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The comparative strontium isotopic composition of alkaline rocks: New data from Southern Portugal and East Africa

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Abstract

Rb-Sr whole rock isochrons for the Monchique and Mt. Kenya complexes, together with a series of 87Sr/86Sr ratios for 7 other alkaline suites, reveal limited initial ratio variation within the oceanic basalt range. Such variation is ubiquitous in alkaline suites and renders interpretation of their Sr behaviour highly subjective; this has led to major interpretive inconsistencies between previous studies. A consideration of wall-rock reaction, responsible for a few anomalous ratios in the present data, is believed to reconcile many of these inconsistencies; the subjective element can also be minimised by using the total of available data as a basis for comparative interpretation instead of interpreting individual sets of data absolutely. This approach shows available data to be remarkably unified, implying less diversified petrogenesis than is apparent from the sum of interpretations for individual alkaline complexes. In particular, Sr data lend little support to various models invoking crustal participation in the origin of nephelinites, nepheline syenites, ijolites or peralkaline oversaturated rocks, but suggest that these generally are straight-forward derivatives of mantle liquids. The consanguinity of most alkaline complexes is also confirmed.

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  1. Department of Mineralogy and Petrology, University of Cambridge, England

    N. M. S. Rock

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Rock, N.M.S. The comparative strontium isotopic composition of alkaline rocks: New data from Southern Portugal and East Africa. Contr. Mineral. and Petrol. 56, 205–228 (1976). https://doi.org/10.1007/BF00399605

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