The Los Archipelago nepheline syenite ring-structure; a magmatic marker of the evolution of the Central and Equatorial Atlantic

The Los Archipelago ring structure in Guinea, W Africa, is composed of peralkaline nepheline syenite, which crops out in the long crescent-shaped islands of Tamara and Kassa, five islets and a central island (Roume). A new geological map is presented and has led to the recognition of two petrographic suites: one is miaskitic and consists of hastingsite-augite nepheline syenite, and the other is agpaitic and mainly composed of arfvedsonite-aegirine nepheline syenite. These formations are intruded by microsyenite of the same composition, and then by ring-shaped monchiquitic and radial phonolitic dykes; late pegmatites cross-cut all the rocks. EPMA results are given for pyroxene, amphibole, mica, catapleiite, eudialyte, sodalite, nepheline, analcite, astrophyllite, kupletskite, lavenite, Ti-magnetite and pyrophanite. A Rb-Sr whole-rock isochron gives an age of 104.3 + or - 1.7 m.y.; the initial Sr isotopic composition ( (super 87) Sr/ (super 86) Sr (sub 0) 0.7040) is compatible with a mantle origin with minor or no crustal influence. The production of Los Archipelago is considered to have occurred during the continental breakup between W Africa and S America and early rifting of the Equatorial Atlantic Ocean.