The direction of migration of the recent alkaline-basalt plumes of West African alkaline province
Lazarenkov V. G,* Mamayeva H.I., ** Zinchenko V.N.*
* St.-Petersburg State Mining Institute (technical university) (SPSMI), St.-Petersburg, Russia ** Federal State Enterprise «Academician I.S. Gramberg Russian Research Institute of Geology and Mineral Resources of the World Ocean» («VNIIOkeangeologia of I.S. Gramberg»), St.-Petersburg, Russia
Problems of alkaline magmatism of Western Africa were considered in L.N. Kogarko's works (2008). The West African province of later Mezo-Cenozoic alkaline rocks is stretched along all Atlantic coast of the Western Africa, beginning from massifs of nepheline syenite with carbonatites Tamazert (Morocco) (44 million years old) and finishing the same massifs in Angola (80-120 million years) and modern kimberlites of Namibia and Southern Africa (Lazarenkov V. G., 1988). West African province on its length approximately corresponds to a known East African province, but considerably concedes to it on the area of wedging out of alkaline rocks. It alkaline-basalt volcanism and alkaline-granite, nepheline-syenite and kimberlite plutonisme form numbers of alkaline formations of Atlantic type passive margin. In regard to tectonic West African alkaline province is dated to the coupled system pericontinental and perioceanic rifts in transition continent-ocean zone. From Atlantic ocean it is framed by perioceanic rifts (V.E.Hain), which general contour coincides with borders of the Atlantic coast of the Western Africa. From continent it is supervised by rift zones of mainly northeast direction – «a line of Cameroon», a zone of Conakry-Bomako, linear zones of alkaline massifs of Angola, Namibia and others liniment. Perhaps, they have a continuation in Atlantic ocean as a transform faults, sometimes marked by chains of islands of gulf of Guinea («a line of Cameroon»), Canary, islands of Cape Verde, Los and others.
Signs of linear temporal and lateral zonality are remarkable enough feature of chains of the alkaline massifs which are settling down in these liniment. They are shown in Guinea where feldspathoid syenite massifs of Los (105-80 million years) are located at the west end of oceanic rift zone Conakry-Bomako which extends more than 700 km. This zone is filled with stratified mafic-ultramafic massif Calum of Great Dyke type 150 million years old (Lazarenkov V. G., 1988). Probably, rift zone of Conakry-Bomako is traced further on the east through all African continent to Red sea [Wilson, Guiraud, 1993]. Elements of temporal and lateral zonality of West African alkaline province are well expressed in meridional paleorift of Nigeria extended for about 400 km. Here is a line of ring massifs of normal and alkaline granites from border with Niger in the north to coast of Atlantic ocean, in which from north to south there is a rejuvenation of age of these massifs from 213 to 141 million years [Rahaman et al., 1984; Yail, 1989].
Signs of temporal and lateral zonalitys are shown even more clearly in «a line of Cameroon» of northeast direction in which its continental piece 1600 km long has in its oceanic termination in the form of a linear chain of volcanic islands of Guinea gulf 700 km long with possible transform oceanic continuation on Saint Elena's island. On continent the chain of ring massifs of normal granites and syenites of Cameroon is stretched from the lake Chad to a volcano Cameroon with the age of 73-60 million years in the north and in the centre of the country to 45-35 million years in the south [Guiraud et al., 1987]. In a chain of volcanic islands of Guinea gulf from a volcano Cameroon to island Pachalu towards Atlantic ocean occurs a rejuvenation of age too - from island Principi (31 million years) to an island San-Tome (13 million years) and to island Pachalu (4,8 million years) [Lee et al., 1994].
Similar examples of temporal and lateral zonality in linear chains of alkaline massifs can be continued on examples of magmatic complexes of Angola, Namibia and other countries of the Western Africa. In Angola in a zone of northeast course of the transcontinental graben Lukapa the chain of kimberlite massifs (more than 60) in an ocean direction is replaced by a chain of 26 massifs of nepheline syenites and carbonatites (80-120 million years) (Lapido-Lurejro, 1968). In Namibia damaraland complex settles down in limits of rift zone of northeast course stretched from continent towards Atlantic ocean on distance about 400 km. The luderits complex of alkaline and nepheline syenite in Namibia (130 million years) is also located in a zone of northeast course. There is a modern kimberlite province (147-84 million years) with carbonatite-kimberlite volcano Gross Bruccaros in the South Africa and Namibia in a zone of northeast course.
It’s remarkable, that while the general northeast orientation continental liniments of the West African alkaline province, each of these linements reveal an individual corner of an inclination of the course to equator (width): 84º – almost meridional course for paelorift of Nigeria, 58º – for «a line of Cameroon», 47º – for linement of Angola, 28º – sublatitudinal for Conakry-Bomako rift and 18º – for linement of Namibia, as a whole showing a wide range of directions of the courses (fig.).
Last circumstance creates chaotic enough picture of moving of coastal line of Western Africa Atlantic coast according to plate-tectonic constructions and static position of feeding them thermal points – plums. It is especially difficult to combine these constructions with the general northeast vector of moving of West African later Mezo-Cenozoic alkaline province and submeridional vector of moving of East African even-age alkaline province. Probably, for this purpose all African continent should be divided in two, but such decision of the question is not provided yet on modern plate-tectonic schemes. It’s easy to avoid all these complexities and mess if to start with idea of plum-magmatism (Dobretsov, 2007) – mobile behaviour alkaline plums, their nuclear roots and stable position of the African platform and its margins – both western and east (Lazarenkov, Mamayeva, 2008). From such a point of view, it is possible to assume, that the tendency of unidirectional moving of later Mezo-Cenozoic alkaline plums in different north-east linements the of Western Africa is covered in substance moving not in top or, maybe, even in the bottom parts of a mantle, but in layer D or in the top parts of Earth core.
References (all in Russian).
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