A chemical, isotopic and geochronological study of the Cameroon line, West Africa.

The Cameroon line is defined by a 1600 km long chain of intraplate volcanoes and older plutonic complexes that extend from the Gulf of Guinea into the continental interior of west Africa. A combined chemical, chronological and isotopic study of the Cameroon line has been undertaken to determine the compositions of the magma sources, how these have developed with time, the differences between oceanic and continental lithospheric mantle in this region and the causes of melting. The oceanic sector of the Cameroon line displays isotopic variations in time and space despite reasonably uniform trace element chemistry. The earliest magmatism within the oceanic sector decreases in age oceanward. This age progression is consistent with the suggested motion of the African Plate over the same period of time, which, combined with isotopic data, implies that the magmatism on each island was initiated by a deep mantle plume with PREMA isotopic signatures that has been gradually modified by a second "St. Helena type" HIMU component, yet to affect Pagalu. Most continental basalts from the Cameroon line show trace element and isotopic compositions similar to the oceanic basalts, implying negligible crustal contamination. Isotopic variations in the continental basalts are explicable by mixing between a plume derived "mixed PREMA-HIMU" component analogous to a mixture of the components resolvable in the oceanic sector and a second HIMU component with higher Sr and lower Nd, typically developed at the continent/ocean boundary ("c.o.b. type HIMU"). The continental lithosphere of the Cameroon line is characterized by very depleted isotopic compositions and was not involved in the genesis of the Cameroon line magmas. Some xenoliths display evidence of metasomatism, resulting U and LREE enrichments and the presence of trace amphibole. The timing of the metasomatism has been estimated as Mesozoic, similar in age to an event recorded in the Pb isotopic data for the Cameroon line basalts. This study has resulted in a new model for the Cameroon line and shed light on the manner in which heterogeneities are produced within the upper mantle.PhDGeologyUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/103985/1/9423242.pdfDescription of 9423242.pdf : Restricted to UM users only