Geochronology and geochemistry of igneous rocks of the Dassa region, Central‑Benin: evidence of an Ediacaran emplacement of alkali‑calcic and alkaline plutonic and volcanic magmas

Abstract

metacraton. This region, with widespread granitoids and volcanites, is cut by the Kandi Shear Zone (KSZ) that could be connected to the 4°50’ shear zone northwards in the Hoggar mountains and to the “Transbrasiliano lineamento” Northeast Brazil. The LA-ICP-MS U–Pb isotopes of zircon, whole-rock geochemistry and Sr–Nd isotope were analysed to constrain the emplacement ages and chronology of granitoids and volcanic rocks in the Dassa region in relation to the KSZ dynamics and the geochemical signatures and genesis of the magmas. Five intrusions (Dassa, Tré, Gobada, Tchetti and Fita) with the igneous rocks of the volcano-sedimentary basin of Idaho-Mahou have been investigated. Radiometric ages (U–Pb) obtained on zircons from the intrusions and the alkaline rhyolites from the Idaho-Mahou volcano-sedimentary basin indicate the Ediacaran timing emplacement in the Pan-African Dahomeyide orogen. The Dassa intrusion is dated at 633 ± 19 Ma while the Tchetti, Gobada and Tré intrusions yield 607 ± 11 Ma, 603 ± 10 Ma and 595 ± 12 Ma, respectively. The alkaline Fita granite and rhyolite of the Idaho-Mahou volcano-sedimentary basin provide U–Pb on zircon ages of 594 ± 32 Ma and 583 ± 9 Ma, respectively. Geochemical investigations reveal that the granitoids are ferroan and metaluminous; alkali-calcic (Dassa and Tré) and alkali-calcic to alkalic (Gobada, Tchetti and Fita). The mafic to intermediate volcanites and trachyte of Idaho-Mahou volcano-sedimentary basin are alkali-calcic whereas the rhyolite displays an alkalic signature. Sm–Nd isotopic analyses reveal that the ε Nd values at c. 600 Ma in the Fita granite and alkalic rhyolite of Idaho-Mahou basin are the highest (− 6.94 and − 6.07). In the other intrusions, the ε Nd values at c. 600 Ma are lower, ranging between − 9.66 and − 9.28 (Tchetti and Godada granites) and − 8.84 and − 8.56 (Dassa and Tré granites). The Tré monzonite shows an intermediate value around − 7.70. The calculated TDM model ages are very close and range between 1.80 and 2.05 Ga. All this suggests hybridization processes between an Archean to Paleoproterozoic continental crust source and a juvenile mantle (600 Ma) source for the parent magmas. But the significantly high ε Nd values of Fita granite and the alkaline rhyolite are in agreement with higher mantle contribution and a distinct more enriched mantle source for the parent magmas of alkaline rocks compared to the alkali-calcic felsic ones for which the mantle contribution is from the depleted mantle. According to these data, the Ediacaran magmatic events in the Dassa region can be divided into two main episodes with respect to the main transcurrent shearing event that started at about c. 610 Ma: a pre-transcurrent shear alkali-calcic plutonic magmatism c. 635–610 Ma and the transcurrent shear magmatism c. 610–580 Ma with contemporaneous alkali-calcic and alkaline plutonic and volcanic magmatism. The integration of existing U–Pb on zircon dating in the Benin basement indicates that in the Dahomeyide orogen, the reactivation of the Kandi shear zone toward the end of Ediacaran timing at around c. 550–540 Ma allowed emplacement of the third suite of magmatism as intrusions and volcanoes.