Mesozoic igneous activity in the Maranha˜o province, northern Brazil: 40Ar/ 39Ar evidence for separate episodes of basaltic magmatism

Low- and high-Ti basaltic rocks of the Maranhao province in northern Brazil appear to be related to the opening of the Atlantic Ocean. Ar-40/Ar-39 dating was carried out on powdered whole-rock basalts, both washed and unwashed in dilute nitric acid, to document this relationship. Acid treatment does not alter the argon retention properties of retentive phases, but is effective in removing alteration phases (high Ca/K) degassed at low laboratory temperatures. Our results identify that at least two periods of volcanic activity formed the Maranhao province. Low-Ti basalts from the western sector are similar to 200-190 Ma and are related to the opening of the North Atlantic. High-Ti basalts from the eastern sector are similar to 129-124 Ma, slightly younger than compositionally similar rocks in the Serra Geral flood basalt province of southern Brazil, and are related to the opening of the South Atlantic. Other flood basalts, such as the Columbia River, Deccan, Rajmahal, Ferrar and Siberian provinces, were each predominantly formed in 1-2 Ma. The Maranhao basalts, remnants of separate igneous events spanning similar to 70 Ma, should not be classed as a single flood basalt province. The temporal complexities noted in the Maranhao basaltic rocks appear to be related to the tracks of hotspots that led to the rifting of the Atlantic Ocean. Two sets of Mesozoic basaltic rocks occur in southern Africa, India and South America, resulting from events leading to the fragmentation and dispersal of sections of Gondwanaland; in each (sub)continent the two provinces were formed by events separated by similar to 50-70 Ma. Of these pairs, only the Maranhao and Serra Geral show geochemical similarities, indicating compositionally distinct upper-mantle source regions under sections of Brazil separated by similar to 3000 km. (C) 1997 Elsevier Science B.V.