Western Hoggar

From an area of about 4000 km2 of the Pan-African belt of western Hoggar, west of the In Ouzzal granulite block, Dostal et al. (1979) have described a sequence of metamorphosed alkaline intrusions and dyke complexes. Within the Proterozoic terrains they distinguish five generations of subalkaline, alkaline and peralkaline igneous rocks. The first generation rocks are pre-tectonic intrusions of middle Proterozoic age which form still recognisable metamorphosed sills and laccoliths, the sills being subalkaline rhyolite but the laccoliths fluorite-bearing alkaline microgranite, albite-biotite granite and rare syenite. The intrusive rocks in places grade laterally into layered and banded orthogneisses, in which magmatic layering, sometimes rhythmic, is well preserved up to the sillimanite zone. They consist essentially of quartz, K-feldspar and albite with minor biotite, amphibole, and/or pyroxene and opaques; accessories include allanite, fluorite, zircon and rare Zr silicates. Although the amphibole is usually ferrohastingsite and the pyroxene hedenbergite riebeckite-arfvedsonite and aegirine-augite and aegirine also occur. The second generation rocks are pre-tectonic, metamorphosed dykes which outcrop over an area of about 500 km2 along the western margin of the Pan-African belt in a north-south zone. The dykes include rhyolite, felsite, albite trachyte, granite, syenite and phonolite. The finer grained rocks, particularly trachytes and phonolites, still preserve their igneous textures and the mineralogy varies from primary igneous to metamorphic. K-feldspar and albite are the principal feldspars with either quartz or nepheline, ferrohastingsite and Fe-rich biotite but, in peralkaline varieties, riebeckite and aegirine. Syenites and trachytes are all rich in fluorite and zircon. The third generation also comprises dykes but these are syntectonic and peralkaline and confined to a 120 km2 area. They are up to several kilometres in length and vary in thickness from 10 to several 10s of metres. The dykes have gneissose structures having been deformed and metamorphosed to greenschist facies with metamorphic growth of aegirne-augite and aegirine and riebeckite-arfvedsonite. They contain quartz, albite and microcline and were originally probably pantellerites and microgranites. The fourth generation is represented by pre- to syn-tectonic peralkaline granites of the Timgaouine area but little detail is given by Dostal et al. (1979) except for the facts that they form an elongated, boudinaged body and contain iron-rich biotite, riebeckite and rare aegirine. The fifth and youngest generation consists of a post-tectonic peralkaline granite in the northern part of the area near Taoudrart with spatially associated flows and ignimbrites of rhyolite and subordinate pantellerite. The granite is 5x1 km and comprises a complex of dykes 1-5 m thick of granite, microgranite and rare gabbro. The granites are leucocratic and consist of quartz, perthite and minor riebeckite, aegirine and stilpnomelane. Large numbers of rock analyses, including some trace elements, have been obtained by Dostal et al. (1979).