A Pb, Sr and Nd isotope study of the basement and mesozoic ring complexes of the Jos Plateau, Nigeria

Abstract

Combined Pb, Sr and Nd isotope determinations on several Nigerian Mesozoic (∼ 170 Ma) ring complexes, studied previously by van Breemen and co-workers, indicate a multistage petrogenetic process. Mantle-derived differentiated magmas assimilated crustal basement of average Early Proterozoic age. After crystallisation, some plutons were subjected to a second stage of crustal contamination by circulating hydrothermal fluids.

Crustal compositions were constrained by isotopic analysis of the Proterozoic basement of the Jos Plateau. Sm/Nd analysis of six gneisses yielded an average crustal residence age of 2 Ga, corresponding to the Burkinian event recognised elsewhere in western Africa. However, one sample yields a model age of 3 Ga, suggesting the presence of Archean crustal remnants. Pan-African granitoids yield a similar range of Nd model ages to the gneisses, suggesting that they were largely generated by crustal melting.

The Zaranda anorogenic complex has relatively radiogenic initial Nd and Pb isotope compositions and unradiogenic Sr (∼ 0.5126, ∼ 18.4 and ∼ 0.705, respectively), attributed to a mantle-derived differentiated magma which suffered moderate contamination during ascent through the crust. Other ring complexes trend toward less radiogenic Nd and Pb isotope ratios and more radiogenic Sr, indicative of an increasing crustal contribution. Initial Pb isotope compositions yield a well-defined Pb/Pb isotope array with a slope age of ∼ 1.8 Ga which is consistent with the average Nd crustal residence ages of basement gneisses and granitoids.

The arfvedsonite albite apogranite from the Ririwai anorogenic complex has isotope ratios resembling Pan-African basement, probably resulting from hydrothermal overprinting with fluids equilibrated in the continental crust. Other Ririwai intrusions and one unit from the Shere Hills display evidence of hydrothermal overprinting of Sr and to some extent Nd isotope compositions, but only the Ririwai apogranite has been significantly overprinted by hydrothermal Pb.

The isotopic evidence supports a model for the Mesozoic anorogenic (“A-type”) granites of Nigeria in which mantle-derived magmas suffered crustal contamination during magmatic differentiation to syenitic compositions, followed by sub-solidus hydrothermal alteration in the continental crust.