. The studied samples come from the Paulínia, Assistência, Taguaí and Itapeva regions, in the State of São Paulo. In Itapeva, the temperature attained during the recrystallization is estimated at about 740-820º C, corresponding to 500-1000 PH2O- The mineralogical paragenesis of the remaing group of hornfelses provides informations only about the minimum temperatures of recrystallization. From chemical and mineralojpcal analyses, some inferences were made about the nature of the parent rocks, particularly in the Paulínia occurrence. All the studied hornfelses have small contact aureoles, consisting of fine-grained rocks which show no grain size decrease with increasing distance from the contact. These features are analysed and discussed.Foram estudados no presente trabalho efeitos termais provocados por intrusões de sills e diques de diabásio, relacionados ao magmatismo basáltico da bacia do Paraná, nas regiões de Paulínia, Assistência, Taguaí e Itapeva, SP. As associações mineralógicas observadas permitiram inferências tâo somente quanto às temperaturas mínimas de recristalização, à exceção de Itapeva onde a paragênese observada permitiu estimar o clímax do evento termal em cerca de 740-820ºC, correspondente a 500-1000 bars de PH2O. Análises químicas e mineralógicas dos hornfels formados permitiram ressaltar aspectos relativos às rochas originais, particularmente no caso de Paulínia, onde se comparou o hornfels a vários sedimentos argilosos do Grupo Tubarão. Todos os hornfels estudados têm pequenas auréolas de contato, formadas de rochas de granulação fina, que não exibem diminuição da granulação com o aumento de distância do contato. Essas feições são analisadas e discutidas

U-Pb baddeleyite ages of key dyke swarms in the Amazonian Craton (Carajás/Rio Maria and Rio Apa areas): Tectonic implications for events at 1880, 1110 Ma, 535 Ma and 200 Ma

Abstract U-Pb baddeleyite ages for key mafic dykes of the Amazonian Craton reveal four significant intraplate episodes that allow connections with global igneous activity through time and supercontinent cycles. The oldest dykes (Carajas-Rio Maria region) are diabases with ages of 1880.2 ± 1.5 Ma and 1884.6 ± 1.6 Ma, respectively, corresponding with the Tucuma swarm which crops out to the west and is age-equivalent. The magmatic activity has a genetic link with the ca. 1.88 Ga Uatuma Silicic Large Igneous Province (SLIP), characterized by felsic plutonic-volcanic rocks. There is an age correlation with LIP events (ca. 1880 Ma) in the Superior, Slave, Indian and other cratons. This magmatism could be derived from significant perturbations of the upper mantle during the partial assembly of Columbia. Gabbronorite of the Rio Perdido Suite (Rio Apa Terrane) crystallized at 1110.7 ± 1.4 Ma, and is identical to that of the Rincon del Tigre-Huanchaca LIP event of the Amazonian Craton. This event was synchronous with the initiation of Keweenawan magmatism of central Laurentia (Midcontinent Rift) and also with coeval units in the Kalahari, Congo and India cratons. The two youngest U-Pb dates (535 and 200 Ma) occur in the Carajas region. Diabase of the Paraupebas swarm yields an age of 535.1 ± 1.1 Ma, which may be correlative with the giant Piranhas swarm located ca. 900 km apart to the west. The Paraupebas swarm is correlative with post-collisional plutonism within the Araguaia marginal belt. Therefore, the Cambrian dykes may reflect reactivation of cooled lithosphere, due to crustal extension/transtension active along the craton’s margin during assembly of West Gondwana. This magmatism is also contemporaneous with the 539–530 Ma Wichita LIP of southern Laurentia. The youngest studied Carajas region dyke was emplaced at ca. 200 Ma, corresponding with 40 Ar/ 39 Ar ages for the Periquito dykes west of Carajas and with most K-Ar ages of the giant Cassipore swarm, located north of the study area. The newly dated ca. 200 Ma dyke fits well into the known, brief span of ages for the CAMP Large Igneous Province event, around the present central and northern Atlantic Ocean

The growth and contamination mechanism of the Cana Brava layered mafic-ultramafic complex: new field and geochemical evidences

The Cana Brava complex is the northernmost of three layered complexes outcropping in the Goiás state (central Brasil). New field and geochemical evidences suggest that Cana Brava underwent hyper- to subsolidus deformation during its growth, acquiring a high-temperature foliation that is generally interpreted as the result of a granulite- facies metamorphic event. The increase along the stratigraphy of the incompatible elements abundances (LREE, Rb, Ba) and of the Sr isotopic composition, coupled with a decrease in εNd(790), indicate that the complex was contaminated by the embedded xenoliths from the Palmeirópolis Sequence. The geochemical data suggest that the contamination occurred along the entire magma column during the crystallization of the Upper Mafic Zone, with in situ variations determined by the abundance and composition of the xenoliths. These features of the Cana Brava complex point to an extremely similarity with the Lower Sequence of the most known Niquelândia intrusion (the central of the three complexes). This, together with the evidences that the two complexes have the same age (c.a. 790 Ma) and their thickness and units decrease northwards suggests that Cana Brava and Niquelândia are part of a single giant Brasilia body grown through several melt impulses

Mafic Dykes: Petrogenesis and Tectonic Inferences

The main mafic dyke swarms of the São Francisco craton range in age from Late Archean to the Early Neoproterozoic, and record extensional and transpression tectonic events. The Uauá and Lavras regions include norite (2.73 and 2.66 Ga) and tholeiite swarms (2.62 and 1.97 Ga) respectively. Other swarms are made up of tholeiites: Paraopeba (2.10 Ga), Pará de Minas (1.71 Ga), Curaçá and Chapada Diamantina (1.5 Ga), Diamantina (0.93 Ga) and Salvador-Olivença (0.92 Ga). Geochemical and Sr–Nd isotopic data indicate variable scales of mantle heterogeneity, caused by recurrent metasomatic effects since Archean times. The main processes are attributed to the action of slab fluids from recycling of oceanic crust and overlying sediments on the sub-lithospheric mantle in continental settings, and by upwelling of OIB-like material. These processes are consistent with the protracted geologic history of the country rocks, given by episodic juvenile accretion and successive continental amalgamations from Archean to ca. 2.0 Ga. The adopted mixing geochemical model permits to evaluate the variation range of N-MORB melts, OIB and slab fluids and melts, which are considered to represent the main components of the parental mantle composition of the studied dykes