LAUSR

Abstract The intracratonic Parnaiba basin, in northeast Brazil, records episodes of tholeiitic magmatism characteristic of Large Igneous Provinces (LIP's). They are represented by the Mosquito (Triassic-Jurassic) and Sardinha (Cretaceous) formations related to tension and rupture processes of Pangea and Gondwana, respectively. The Mosquito (mostly basaltic flows) and Sardinha (mostly diabase sills) formations include low- and high-TiO2 suites. This paper presents the results related with the petrogenesis of a 43 m-thick diabase sill sampled from the 1UN-26 borehole within the eastern portion of the Parnaiba basin. This stratified magmatic body comprises three petrographic facies: fine grained basalt (chilled margin); medium-grained diabase (intermediate portion) and coarse-grained gabbro (inner portion of the sill). The rocks are tholeiitic basalts and andesitic basalts which define two chemically distinct groups: high-TiO2 tholeiites (TiO2 > 2 wt %, inner body portion) and low-TiO2 tholeiites (TiO2