The Archenu complex intrudes Precambrian metamorphic basement rocks and overlying Cambro-Ordovician sediments. The 20x14 km intrusion consists of a series of ring-dykes and sheets, which were emplaced about several centres, remnants of pyroclastic rocks and numerous phonolite sills lying within sediments to the north of the central complex (Hunting Geology and Geophysics Ltd, 1974). The volcanic rocks are preserved principally within the eastern part of the complex and consist of phonolitic and trachytic tuffs and agglomerates, some of which include pebbles of sedimentary as well as igneous rocks. The earliest intrusive phases are microsyenite and phonolite ring-dykes which may contain feldspar or nepheline phenocrysts. These earliest rocks are cut by fine- to medium-grained syenite sheets which, however, also cut a syenite that is generally considered to belong to a later phase, indicating an extended period of emplacement of the syenite sheets. The next major event was emplacement of a large nepheline syenite ring-dyke. Subsequent intrusions, also ring-dykes, are syenites and nepheline syenites which reach 1500 and 1750 m in thickness and are variably porphyritic with pegmatitic segregations common. There is almost a continuum from perthosites to nepheline syenites with >20% nepheline. The whole complex is cut by a variety of dykes and shallow-dipping sheets of syenite, microsyenite, trachyte and occasionally basalt. Flinn et al. (1991) give petrographic details of a range of rocks together with data on feldspar, nepheline, analcime, sodalite, zeolites, pyroxene, amphibole, biotite, titanite, lorenzenite, eudialyte, catapleite, lavenite, cancrinite, magnetite and ilmenite. Modal and chemical data for 19 specimens, including trace elements and a full range of REE for eight of them, are presented and discussed by Flinn et al. (1991).
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