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Olivine nodules and related inclusions in trachyte from the Jos Plateau, Nigeria

Published online by Cambridge University Press:  05 July 2018

J. B. Wright
J. B. Wright*
Affiliation:
Department of Geology, Ahmadu Bello University, Zaria, Nigeria
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Summary

A trachyte plug near Jos contains a suite of inclusions, commonly rounded and of large size, including olivine nodules, pyroxenites, and a variety of basic to intermediate plutonic rocks, as well as albite megacrysts.

The olivine nodules are typical of those found elsewhere in alkaline basic lavas and widely interpreted as upper mantle fragments. Most of the other inclusions contain evidence of crystallization at high pressures, even the syenites, which are interpreted as plutonic equivalents of the trachyte; for the feldspar in them is sanidine-microperthite.

Discovery of this inclusion assemblage in a trachyte provides further support for the thesis that alkaline salic lavas can be generated at sub-crustal depths, by partial melting beneath domically uplifted continental shield areas (Bailey, 1964).

The Nigerian Cenozoic volcanic province lies in an area of regional uplift, and is characterized by scattered small volcanic centres rather than by major eruptive foci. High pressure inclusions are being found at an increasing proportion of these centres, suggesting that the magmas rose directly and rapidly to the surface from the upper mantle levels at which they were all (including the trachyte) generated.

Type
Research Article
Information
Mineralogical Magazine , Volume 37 , Issue 287 , September 1969 , pp. 370 - 374
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1969

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