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

The Paraná Continental Flood Basalt Province (PCFB), one of the largest magmatic provinces in Planet Earth, is composed of lava flows, dike swarms and sills that occupy thousands of kilometers across central-eastern South America. The Campinas-Jaguariúna Sill (CJS) is a large (<50 m thick and >100 km 2 ) diabase intrusion showing frequent medium to coarse-grained gabbroic segregations and was emplaced at the interface between the Precambrian basement and sedimentary rocks in the northeastern Paraná Basin. This paper applies aeromagnetic and geochemical modeling to help understand the emplacement mechanisms and petrological evolution of the CJS. Magnetic field filtering techniques demonstrates that the CJS continues for tens of kilometer in the subsurface; analogous features are shown to be associated with other neighbor diabase occurrences, creating lobe-shaped structures that seem to freeze their original geometry and are not fully exposed by the current erosion levels. Petrological modeling indicates that the parental magma of the CJS was emplaced at ~1 kbar, 1000 ppm H 2O and oxygen fugacity near FMQ-1. The gabbroic segregations, exposed as cm-to dm-sized sheets and eyeshaped lenses, are more evolved than the host diabase and can be shown by geochemical modeling to correspond to residual melts formed by crystal fractionation in a closed-system. The parent magma of the CJS is akin to the more primitive Paranapanema magma-type of the PCFB and we tested the possibility that sills intruding upper sedimentary layers may be part of a single sill complex. For this purpose we used geochemical data from the well-studied Limeira Sill, which is akin to the more evolved Pitanga magma-type. Our modeling shows that under crystal fractionation in the representative system condition a CJS starting compositions cannot generate the Limeira rocks.

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