Geochemistry and Sm-Nd isotopic composition of the Agou Igneous Complex (AIC) from the Pan-African orogen in southern Togo, West Africa: Geotectonic implications

摘要

Major, trace element concentrations and Nd, Sr isotopic data are reported for Neoproterozoic high-pressure (HP) mafic granulites from the Agou Igneous complex (AIC), which marks the suture zone of the Pan-African Dahomeyide orogen in southwestern Togo. The main rock types consist of garnet-free and garnet-bearing metagabbronorites (GFM and GBM) that are cross cut by subordinate leucocratic to melanocratic veins and dykes of garnet-bearing microgabbroic rocks. Geochemical data suggest that the HP granulites may have preserved the geochemical imprints of their igneous protoliths. They display either cumulitic features or liquid compositions varying from primitive to evolved basalts and andesites and then to more differenciated dacitic magmas as supported by major and trace element contents. Primary basalts display LREE and LILE enriched patterns and no Eu anomaly, but are depleted in Y and Yb suggesting a slightly enriched mantle source or a weak crustal contamination. Mafic veins and dykes display REE flat patterns (La_N/Yb_N = 0.58-1.7) and a slightly negative Eu anomaly and ΣREE content ranging between 10 and 20. These features are consistent with protoliths of N- to E-MORB affinities. Mafic cumulates and andesitic basalts are characterized by various REE patterns which could be compared to those of the primary and/or the evolved basalts, probably identifying various igneous suites. The fractionated REE patterns (Ce/Yb_N ≈11.0-17.9) with slight LREE enrichment (Ce/Sm_N≈ 3.1-3.8) and HREE depletion (Sm/Yb_N ≈ 3.6-4.8) could be induced by a garnet residue in the melt source. Neodymium isotopic data show that most mafic cumulative rocks have similar ~(143)Nd/~(144)Nd ratios between 0.5127 and 0.51289, corresponding to initial ε_(Nd)(T) values between +2.26 and +6.33 that yielded T_(DM) ages from 1.2 to 0.94 Ga. The mafic mesocratic vein displays ~(143)Nd/~(144)Nd values of 0.51227, corresponding to initial ε_(Nd)(T) values of +6.87 that yielded T_(DM) = 0.94 Ga, and constrains the minimum age. However primitive basalts samples A17 (GFM) and other mafic cumulate Ag224 (GBM) give an older depleted mantle source derivation age of 2.31 and 1.68 Ga respectively, suggesting crustal contamination by the surrounding gneissic Paleoproterozoic basement. The Nd isotopic compositions suggest that the parental magmas of the Agou Igneous Complex in southern Togo were derived from time-integrated depleted mantle source. Such geochemical features are consistent with an active continental margin environment and suggest that the AIC represents a magmatic body emplaced and cooled at the base of a continental crust above a subduction zone.