Abstract
The elements Rb, Sr, Yt, Zr, Nb and Ta have been determined in a variety of alkali igneous rocks and concentration values have been compared with those found in some of the Younger Granites of N. Nigeria. Na-rhyolites, comendites, pantellerites and related rocks from different localities show some close similarities. Nb and Ta both tend to become enriched in (these) late stage differentiates and can exceed 200 p.p.m. and 10 p.p.m. respectively; Zr is similarly high and can exceed 2000 p.p.m. There is appreciable variation in the K/Rb ratio but less in that of Nb/Ta for rocks from the same petrographic province. Data for acid rocks containing Na-amphibole are compared. Nb increases as Sr decreases and becomes conspicuous when Sr is below the detection limit of 40 p.p.m.; this tends to correspond to the appearance of riebeckite. In contrast Sr and Nb can both be high in under-saturated alkali plutonic and volcanic rocks.
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