Chemical and Sr-Nd compositions and 40Ar/39Ar ages of NW-trending dolerite dikes of Burkina Faso: Evidence for a Mesoproterozoic magmatism in the West African Craton

Abstract

The Paleoproterozoic basement of the northeastern part of the Leo-Man craton is intruded by generally NW-trending dikes. These regional scale dikes extend over 1000 km in Burkina Faso, Mali and Niger. We present chemical and Sr-Nd isotope compositions, as well as 40Ar/39Ar ages of these dikes with the following strikes N98¢X¡VN112¢X and N114¢X¡VN124¢X in NE Burkina Faso. Field relationships show that the dikes are posterior to all other rock types dated between 2.26 Ga and 2.0 Ga. Chemical data indicate that the dikes are continental flood basalts and composed of low-Ti (TiO2 ? 2 wt.%) sub-alkaline basalts and andesites. They exhibit a minor negative Europium anomaly (0.86¡V0.99) and slightly fractionated REE patterns ((La/Yb)N = 2.5¡V9.1; YbN = 9.5¡V19.9). The ratios of Th/Ta (1.3¡V11.4) and Ce/Pb (5.2¡V58.5) suggest a varying crustal assimilation of the dike magmas during ascent in the continental crust for all studied samples. Calculated P-T conditions indicate that the magma reached temperatures of 1285 ¢XC (calculated from olivine compositions) and pressures of 6.9 kbar (calculated for pyroxene minerals). Calculated initial 87Sr/86Sr (0.70040¡V0.70260) and ?Nd(t) = +2.1 to ?3.5 at 1575 Ma, also point to a crustal contamination with the most primitive samples showing TDM values of 1946 Ma and 2154 Ma. The low values of La/Ba (<0.2) and Nb/La (<1.0), contrast with the low Th/Nb (<0.9), and suggest a lithospheric mantle or subduction-modified mantle as possible source for the dikes. Sr-Nd data, Mg# and Nb-Ta-Zr-Y-Th-Tb-Yb compositions further suggest that the most primitive samples were emplaced in a none orogenic setting and their magmas were subjected to variable crustal contaminations. Literature and the present whole rock 40Ar/39Ar age determinations show that the dikes were emplaced during a widespread Mesoproterozoic magmatism between 1.6 Ga and 1.2 Ga, and were affected by a thermal event causing the argon systematics resetting, best constrained by the date of sample KK1 (1236 ¡Ó 20 Ma, 40Ar/36Ar = 294 ¡Ó 13, MSWD = 2.2). Contemporaneous 1590¡V1570 Ma extensive magmatism is reported in other crustal blocks in Baltica (Sveconorw-Goth, svecofennian) NW Laurentia (Slave craton, Yukon), and Australia (Gawler craton), and together with the 1575 studied dikes, are related to the breakup of the supercontinent Nuna. ? 2018 China University of Geosciences (Beijing) and Peking University