Transfusion of quartz xenoliths in alkali basic and ultrabasic lavas, south-west Uganda

Abstract

The occurrence in basic lavas of quartz xenocrysts and xenoliths and the various types of reaction phenomena which they exhibit have been well described by Lacroix (1893, pp. 17-48). The modified quartz is often surrounded by a zone of pyroxene granules or prisms and on the inner side of this zone the quartz may be replaced by glass, both peripherally and through an anastomosing system of vein-like channels. The glass may be clear or charged with gas inclusions; some examples contain minute quartz-pseudomorphs after tridymite, while in others microliths of alkali-felspar occur. In the basanitoid of Hirzstein, Hesse, nepheline occurs around quartz in place of the usual vitreous shell (Fromm, 1891, pp. 58, 65, and 70). Despite these observations, it seems to have been tacitly supposed that the glass is a more or less hydrous variety of vitreous silica.