ISSN:
1432-0967
Source:
Springer Online Journal Archives 1860-2000
Topics:
Geosciences
Notes:
Abstract Sixty-four Rb-Sr and two K-Ar isotopic measurements from seven ring complexes in central Nigeria provide evidence for a systematic age trend along a 200 km zone ranging from 174±5 m.y. in the north to 154±4 m.y. in the south. A peak of anorogenic magmatism occurred in the Jos Plateau region about 164±4 m.y. ago. Although a small syenitetrachyte complex at Zaranda, near Bauchi, gives an age of 190±15 m.y., unpublished ages of 290–330 m.y. for the southern Niger ring complexes confirm the existence of an overall southerly decreasing age trend in the Niger-Nigeria province of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 87Sr/86Sr 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 87Sr/86Sr initial ratios characteristic of the earth's crust. This variation of initial 87Sr/86Sr 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 87Sr/86Sr 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 87Sr/86Sr 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 “dioritio” lower crust. Although the magmatic trends show small variations in the initial 87Sr/86Sr ratio, much higher initial ratios are recorded in granites which have been modified within their roof zone by deuteric (autometamorphic) and/or metasomatic processes.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1007/BF00371037
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