Los Islands (Iles De Los)

The Los Islands are located just off the Guinea coast 5 km southwest of the capital Conakry. The archipelago comprises three major islands together with five islets. The two largest islands, Tamara and Kassa, are arcuate in outline and concentrically arranged around the central island of Roume, the whole group having a diameter of about 12 km. The islands are built essentially of nepheline syenites which are cut by a dyke series which includes ultrabasic rocks and lamprophyres: there are numerous enclaves of basic rocks in the syenites. Lacroix (1911), who describes the rocks in detail and gives numerous photomicrographs, divided the syenites into aegirine- and amphibole-bearing types, which is essentially the classification followed by Moreau et al. (1986) who, however, in a later paper (Moreau et al., 1996) distinguished agpaitic and miaskitic series. In contrast, Lazarenkov (quoted in Kuznetsova and Zagryazhskaya, 1983) delimited and mapped 10 syenite types and extrapolated them between the islands to produce a clear concentric structure. Moreau et al. (1986 and 1996) also recognised a concentric pattern which is reflected in the rocks as a shallow, inward dipping planar fabric. They describe central and external breccias and interpret the occurrence as being a sub-volcanic ring complex. Amphibole-bearing syenites are the principal rock types on the islets and the inner parts of Kassa and Tamara, while pyroxene-bearing syenites define an outer zone to the two main islands and also occur as breccias which form the central island of Roume. Both syenite types have coarse- and fine-grained facies. There are leucocratic and mesocratic varieties of the amphibole syenites which consist of perthite, interstitial nepheline, and sometimes nosean/hauyne, poikilitic amphibole and titanite. Lacroix (1911) describes the amphibole as barkevikite and hastingsite and identified sodalite, wohlerite, lavenite, rinkite, zircon, pyrochlore and melanite in these rocks. The aegirine-bearing syenites are characterised by a trachytic texture and comprise aligned K-feldspar tablets with interstitial nepheline and fine-grained aegirine. The aegirine syenite from the island of Roume is particularly rich in lavenite and astrophyllite and Lacroix (1911) also described from these rocks sodalite, rinkite, biotite, eudialyte, catapleiite, pyrochlore, fluorite, galena, analcime, vishnevite and villiaumite, for which this is the type locality. There are amphibole-bearing aegirine syenites, which form a zone between the two main syenite types (Moreau et al., 1986) in which the amphibole is arfvedsonite. The dykes, of which Moreau et al. (1996) distinguish ring dykes and radial dykes, are numerous, encompass many rock types and include varieties akin to nepheline syenite including microsyenites, pegmatites, aplites and tinguaites; mesocratic and melanocratic rocks include pyroxene-rich types (shonkinite) and essexite, and lamprophyres are represented by camptonite and monchiquite; petrographic descriptions are given by Lacroix (1911). Rock analyses, including a range of trace elements, are available in Gerasimovskii et al. (1970) and further analyses, including Rb-Sr isotopic data, are in Moreau et al. (1996), these authors also giving numerous microprobe analyses of pyroxene, amphibole, mica, astrophyllite, kupletskite, lavenite, titaniferous magnetite, pyrophanite, catapleiite, eudialyte, sodalite, nepheline and analcime. Sulphur isotopic compositions for many rocks are given by Kuznetsova and Zagryazhskaya (1983) who, applying isotope thermometry, obtained temperatures of 920-880°C, 870-860°C and 790-780°C for the three phases of crystallization they identify and define.