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Journal of Petrology Advance Access published online on October 1, 2004
Journal of Petrology, doi:10.1093/petrology/egh067
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Received August 26, 2002
Accepted August 3, 2004

Article

The Role of Continental Crust and Lithospheric Mantle in the Genesis of Cameroon Volcanic Line Lavas: Constraints from Isotopic Variations in Lavas and Megacrysts from the Biu and Jos Plateaux

K. RANKENBURG 1*, J. C. LASSITER 2, and G. BREY 3

1 MAX-PLANCK-INSTITUT FÜR CHEMIE, ABT. GEOCHEMIE, POSTFACH 3060, 55020 MAINZ, GERMANY; INSTITUT FÜR MINERALOGIE, SENCKENBERGANLAGE 28, 60054 FRANKFURT/MAIN, GERMANY
2 MAX-PLANCK-INSTITUT FÜR CHEMIE, ABT. GEOCHEMIE, POSTFACH 3060, 55020 MAINZ, GERMANY
3 INSTITUT FÜR MINERALOGIE, SENCKENBERGANLAGE 28, 60054 FRANKFURT/MAIN, GERMANY

* To whom correspondence should be addressed. E-mail: kai.rankenburg1.jsc.nasa.gov.


  Abstract

We present a combined Sr, Nd, Pb and Os isotope study of lavas and associated genetically related megacrysts from the Biu and Jos Plateaux, northern Cameroon Volcanic Line (CVL). Comparison of lavas and megacrysts allows us to distinguish between two contamination paths of the primary magmas. The first is characterized by both increasing 206Pb/204Pb (19·82-20·33) and 87Sr/86Sr (0·70290-0·70310), and decreasing {varepsilon}Nd (7·0-6·0), and involves addition of an enriched sub-continental lithospheric mantle-derived melt. The second contamination path is characterized by decreasing 206Pb/204Pb (19·82-19·03), but also increasing 87Sr/86Sr (0·70290-0·70359), increasing 187Os/188Os (~0·130-0·245) and decreasing {varepsilon}Nd (7·0-4·6), and involves addition of up to 8% bulk continental crust. Isotopic systematics of some lavas from the oceanic sector of the CVL also imply the involvement of a continental crustal component. Assuming that the line as a whole shares a common source, we propose that the continental signature seen in the oceanic sector of the CVL is caused by shallow contamination, either by continent-derived sediments or by rafted crustal blocks that became trapped in the oceanic lithosphere during continental breakup in the Mesozoic.

Keywords: crustal contamination; CVL; megacrysts; ocean floor; osmium isotopes.
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