Abstract
Sr-Nd-Pb isotopic studies have been carried out on the lavas of Mt. Oku, the central part of the continental sector of the Cameroon volcanic line (CVL). The lavas define a considerable range of Sr-Nd-Pb isotopic compositions characterized by 143Nd/144Nd = 0.512626 – 0.512937,87Sr/86Sr = 0.699411 - 0.704279 and more radiogenic isotopic ratios 206Pb/204Pb = 18.3800 – 19.4480. 207Pb/204Pb = 15.4216 – 15.6448, 208Pb/204Pb = 37.1114 – 39.7507, with radiogenic Pb similar to the HIMU of the continent ocean boundary of the Cameroon volcanic line. These isotopic data display a linear array on the 143Nd/144Nd vs 87Sr/86Sr diagrams trending from MORB towards bulk earth. 143Nd/144Nd and 87Sr/86Sr vs. 206Pb/204Pb show a continuum of compositions that suggest differing contributions to the parental magmas from three end-members. One end-member corresponds to a mixture of high uranium high lead (HIMU) and depleted mantle (DM) source components while the other end-member corresponds to an enriched mantle source thought to be the MORB source or sub-lithospheric source. The enriched geochemical signatures (EM) of the Oku magmas are unlikely to be the result of crustal assimilation during magma ascent. Rather, they are thought to be derived from a sub-continental lithospheric mantle enriched in incompatible trace elements by ancient metasomatic processes. The HIMU component is thought to have been inherited from the fossil plume that underlay the Equatorial Atlantic during the period 130 to 100 Ma before migrating to its present site. The radiogenic initial 206Pb/204Pb ratios of the samples (~18.3800 – 19.44804) furthermore require the involvement of an ancient HIMU mantle plume in the magmatism and this may have been formed by metasomatism of fluids derived from St. Helena hot plume ca 120 Ma. Key words: Oku, isotopes, metasomatism, HIMU, enriched geochemical signatures (EM), mantle, enrichment.
Highlights
Basaltic magmas erupted in continental intra plate setting are believed to originate from the asthenospheric mantle and their compositional diversity is attributed to fractional crystallization and interaction of melts with the heterogeneous continental lithosphere (Fitton, 1987; Fitton and Dunlop, 1985; Halliday et al, 1988, 1990; Chung et al, 1994; Peng et al, 1994; Haase et al, 2000; Gertisser and Keller, 2003)
Highlands basanites and alkali basalts have anomalously high concentrations of Sr, Ba and P and low concentrations of Zr, which are exclusive features of continental Cameroon basalts. The genesis of these latter magmas is consistent with derivation from an incompatible element enriched, amphibole-bearing lithospheric mantle source (Marzoli et al, 2000)
Since the mobility of Rb and Sr in the melt (H2O + CO2 and chlorite medium) interaction is considerably higher than Sm and Nd Light Rare Earth Elements (LREE), the fluid induced melt in the upper mantle becomes isotopically more homogeneous with respect to Rb-Sr compared to Sm-Nd causing variable ٤Nd (~Nd +5.3 to -3.24)
Summary
Basaltic magmas erupted in continental intra plate setting are believed to originate from the asthenospheric mantle and their compositional diversity is attributed to fractional crystallization and interaction of melts with the heterogeneous continental lithosphere (Fitton, 1987; Fitton and Dunlop, 1985; Halliday et al, 1988, 1990; Chung et al, 1994; Peng et al, 1994; Haase et al, 2000; Gertisser and Keller, 2003). Basalts from Ngaoundere (Miocene to Quaternary) and from the early activity (31 to 14 Ma) of the Western Highlands (Bambouto and Oku) have incompatible trace element and Sr–Nd isotopic compositions similar to those of oceanic Cameroon Volcanic Line (CVL) basalts, pointing to a similar asthenospheric mantle (Marzoli et al, 2000; Rankenburg et al, 2005; Moundi et al, 2007; Njilah et al, 2008) source. Highlands basanites and alkali basalts have anomalously high concentrations of Sr, Ba and P and low concentrations of Zr, which are exclusive features of continental Cameroon basalts The genesis of these latter magmas is consistent with derivation from an incompatible element enriched, amphibole-bearing lithospheric mantle source (Marzoli et al, 2000). About 2000 m of lava pile would have been erupted ranging from basalt through hawaitte, mugearite to trachyterhyolites, high level intrusions and intercalated pyroclastics. Njilah (1991) and Njilah et al (2007) give KAr ages, major and trace element including Rare Earth Elements (REE) geochemistry as well as petrographic studies of the spectrum of lavas of this massif
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