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Geology and geochronology of Inaccessible Island, South Atlantic

Published online by Cambridge University Press:  01 May 2009

L. Chevallier ,
D. C. Rex  and
W. J. Verwoerd
L. Chevallier
Affiliation:
Department of Geology, University of Stellenbosch, Stellenbosch 7600, South Africa
D. C. Rex
Affiliation:
Department of Earth Sciences, University of Leeds, Leeds LS2 9JT, UK
W. J. Verwoerd
Affiliation:
Department of Geology, University of Stellenbosch, Stellenbosch 7600, South Africa
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Abstract

Inaccessible Island is the eroded remnant of an extinct, comparatively small intraplate volcano dominated by flows of alkaline olivine basalt. The oldest stratigraphie unit is a hydrothermally altered basement of somewhat questionable early Pliocene (6.5 Ma) age. This is unconformably overlain by a volcanic superstructure built up during the last three million years. The two formations have different trace element signatures that may be attributed to different mantle sources. Boulders of gabbro are common but the presence of an in situ plutonic intrusion could not be confirmed. Their K-Ar age of 12.8 Ma may be spurious and their possible relationship with the volcano is uncertain. Reliable age determinations of 0.95–0.72 Ma were obtained on lava flows of the second volcanic stage, subdivided into four units or stratigraphie members. The latest unit consists of plugs, sills and flows of an evolved magma fraction (benmoreite and trachyte) of which benmoreite is considered to be the more voluminous. Several dyke swarms of different ages reveal the internal structure of the volcano. It is concluded that the main volcanic centre was located immediately offshore to the northwest and that the edifice was attached to an east–west volcanic rift zone. Apart from marine erosion, massive land-sliding probably took part in shaping the island and its submarine platform.

Type
Articles
Information
Geological Magazine , Volume 129 , Issue 1 , January 1992 , pp. 1 - 16
Copyright
Copyright © Cambridge University Press 1992

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