Located on the western continental tip of the Fernando de Noronha transform fracture (FNTF), northeastern Brazil, the Eocene-Oligocene Mecejana Phonolitic Province (MPP) was subjected to a multi-methodological study, comprising field and microscopic characterization, mineral and whole rock chemistry, and Sr-Nd isotopes. These rocks intrude the Precambrian crystalline basement, forming mainly plugs oriented in the NE-SW direction. They are composed of alkali basalt, phono-tephrite, tephri-phonolite, and phonolite with phenocrysts of nepheline and sanidine, and minor diopside, aegirine-augite, aegirine, kaersutite, biotite and titanite. Chemically, they are sodic (Na2O/K2O > 1) and display LREE enrichment, concave upward or flat HREE patterns, weak to strong Eu anomaly, and high contents of Nb, Ta and Zr. They delineate two chemical groups, the less evolved one comprising basalt to tephri-phonolites and the more evolved constituted by phonolites. Oxide and trace element modeling points out to an evolution by fractional crystallization, leaving olivine-rich gabbro and foidolite / nepheline syenite cumulates, respectively. The analyzed samples have Sr-Nd isotopic ratios akin to OIB rocks (positive εNd of +3.0 to +0.9 at 30 Ma) and low to high 87Sr/86Sri (at 30 Ma) of 0.70264-0.70955. The data suggest a three stages evolution model for the MPP: (1) <5% partial melting of a metasomatized phlogopite-amphibole-garnet peridotite generates an alkali basalt melt; (2) polybaric differentiation of this melt at crustal level (~570-220 MPa, 1040-645 oC) leads firstly to phono-tephrites and tephri-phonolites and then to (3) foidolite and nepheline syenite. Petrographic, chemical, and isotopic characteristics of the MPP and similar rocks of the Fernando de Noronha islands suggest the control by the FNTF and reactivation of NE-SW trending deep-rooted Precambrian faults allowing channeling and emplacement of the alkaline magmas.
No nordeste brasileiro, sistemas vulcânicos de pequena escala constituem uma ampla província cenozoica tectono-magmática, o Alinhamento Macau-Queimadas, composto de rochas vulcânicas máficas alcalinas. O objetivo desta pesquisa foi caracterizar a petrogênese e o mecanismo de colocação de uma dessas ocorrências, localizada na borda sul da Bacia Potiguar. Para isso, são apresentados dados de mapeamento geológico, petrográficos e geoquímicos de rocha total. Na área estudada, três corpos vulcânicos ocorrem com diferentes geometrias. O mais expressivo tem alojamento magmático controlado tectonicamente, gerando um sistema de diques nas direções NNW-SSE e NE-SW. Petrograficamente, o conjunto mostra feições vulcânicas e intrusivas rasas, constituídas de olivina basaltos e nefelina microgabros. As fácies texturais observadas indicam resfriamento final em nível crustal raso de magmas alcalinos insaturados em sílica, gerando basanitos, melanefelinitos e olivina basaltos. Em termos litoquímicos, são rochas relativamente primitivas (#mg = 65–79), com baixo SiO2 (39–43%) e elevados MgO (12–19%) e Ni (> 200 ppm). Apresentam enriquecimento em terras raras leves (LaN/YbN ~ 35–17) e em elementos incompatíveis, destacando-se anomalias negativas de Rb, K e Hf. Modelamento geoquímico de óxidos com base em equações de balanço de massa e simulações com MELTS mostra taxa de cristalização de ~64% e cumulato composto de olivina, melilita, nefelina, magnetita, apatita e perovskita. Modelamento geoquímico de elementos traço para fusão parcial em equilíbrio sugere taxa de fusão menor que 5% de fonte do tipo granada-lherzolito enriquecido em elementos incompatíveis e contendo anfibólio e/ou flogopita. O mecanismo de colocação é associado à instabilidade do campo de tensões local no Mioceno, que reativou estruturas das unidades encaixantes favorecendo abertura de canais para escoamento de magmas de origem mantélica...
The Rio Ceará-Mirim dike swarm (RCMDS) of the Borborema Province is a suite of subvertical intrusions classically described in the state of Rio Grande do Norte as E-W-trending dikes, up to southern Ceará, progressively deflecting to NE-SW. The rifting processes involving the Atlantic Ocean opening in the Early Cretaceous is responsible for the Northeast Brazilian Rift System (NBRS) and RCMDS development. This paper investigates the morphological styles and stress states in eastern RCMDS, focusing on well-preserved dikes at the localities of Rio Salgado and Lajes (state of Rio Grande do Norte). Remote sensing techniques, fieldwork, and numerical models aimed to obtain data to propose correlations between the eastern RCMDS emplacement and the NBRS tectonic settling in the Early Cretaceous. The studied dikes are predominantly tholeiitic basalt-diabase ranging from a few centimeters to 150 meters in thickness, locally achieving tens of kilometers in length. Their morphological styles vary from symmetrical to asymmetrical, from sharp and straight to anastomosing/braided dikes, showing diverse en echelon patterns, steps, horns, bridges, and bridge xenoliths. Fractal analysis of the dikes framework indicate syn-magmatic strike-slip components. The majority of morphological markers and the average orientation of the Rio Salgado dikes indicate a NNS-SSW (010 Az) orientation for the least compressive axis and dextral displacements. Based on some occurrences of en echelon dikes showing syn-emplacement stages in the extension direction, we propose three main stages for the least compressive axis -initially oriented to NW-SE, changing to N-S, and finally to NNE-SSW. Mechanical models indicate that the eastern RCMDS were emplaced in deviatoric stress with low/intermediate fluid pressure (Pf < σ2), which is in agreement with the observed morphological patterns.Stress states during the emplacement of the eastern Rio Ceará-Mirim dike swarm, Borborema Province, northeastern Brazil
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