Petrology and geochemistry of metamorphosed komatiites and basalts from the Sula Mountains greenstone belt, Sierra Leone |
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| Authors: | Hugh Rollinson |
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| Institution: | (1) Geography and Environmental Management Research Unit Cheltenham and Gloucester College of Higher Education, Francis Close Hall, Swindon Road, Cheltenham, GL 50 4AZ, UK; e-mail: hrollinson@chelt.ac.uk, GB |
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| Abstract: | The Sula Mountains greenstone belt is the largest of the late-Archaean greenstone belts in the West African Craton. It comprises
a thick (5 km) lower volcanic formation and a thinner (2 km) upper metasedimentary formation. Komatiites and basalts dominate
the volcanic formation and komatiites form almost half of the succession. All the volcanic rocks are metamorphosed to amphibolite
grade and have been significantly chemically altered. Two stages of alteration are recognised and are tentatively ascribed
to hydrothermal alteration and later regional amphibolite facies metamorphism. Ratios of immobile trace elements and REE patterns
preserve, for the most part, original igneous signatures and these are used to identify five magma types. These are: low-Ti
komatiites – depleted in light REE; low-Ti komatiites – with flat REE patterns; high-Ti komatiitic basalts – with flat REE;
low-Ti basalts – depleted in light REE; high-Ti basalts – with flat REE patterns. Much of the variation between the magma
types can be explained in terms of different melt fractions of the mantle source, although there were two separate mantle
sources one light REE depleted, the other not. The interleaving of the basalts and komatiites produced by this melting indicates
that the two mantle sources were melted simultaneously. The simplest model with which to explain these complex melting processes
is during melting within a rising mantle plume in which there were two different mantle compositions. The very high proportion
of komatiites in the Sula Mountains relative to other greenstone belts suggests either extensive deep melting and/or the absence
of a thick pre-existing crust which would have acted as a “filter” to komatiite eruption.
Received: 10 February 1998 / Accepted: 28 July 1998 |
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