Major, Trace Elements and Sr-Nd Isotopic Characteristics of High-Pressure and Associated Metabasites from the Pan-African Suture Zone of Southern Togo, West Africa

摘要

Eclogites and garnet-bearing or garnet-free amphibolites from Lato, Toutouto and Ahito-Meliendo hills in southern Togo, were analyzed for their bulk rock geochemistry and Sr and Nd isotopic compositions. Major and trace element compositions for these rocks indicate that precursor magmas belong to tholeiitic to subalkaline series which may have evolved by fractional crystallisation. However, some intense sheared eclogites from southern Lato hills display secondary LREE-loss patterns not typical of magmatic protoliths. Measured ratios and epsilon values have been recalculated at 800 Ma. 87 Sr/ 86 Sr initial values for the eclogites and amphibolites range from 0.70215 to 0.70460 and from 0.70126 to 0.70307, respectively. 147 Sm/ 144 Nd values for both eclogites and amphibolites range from 0.14510 to 0.183320 and are lower than chondritic values of 0.1966. Overall 143 Nd/ 144 Nd values range from 0.512804 to 0.512862, corresponding to initial ? Nd (T) from +4.9 to +8.8 that yielded T DM model ages from 1.6 to 0.72 Ga. One eclogite from south Lato hill displays 143 Nd/ 144 Nd values of 0.512862, corresponding to initial ? Nd (T) values of +8.83 and T DM model age of 0.72 Ga, that constrains its minimum age. However, eclogites from northern Lato and garnet amphibolites yielded older model ages of 1.65 and 1.43 Ga respectively, suggesting crustal contamination of the surrounding metasedimentary pile. Key words: Eclogites, Sr-Nd isotopes, Pan-African suture zone, Togo.