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Grant Institute of Geology, University of Edinburgh, West Mains Road, Edinburgh EH9 3JW, U.K.
Alkaline igneous rocks may be defined as those which have higher concentrations of alkalis than can be accommodated in feldspars alone, the excess appearing as feldspathoids, sodic pyroxenes, sodic amphiboles and other alkali-rich phases. These rocks are, therefore, deficient in silica and/or alumina with respect to alkalis and will have nepheline and/or acmite in their norms. In practice the term ‘alkaline’ is used to encompass a wide range of igneous rocks, not all of which conform to this rigid definition. Carbonatites, for example, are certainly silica-deficient but are rarely alkali-rich. True (nepheline-normative) alkali basalts grade into hypersthene-normative transitional basalts without any obvious change in mineralogy. Since transitional basalts are often closely associated with alkali basalts in the field, they are traditionally regarded as alkaline. It is now usual practice to define alkaline igneous rocks simply in terms of their alkali (Na2O + K2O) and silica contents (see, for example, Le Bas et al. 1986). We have not attempted to review the classification of alkaline igneous rocks in this volume as this has been dealt with elsewhere (e.g. Sørensen 1974; Streckeisen 1967, 1980). The only alkaline rocks not covered in previous reviews are those hydrous mafic to ultramafic hypabyssal rocks known as the lamprophyres. The present volume includes three papers on this group. A comprehensive overview of lamprophyres is given by Rock and of the sub-group of lamproites by Bergman. The relationship between lamproites and kimberlites (which arguably belong to the lamprophyres) is discussed by Dawson.
Volumetrically, alkaline rocks account for
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