Pseudoleucite from the Gardar of South Greenland

Abstract

Igneous pseudoleucite has been found for the first time in the Gardar alkaline province of South Green­land. The 10 cm diameter pseudoleucites occur in a trachyte dyke and are comprised of radiating inter­growths of K-feldspar with sericite and cancrinite. The textures are interpreted as being a direct reflection of original nepheline/K-feldspar intergrowths. Major element and trace element data are presented and these show that the pseudoleucite retains the structural formula of leucite but with considerable alkali loss. Consideration of the textural relationships is very important in determining possible and impossible modes of genesis although in this case interpretation is constrained by the very advanced state of altera­tion of the rock. It is proposed that the Gardar pseudoleucites formed by a breakdown reaction of an ori­ginally sodic leucite by reaction involving deuteric waters.