Contributions to Mineralogy and Petrology

, Volume 71, Issue 2, pp 125–131 | Cite as

A K-Ar and Sr-isotopic study of the volcanic rocks of the island of principe, West Africa — Evidence for mantle heterogeneity beneath the Gulf of Guinea

  • H. M. Dunlop
  • J. G. Fitter


K-Ar dating on a suite of volcanic rocks from the island of Principe gives the following chronology.
  1. 1.

    Basal palagonite breccia (30.6 ± 2.1 Ma).

  2. 2.

    Older Lava Series (OLS) basalt (23.6±0.7 Ma) and hawaiite (19.1±0.5 Ma).

  3. 3.

    Younger Lava Series (YLS) nephelinite (5.60±0.32 Ma) and basanite (3.51 ±0.15).

  4. 4.

    Intrusive phonolite (5.32±0.17 Ma, 5.48±0.19 Ma), tristanite (4.89±0.15 Ma) and trachyphonolite (6.93±0.68 Ma) plugs.


Phonolites and YLS samples plot on a 5.9±0.3 Ma Rb-Sr isochron. The tristanite-trachy-phonolite suite samples also lie on this isochron. This lends support to the suggestion that the YLS basanite magmas were parental to the phonolites but rules out a similar relationship between the OLS magmas and the tristanite-trachyphonolite suite. The mean initial 87Sr/86Sr ratio for the YLS nephelinites and basanites is 0.70297.

The basalts and hawaiites of the OLS show a positive 87Sr/86Sr vs. Rb/Sr correlation which may be interpreted as a 244±43 Ma pseudoisochron. This could be the result of a large-scale heterogeneity generated in the mantle during the early stages in the break-up of Gondwanaland. The mean initial 87Sr/ 86Sr ratio (at 21 Ma) for the OLS (0.70326) is significantly higher than that for the YLS and implies an isotopically distinct mantle source.


Mineral Resource Volcanic Rock Breccia Similar Relationship Gondwanaland 
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Copyright information

© Springer-Verlag 1979

Authors and Affiliations

  • H. M. Dunlop
    • 1
  • J. G. Fitter
    • 2
  1. 1.Scottish Universities Research and Reactor CentreEast KilbrideScotland
  2. 2.Grant Institute of GeologyUniversity of EdinburghScotland

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