Silicic magmas from the continental Cameroon Volcanic Line (Oku, Bambouto and Ngaoundere): Ar-40-Ar-39 dates, petrology, Sr-Nd-O isotopes and their petrogenetic significance

The intraplate Cameroon Volcanic Line (CVL) straddles the African-South Atl antic continent-ocean boundary and is composed mainly of alkaline basic vol canic rocks. Voluminous silicic volcanics characterize the continental sect or of the CVL. We present here new geochemical, isotopic (Sr-Nd-O) and Ar-4 0/Ar-39 geochronological data on the main silicic volcanic centres of the W estern (Mt. Oku, Sabga and Mt. Bambouto) and Eastern (Ngaoundere plateau) C ameroon Highlands. The silicic volcanism of Mt. Oku, Sabga and Mt. Bambouto occurred between 25 and 15 Ma and is represented by voluminous quartz-norm ative trachytes and minor rhyolitic ignimbrites. At Mt. Bambouto central vo lcano about 700 m of silicic volcanics erupted in less than 2.7 million yea rs. These silicic volcanics are associated with slightly to moderately alka line basalts and minor basanites. In general, onset of the silicic volcanis m migrated from NE (Oku: 25 Ma) to SW (Sabga: 23 Ma; Bambouto: 18 Ma; and M t. Manengouba: 12 Ma). The silicic volcanism of the Ngaoundere plateau (eas tern branch of the CVL) is instead dominated by nepheline-normative trachyt es which are associated with strongly alkaline basalts and basanitic rocks. These Ne-trachytes are younger (11-9 Ma) than the Q-trachytes of the Weste rn Highlands. The least differentiated silicic volcanics are isotopically s imilar (Sr-87/Sr-86 < 0.70380; Nd-143/Nd-144 > 0.51278) to the associated a lkaline basalts suggesting differentiation processes without appreciable in teraction with crustal materials. Such interactions may, however, have play ed some role in the genesis of the most evolved silicic volcanics which hav e Sr-87/Sr-86 as high as 0.705-0.714. Fractional crystallization is the pre ferred mechanism for genesis of the silicic melts of both Western and Easte rn Highlands, as shown by modeling major and trace element variations. The genesis of the least evolved Q-trachytes from the Western Highlands, starti ng from slightly to moderately alkaline basalts, is compatible with fractio nation of dominantly plagioclase, clinopyroxene and magnetite. Crystal frac tionation may have occurred at low pressure and at QFM buffer f(O2) conditi ons. Parental magmas of the Ngaoundere Ne-trachytes are likely instead to h ave been strongly alkaline basalts which evolved through crystal fractionat ion at higher P (6-2 kbar) and f(O2) (QFM + 2). The migration (25 to 12 Ma) of the silicic volcanism from NE to SW in the continental sector of the CV L is reminiscent of that (31-5 Ma) of the onset of the basic volcanism in t he oceanic sector (Principe to Pagalu islands) of the CVL. These ages, and that (11-9 Ma) of the silicic volcanism of the Ngaoundere plateau, indicate that the Cameroon Volcanic Line as a whole may not be easily interpreted a s the surface expression of hot-spot magmatism.