Abstract
K-Ar dating on a suite of volcanic rocks from the island of Principe gives the following chronology.
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1.
Basal palagonite breccia (30.6 ± 2.1 Ma).
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2.
Older Lava Series (OLS) basalt (23.6±0.7 Ma) and hawaiite (19.1±0.5 Ma).
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3.
Younger Lava Series (YLS) nephelinite (5.60±0.32 Ma) and basanite (3.51 ±0.15).
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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.
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Dunlop, H.M., Fitter, J.G. 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. Contrib. Mineral and Petrol. 71, 125–131 (1979). https://doi.org/10.1007/BF00375428
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DOI: https://doi.org/10.1007/BF00375428