Qagssiarssuk

The uppermost parts of a number of diatremes and associated effusives interstratified with sandstones have been preserved in a downfaulted area of approximately 2 km2 south of Qagssiarssuk. To the northwest of this downfaulted area within Precambrian granites and gneisses lower sections through a number of diatremes are exposed. The extrusive rocks are predominantly pyroclastic but with some rubbly flows: they lie above a lower sandstone, within which a basaltic flow occurs, and below an upper sandstone. They change rapidly vertically and horizontally and probably have a thickness of about 375 m, thinning away from the eruptive centres which are concentrated in the eastern part of the area. The tuffs have a high proportion of carbonate and there are numerous layers of inclusions throughout the succession, including angular pieces of orthoclasite and spheroidal bombs of carbonate tuff, 'amygdaloid', and massive carbonate rocks including sovite and lamprophyre. Beds of lapilli tuff include varieties with lapilli of alnoite in a carbonate matrix. The numerous minor intrusions cutting basement north and west of Qagssiarssuk are usually heavily carbonated, steep pipe-like bodies and low angle sheets. Rock types represented include monchiquitic alnoite, mica peridotite, mica pyroxenite and melilite rocks, while breccias fill most of the pipes and consist of country rock fragments, together with lamprophyre, in a carbonate-rich matrix. Intrusions within the downfaulted area south of Qagassiarssuk include tuffisite bodies containing sand-sized grains, coarse breccias, two volcanic vents and a stock of carbonitized melilite rock. Throughout the igneous sequence extensive replacement of olivine, pyroxene and perovskite by carbonate, usually ankeritic/dolomitic, has taken place, but rectangular crystals pseudomorphed by carbonate and interpreted by Stewart (1970) to be after melilite are considered by Deans and Roberts (1984, p. 572) to be after nyerereite. These rocks, now comprising as much as 80% carbonate, have chemical compositions closely comparable with carbonatites (Stewart, 1970, Table 6). Potassium metasomatism of basement rocks around the diatremes has produced rocks with over 11% K2O, while an intrusive orthoclasite gave 13.50% K2O. Stewart (1970, p.62) postulates the presence of a carbonatite body beneath the area.