Alkalic Rocks and Carbonatites

Abstract

Alkalic rocks and carbonatites are among the rarest igneous rock-types. They tend to occur in stable platform areas and in Precambrian shields, rather than in orogenic belts. Barker (1969) estimated that in North America, alkalic rocks occupy a total surface area of only about 1500 square miles, and Heinrich (1966) concluded that the worldwide areal exposure of carbonatites is only about 200 square miles. Yet Daly (1933) wrote that as many as one-half of the names that have been given to igneous rocks refer to alkalic types. The literature of igneous petrology also reflects the interest which these rocks have held: one complete volume on alkalic rocks (Sorensen, 1972) and two volumes on carbonatites (Heinrich, 1966; Tuttle and Gittins, 1966) are available, and journal articles are numerous. Hamilton (1968) and Powell and Bell (1972) have reviewed the application of strontium isotope studies to the origin of alkalic rocks.