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Mantle Dynamics: A Nd, Sr, Pb and O Isotopic Study of the Cameroon Line Volcanic Chain |
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| Authors: | HALLIDAY A N; DICKIN A P; FALLICK A E; FITTON J G |
| Institution: | 1Department of Geological Sciences, University of Michigan Ann Arbor, Michigan 48109-1063 2Department of Geology, McMaster University Hamilton, Ontario L8S 4M1 3Isotope Geology Unit, Scottish Universities Research and Reactor Centre East Kilbride, Glasgow, G75 OQU 4Grant Institute of Geology, University of Edinburgh West Mains Road, Edinburgh, EH9 3JW |
| Abstract: | The Cameroon line comprises a 1600-km long Y-shaped chain of< 30 m.y. old volcanoes and <70 m.y. old plutons extendinginto mainland Africa from the Atlantic island of Pagalu. Thedistribution of basaltic volcanic centres is ideal for comparingsub-continental and sub-oceanic sources for basalts and forstudying the influence of the lithosphere on magma generation.We report Nd, Sr, Pb and O isotopic data for more than thirty(principally basaltic) samples from all the main volcanic centrestogether with data for two granulite facies xenoliths. Thosebasalts which display no obvious evidence of crustal contaminationyield initial 87Sr/86Sr ratios ranging from 0.7029 to 0.7035, Nd between +2 and +7 and 206Pb/204Pb between 19?0 and 20?6.The Nd and Sr isotopic compositions define a field on the lefthand side of the ‘mantle array’ (that is with relativelyunradiogenic Sr) and include some data which show overlap withcompositions observed for St. Helena. 208Pb/204Pb ratios extendto 40?4—amongst the more radiogenic observed for alkalibasalts. The Nd and Sr isotopic data are similar in oceanicand continental sectors indicating that the magmas are derivedfrom generally similar mantle sources. Despite this overallsimple picture, the source of the Cameroon line volcanics hasin fact been variable in both time and space. Pb is less radiogenicand Sr is more radiogenic in transitional to hypersthene-normativecompositions. There is a progression to more radiogenic leadisotopic compositions with time for the Cameroon line as a wholethat is most strikingly displayed in the 30 m.y. eruptive historyof Principe. These space-time data are difficult to reconcilewith conventional plume models or with some dispersed ‘plumpudding’ models. The heterogeneities require isolationtimes considerably longer than the age of the south Atlanticsea floor (120 Ma). The eruptive lavas with the most radiogenicPb observed (accompanied by unradiogenic Nd) precisely straddlethe continental edge (i.e. occur in both oceanic and continentalsectors) with no dependency on Nd and Pb concentrations. A modelis proposed which links these observations with the destructionof lithosphere, and the impregnation of the uppermost mantleby the St. Helena hot spot during the formation of the SouthAtlantic ocean. This mantle was subsequently melted to formthe Cameroon line which appears to be derived from a risinghot zone initiated by the early plume activity. The magmaticproducts reflect the mantle mixing that took place during continentalbreakup, the consequent cooling and thickening of the lithosphereand the continued interaction between rising plume componentsand this lithosphere. The depth from which magmas are currentlybeing tapped at the continent/ocean boundary is estimated atless that 150 km. |
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