Alkalic Igneous Rocks on the Continents

Abstract

Alkalic igneous rocks are characterized by elevated concentrations of Na₂O and K₂O and typically have low concentrations of SiO₂, CaO, MgO, and FeO compared to silica-saturated granitic rocks. They are also characterized by high abundances of relatively rare elements including P, F, CI, Zr, Ti, Nb, Ta, and REEs. Alkali-rich plutons tend to be elliptical or circular in plan and are smaller in volume than plutons of granitic rocks. They are composed of alkali feldspar, feldspathoids (e.g. nepheline and leucite), and alkalic pyroxenes or amphiboles. Above all, the alkali-rich volcanic and plutonic rocks are characterized by a wide range of chemical and mineral compositions requiring elaborate classification schemes (e.g. Streckeisen 1967; Middlemost 1975, 1980; LeBas et al. 1986; Wilkinson 1986). Cox et al. (1979) proposed to classify alkali-rich volcanic rocks on the basis of Na₂O + K₂O and SiO₂ (Fig. 2.2, Sect. 2.1). In addition, kimberlites and carbonatites are included among the alkalic rocks because kimberlites have high concentrations of Na₂O and K₂O, whereas carbonatites are commonly associated with differentiated alkali-rich plutons (Mitchell _1995; Harmer 1997; Bell 1989; Heinrich 1966). Alkali-rich igneous rocks are considered in all textbooks on igneous petrology (e.g. Turner and Verhoogen 1960; Cox et al.1979; Best 1982; Wilson 1989). In addition, numerous monographs have been published on alkali-rich rocks (e.g. Sørensen 1974; Palmason 1982; Fitton and Upton 1987).