Faciological and petrological characterization of basic sillsin the Picos-PIregion, eastern portion of the Parnaíba Basin

Two major basic magmatic events from the Mesozoic have been identified in the Parnaíba Basin. The first, associated with the Jurassic - Triassic boundary, is correlated with the Central Atlantic Magmatic Province (CAMP) and is represented in the basin by the Mosquito Formation. This event occurs mainly as lava flows and intrusions of dikes and sills in the western portion of the Basin. The second event is linked to the Cretaceous and is represented by the Sardinha Formation. It consists predominantly of intrusive rocks such as dikes and sills in the eastern part of the basin; however, its magmatic origin remains debated. This study aimed to characterize the intrusive rocks in the Picos region of the eastern Parnaíba Basin through facies identification and classification, detailed petrographic analysis of the bottom, core, and top sections, and major-element geochemistry. Dolerite sills were described and categorized into regular columnar facies characterized by cooling fractures and massive dolerite. Additionally, the rocks were subdivided into phaneritic and aphanitic types. Subophitic, intergranular, and granophyric textures were observed in the phaneritic dolerite, while the aphanitic ones exhibited porphyritic, intersertal, and intrafascicular textures, typical of rapid crystallization. Geochemically, the rocks were classified as basic to intermediate, metaluminous, high-TiO₂, and tholeiitic. These results indicate that the dolerite underwent distinct crystallization processes during emplacement, with the aphanitic types suggesting that aphanitic types experienced higher cooling rates. Based on petrographic and geochemical similarities with previous studies, we suggest that these rocks belong to the Sardinha Formation.