Structural controls and 40Ar/39Ar geochronological data of basic dike swarms in the eastern domain of the Parnaíba Basin, northeast Brazil

Abstract

Two important magmatic events related to the breakup of the Pangea supercontinent and the opening of the Atlantic Ocean are represented, in the Parnaíba Basin (Northeast Brazil), by volcanic intrusive and subordinate extrusive rocks ascribed to the Mosquito (Early Jurassic) and Sardinha (Early Cretaceous) magmatic suites based on K–Ar and Ar/Ar dates. In the southeastern and central-eastern regions of the basin, the basic dike swarms display either a consistent or different orientations at given places. These rocks were elected for Ar/Ar geochronological dating to verify whether they have different ages or different emplacement models, integrating geochronological and tectonic-structural approaches. Structural analysis of remote sensor images and field data allowed to characterize (i) a family of consistent NE-trending lineaments in the southeastern border of the basin and (ii) three sets striking NE-SW, ENE-WSW, and NW-SE, in the central-eastern region. In outcrops, these swarms correspond to diabase dikes and sills associated with fracture and fault systems also recorded in their older host sedimentary rocks of the basin. In the southeastern region, the NE-trending dike system, correlated to the Sardinha Suite was controlled by extensional fractures and faults with the same orientation, arranged adjacent to and along the basin border. These structures indicate a NW extension, well documented further east in the continental rift basins of Northeast Brazil, related to the onset of the South Atlantic opening in the Early Cretaceous. Further north, in the central-eastern region of the basin, the emplacement of a WNW-trending dike, dated at 135 Ma, is interpreted as controlled by a strike-slip sinistral fault also related to the NW extension. The second group of dikes in this region presents older ages (190–200 Ma range), being thus correlated to the Mosquito Suite, controlled by NNW extension and well exposed in the central-western region of the basin. These older dikes, previously poorly known in the eastern domain, display variable orientations but following a NE-trending swarm of fractures along the NE-trending Transbrasiliano Lineament. It is proposed that the Mosquito Suite dikes in this domain were emplaced along coeval and older (Permian to early Triassic) structures reactivated by the NNW-SSE extension during the opening of the Central Atlantic Ocean, at the transition between the Triassic and the Jurassic.