Sixty-four Rb-Sr and two K-At isotopic measurements from seven ring complexes in central Nigeria provide evidence for a systematic age trend along a 200 km zone ranging from 1744-5 m.y. in the north to 154-4-4 m.y. in the south. A peak of anorogenie magmatism occurred in the Jos Plateau region about 164β’ ago. Although a small syenitetrachyte complex at Zaranda, near Bauchi, gives an age of 190β’ 15 m.y., unpublished ages of 290-330 m.y. for the southern Niger ring complexes confirm the existence of an overall southerly decreasing age trend in the Niger-Nigeria province of West Africa.
Isotopic measurements on two small, oversaturated syenite intrusions at Zaranda and Pankshin suggest that syenitic liquids had initial s~Sr/SSSr ratios of 0.7048---not significantly different from the mantle range of values, but that related peralkaline silicic variants from the same complexes are depleted in total Sr and have higher sTSr/S6Sr initial ratios characteristic of the earth's crust. This variation of initial sTSr/S~Sr ratios in syenite-related granitic liquids of the peralkaline spectrum has also been noted at the Shere Hills, near Jos, and at Liruei, near Kano, and may be representative for all syenite-granite occurrences in the Nigerian Younger Granite province. Such isotopic variations in the initial sTSr/S6Sr ratio may be attributed to "crustal enrichment" of syenitic liquids whose source lies in the mantle.
Coarse-grained, peraluminous biotite granites have consistently low initial s~Sr/S6Sr ratios in the range 0.706-0.709 (similar to the ca. 600 m.y. Pan-African granites of the basement), and may represent further modifications of originally syenitic liquids in the crust, or the granites may have originated from an independent source within a "dioritic" lower crust.
Although the magmatic trends show small variations in the initial s~Sr/S6Sr ratio, much higher initial ratios are recorded in granites which have been modified within their roof zone by dcuteric (autometamorphic) and/or metasomatic processes.