THE LOS ARCHIPELAGO NEPHELINE SYENITE RING-STRUCTURE: A MAGMATIC MARKER OF THE EVOLUTION OF THE CENTRAL AND EQUATORIAL ATLANTIC

CHRISTIAN MOREAU
Département des Sciences de la Terre, Pôle Sciences et Technologie, Université de La Rochelle, Avenue Marillac, 17042 La Rochelle Cedex 1, France

DANIEL OHNENSTETTER
Centre de Recherche Pétrographiques et Géochimiques - Centre National de la Recherche Scientifique, B.P. 20, 54501 Vandoeuvre-lès-Nancy Cedex, France

DANIEL DEMAIFFE
Département des Sciences de la Terre, C.P. 160/02, Université Libre de Bruxelles, 50, avenue F.D. Roosevelt, 1050 Bruxelles, Belgique

BERNARD ROBINEAU
Département des Sciences de la Terre, Université de la Réunion, 15, avenue René Cassin, 97489 Saint-Denis Cedex, La Réunion

Abstract

The Los Archipelago ring structure (Guinea, West Africa) is composed of peralkaline nepheline syenite, which crops out over two long crescent-shaped islands (Tamara, Kassa), five islets and a central island (Roume). A detailed geological study, which led to a new geological map, recognizes several series within two main petrographic suites, on the basis of textural and structural field criteria. The first suite is miaskitic and is composed of hastingsite-augite nepheline syenite, whereas the second suite is agpaitic and is 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. A Rb-Sr whole rock isochron gives an age of 104.3 ± 1.7 (2 ) Ma (MSWD = 1.28). The initial Sr isotopic composition (87Sr/86Sr0 = 0.7040) is compatible with a mantle origin with minor or no crustal influence. The geological environment and the emplacement age of the Los Archipelago point to formation during the continental breakup between the West Africa and South America and early rifting of the Equatorial Atlantic Ocean.