Dike control on the morphology and formation of granitic inselbergs in Northeastern Brazil

Granitic inselbergs rise from a plain 160 km away from the coast in Northeastern Brazil. They are located in the structural Borborema Province, within the area of the Quixadá Pluton, one of the igneous intrusions from the later stages of the Brasiliano Orogeny (640 – 570 Ma). In this area, monzonites, diorites, and granites sensu sctricto constitute the main granitic facies, with the former being the predominant lithology supporting these landforms. The granites s.s occur as dikes that vary from a few centimeters to a few meters in thickness and form swarms seen within the inselbergs. This study aims to investigate the structural control exerted by dikes on inselberg morphology. Therefore, four inselbergs with a high density of dikes were selected to carry out detailed analysis. Three petrographic types of dikes were identified: fine-grained isotropic dikes (FID), coarse-grained isotropic dikes (CID), and fine-grained anisotropic dikes (FAD), different in terms of their texture and fabric. X-ray fluorescence analysis shows high silica content of the dikes; the first two types with a higher average (82.9 wt %) and the FAD with slightly lower content (76.2 wt%). In contrast, the host rock has lower average SiO₂ content (64.5 wt %) and high percentage of Fe₂O₃ (10.7 wt %), compared with 1.69 wt % in the dikes. The results of Schmidt hammer tests in situ correlate with petrological and geochemical characteristics, attesting to the higher uniaxial compressive strength for finer-grained and more siliceous types. These data suggest higher resistance of dikes to weathering attack in relation to the host rock, which is evident in relief features such as dikes protruding from the main rock mass and dissolution features being obstructed by the presence of felsic dikes. The petrographic, geochemical, and geomechanical characteristics make the dikes significant features contributing to the morphology of inselbergs, especially when combined with the geometry of these parallel dike swarms. In the SW zone of the pluton, the dikes present low to medium dip angles (8 to 34° / NW and SE), which are followed by slope outlines of various inselbergs. In the eastern sector, the trending orientations of dikes (NE-SW) locally coincide with the orientations of linear features such as runnels, which are developed along the dike-host rock contact and therefore mirror their parallel array. We conclude that resistant dike swarms within inselbergs influence final morphological patterns related to slope outline and the location of various minor features resulting from selective weathering, commonly developed next to the dike/host rock contact. Additionally, a 3D sketch model is proposed regarding the relationship between preferential occurrence of inselbergs in areas of gently dipping and parallel dyke arrays, as they act as levels of obstruction of vertical denudation that shapes granite forms in epigenic conditions.