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RESEARCH PRODUCT

Evidence of magmatic activity related to Middle Jurassic and Lower Cretaceous rifting from northeastern Brazil (Ceará-Mirim): K/Ar age, palaeomagnetism, petrology and Sr/1bNd isotope characteristics

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The Mesozoic magmatic activity in the easternmost part of NE Brazil (Ceara-Mirim) is mainly represented by two-pyroxene tholeiitic dykes; only few dykes have alkaline character. K/Ar ages and palaeomagnetism data indicate that Ceara-Mirim dykes are of Middle Jurassic (175-160 Ma) and Early Cretaceous (140-130 Ma) age. Both Middle Jurassic and Lower Cretaceous dykes have high incompatible-element concentrations and TiO2 with the lower contents generally confined to the Middle Jurassic dykes; rare tholeiitic dykes low in TiO2 and incompatible elements are present. Sr/1bNd isotopic and other chemical data do not support appreciable crustal contamination and in a 87Sr86Sr vs. 143Nd144Nd diagram almost all the dykes plot in a narrow area of the enriched quadrant of the “mantle array”. Tholeiitic and alkaline Ceara-Mirim dykes are related to different parent magmas probably originating by different degrees of melting of garnet-peridotite mantle sources characterized by distinct Sr/1bNd isotope compositions and small-scale chemical heterogeneities. Ceara-Mirim tholeiitic dykes show close mineralogical, chemical and isotopic similarities with the Lower Cretaceous high-TiO2 tholeiites from Maranhao (Sardinha intrusives) and Parana (Northern Province) basins. In general, chemical and Sr/1bNd isotope data for high- and low-TiO2 Brazilian Mesozoic tholeiites appear to be related to time-integrated enriched and depleted mantle components, respectively. Subcontinental mantle heterogeneity is believed to be due to variable “enriched” components related to “metasomatic” processes. The Ceara-Mirim Jurassic dykes correlate well in terms of composition and tectonic setting with the analogue tholeiitic dyke swarms from the Benue trough, and can be related to the opening of the Central Atlantic Ocean. The Lower Cretaceous Ceara-Mirim magmatic activity corresponds to the coeval one of the Benue trough and can be related to the early rifting events responsible for the opening of the Equatorial Domain of the South Atlantic whose oceanic crust formed between 120 and 100 Ma.