Nomenclature and petrochemistry of the peralkaline oversaturated extrusive rocks |
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| Authors: | R MacDonald |
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| Institution: | 1. Dept. of Environmental Sciences, University of Lancaster, U.K.
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| Abstract: | Four-hundred and twenty-one analyses of quartz-normative, peralkaline, extrusive rocks have been collected from the literature and from unpublished sources and are used to examine chemical variation in this group of rocks. Comparisons are particularly made between the full body of data and the variations recorded in the non-hydrated obsidians alone byMacdonald andBailey (1973). It is argued that the compositions of the magmas which formed these obsidians and those which subsequently crystallised were similar as regards the major oxides SiO2, Al2O3, FeO + Fe2O3, Na2O and K2O. Marked variations in the abundances of the minor oxides CaO and TiO2 are shown to be a result of geographical location. Small but significant differences in the distribution of Al and Fe as a function of normative quartz can be recognised between various pantelleritic suites. A new classificatory scheme is proposed, based on the iron (as FeO) and Al2O3 contents. This is simpler than previously employed normative classifications, is more applicable to crystalline rocks, and, happily, in 95 % of cases gives the same rock name as the normative system. |
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