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Lower crustal granulite xenoliths in carbonatite volcanoes of the Western Rift of East Africa

Published online by Cambridge University Press:  05 July 2018

C. W. Thomas  and
P. H. Nixon
C. W. Thomas
Affiliation:
Department of Earth Sciences, The University, Leeds LS2 9JT
P. H. Nixon
Affiliation:
Department of Earth Sciences, The University, Leeds LS2 9JT
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Abstract

Sporadic small garnet granulite and two-pyroxene granulite xenoliths found in the carbonatite tuffs and lavas near Fort Portal, South West Uganda, are chiefly silica-saturated and rich in Al2O3 (> 20 wt. %) and Na2O (c. 2 to 4 wt. %). Three REE patterns are distinguished: LREE enriched—HREE depleted with a positive Eu anomaly; LREE depleted—HREE relatively enriched and flat; and LREE slightly enriched with a very weak Eu anomaly and high overall REE. The xenoliths are considered to represent original basaltic melts and fractional crystallisation products, varying with the dominance of the clinopyroxene, plagioclase or olivine crystallising phase. It is thought that REE abundances were established before metamorphism.

The clinopyroxenes are low-jadeitic augites, the orthopyroxenes, aluminous hypersthenes and the garnets, pyrope-almandine with constant grossularite. Plagioclase varies with increasing metamorphic grade from labradorite to andesine-oligoclase. Scapolite (meionite), alkali-feldspar, quartz, mica, amphibole, rutile and apatite are minor phases and some appear to be metasomatic.

Calculated temperatures of metamorphic equilibration range from 580 to 800°C at pressures > 4 kbar for the two-pyroxene granulites and > 6 kbar for the garnet granulites. A known geophysical discontinuity marking a density change at 16 km in the Western Rift may be due to the presence of two-pyroxene granulite, calculated to become garnet-bearing at depths greater than 23 km. The absence of proven omphacite-bearing eclogite xenoliths (with no plagioclase) indicates that the greatest depth of crustal sampling by the carbonatite in the Fort Portal field is about 25 km which could be the depth of the Moho in this area.

Keywords

lower crustal granulite xenolithscarbonatitewestern riftEast Africa
Type
Petrology and Geochemistry
Information
Mineralogical Magazine , Volume 51 , Issue 363 , December 1987 , pp. 621 - 633
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1987

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Footnotes

*

Present address: British Geological Survey, West Mains Road, Edinburgh EH9 3LA.

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