Leucite- and Feldspar-Bearing Quaternary Joins and Systems

Abstract

The bulk compositions of plagioclase-bearing potassium-rich undersaturated rocks such as leucite tephrites and leucite basanites lie within the system forsterite-diopside-leucite-anorthite. These rocks occur in the Somma-Vesuvius region of Italy (Savelli 1967), the Birunga area of Uganda (Holmes and Harwood 1937; Ferguson and Cundari 1975), and the East Eifel region of West Germany (Duda and Schminke 1978). In these regions these rocks are often associated with phlogopite and/or olivine-bearing leucitites. In Fig. 2.3 compositions of some of these mafic and ultramafic rocks are plotted. A study of the system forsterite-diopside-leucite-anorthite may thus help to understand the paragenetic relationships between leucite-bearing tephrites and basanites and more mafic potassium-rich lavas without plagioclase.