Zuku

This is a ring-complex composed predominantly of granite but there are also significant areas of volcanic rocks. An irregular ring-dyke encloses the complex on all but the northwestern side and in the south and east encompasses an arcuate area of basement rocks; outside the ring-dyke swarms of cone-sheets are developed in three areas. The oldest rocks are volcanics and vent agglomerates which are concentrated in the northern part of the complex. The volcanic rocks comprise glassy and flow-banded rhyolites that generally dip steeply and vary greatly in colour, texture and degree of brecciation. The most extensive mass consists of breccias and agglomerates, mainly of rhyolitic composition, but with some basement blocks. These appear to represent remnants of a large vent. The cone sheets are probably contemporary with the volcanics and are felsites and quartz porphyries many of them being spherulitic and flow banded. The most extensive intrusion is of pyroxene-quartz syenite porphyry which dominates the northern part of the complex and occupies about half the area. It carries numerous xenoliths and large pendants of basement migmatites and is surrounded by a broad aureole of veins and dykes. The syenite is texturally variable and sometimes becomes equigranular rather than porphyritic. The usual rock carries phenocrysts of alkali feldspar in a matrix of feldspar and pyroxene which has green sodic rims; there is a little quartz and rare fayalite. In places the pyroxene is surrounded by reaction rims of amphibole. The ring-dyke is of granite/syenite and over much of its length contains numerous pegmatitic dykes and patches and irregular zones of microsyenite and granite. Quartz varies between 10 and 20% modally and the feldspar is perthite; aegirine and sodic amphibole form clusters up to a centimetre or more in diameter and in the northwest aenigmatite is abundant. Three phases of aegirine granite have been mapped (Jacobson and MacLeod, 1977) but all are mineralogically similar with aegirine as the principal mafic mineral but blue sodic amphibole also generally present. A chemical analysis is given by Jacobson and MacLeod (1977).