Geochronology and petrology of Cretaceous basaltic magmatism in the Kwanza basin (western Angola), and relationships with the Paranà-Etendeka continental flood basalt province

Abstract

Early Cretaceous tholeiites of the Kwanza basin, western Angola, 300 km South of Luanda, are evolved basalts to basaltic andesites, characterized by low TiO 2 (<2 wt%) and incompatible element contents. Their petrographical, major and trace element and Sr-isotopic (0.70448–0.70752) compositions are similar to those of tholeiites of the Paranà-Etendeka flood basalt province, particularly to those of the southern sector. While inland analogues, i.e. northern Paranà tholeiites are high-TiO 2 basalts, Kwanza tholeiites in western Angola, and flood tholeiites of the Campos marginal basin and Cabo Frio dolerites in southeastern Brazil, define a low-TiO 2 (<2 wt%) basalt ‘belt’ in the northernmost Paranà-Etendeka. The main pulse of tholeiitic magmatism in the Kwanza basin (131.9±1.6 and 131.6±1.4 Ma, 40Ar/ 39Ar plateau ages), is contemporaneous with flood volcanism in the Paranà-Etendeka (133–131 Ma), which therefore extends by c. 400 km more to the N-E than previously recognized. Slightly younger Kwanza coast-parallel tholeiitic dykes have an age of 126.1±1.4 Ma, similar to those of other coast-parallel dyke swarms in Brazil and Namibia, and are probably associated with the main rifting stages. A younger magmatic episode in central-western Angola is represented by Late Cretaceous sodic alkaline and transitional volcanic rocks. They are intercalated in Late Cretaceous marine sediments, and one alkaline basalt was dated at 91±2 Ma. Alkaline and transitional rocks have similar, OIB-like incompatible element patterns with mantle normalized Nb/K and Nb/La>1.0. Initial 87Sr/ 86Sr ranges from 0.70297 to 0.70302 and from 0.70287 to 0.70495 for alkaline and transitional rocks, respectively. Sodic alkaline and transitional rocks from the Kwanza basin differ in composition from most coeval alkaline rocks of southeastern Brazil, which are of potassic type.