Geochemistry and petrogenesis of alkali basalts from Tahalra (Hoggar, Northwest Africa)

  • Dautria J
  • Dostal J
  • Dupuy C
 et al. 
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Abstract

The Miocene to Quaternary volcanic complex of Tahalra, Hoggar (N.W. Africa) is composed of alkali basaltic rocks (alkali basalts, basanites and nephelinites) containing ultramafic xenoliths and megacrysts mainly of amphibole, clinopyroxene, ilmenite and zircon. The basalts underwent a variable degree of fractional crystallization dominated in the initial stage by olivine and subsequently by clinopyroxene. The basalts show a progressive increase of the degree of undersaturation and contents of K and incompatible trace elements with decreasing age. The temporal variations are consistent with the derivation of the basalts from an upper-mantle source progressively enriched in incompatible elements over time by repeated interactions with a strongly undersaturated magma, probably of kimberlitic composition. The contamination of the upper-mantle peridotite source by a small but variable amount (3.5-5%) of kimberlitic magma may account for the observed variations of the basalts. The megacrysts of Mg-ilmenite and zircon are probably also related to the kimberlitic magma. Amphibole and some clinopyroxene megacrysts, which have compositions similar to those of minerals from the pyroxenite and amphibole-rich veinlets in the peridotite xenoliths, were probably derived by disintegration of coarse-grained ultrabasic rocks. © 1988.

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