Hostname: page-component-848d4c4894-xm8r8 Total loading time: 0 Render date: 2024-06-22T13:31:13.220Z Has data issue: false hasContentIssue false
  • English
  • Français

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
Get access Rights & Permissions [Opens in a new window]

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

Access options

Get access to the full version of this content by using one of the access options below. (Log in options will check for institutional or personal access. Content may require purchase if you do not have access.)

Footnotes

*

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

References

Bram, K., and Schmeling, B.D. (1976) Structure of crust and upper mantle beneath the Western Rift of East Africa, derived from investigations of near earthquakes. In Afar between continental and oceanic rifting. 2 (Pilger, A., and Rossler, A., eds). Inter-Union Comm. Geodynamics. Sci. Report No. 16.Google Scholar
Brooks, C.K. (1976) The Fe2O3/FeO ratio of basalt analyses: an appeal for standardised procedure. Bull. Geol. Soc. Denmar. 22, 117-20.Google Scholar
Carmichael, I.S.E., Turner, F.J., and Verhoogen, J. (1974) Igneous Petrology. McGraw-Hill. 739 pp.Google Scholar
Christensen, N., and Fountain, D. (1975) Constitution of the lower continental crust based on experimental studies of seismic velocities in granulites. Geol. Soc. Am. Bull. 82, 227-36.Google Scholar
Collerson, K.D., and Bridgwater, D. (1979) Metamorphic development of early Archaean tonalitic and trondhjemitic gneisses; Saglek area, Labrador. In Trondhjemites, dacites and related rocks (Barker F., ed.).Elsevier Scientific Publishing Company, Amsterdam, Netherlands. 207–73.Google Scholar
Collerson, K.D., and Bridgwater, D.and Fryer, B.J. (1978) The role of fluids in the formation and subsequent development of early continental crust. Contrib. Mineral. Petrol. 67, 151-67.Google Scholar
Cox, K.G., Bell, J.D., and Pankhurst, R.J. (1979) The interpretation of igneous rocks. George-Allen and Unwin, London. 450 pp.Google Scholar
Dahl, P.S. (1980) The thermal-compositional dependence of Fe2 + -Mg2 + distribution between co-existing garnet and pyroxene; applications to geothermometry. Am. Mineral. 65, 852-66.Google Scholar
Deer, W.A., Howie, R.A., and Zussman, J. (1966) Introduction to rock-forming minerals. Longman, 528 pp.Google Scholar
Edwards, A.C. Lovering, J.F., and Ferguson, J. (1979) High-pressure basic inclusions from the Kaymunnera Kimberlitic diatreme in New South Wales, Australia. Contrib. Mineral. Petrol. 69, 185-92.Google Scholar
Ehrenberg, S., and Griffin, W.L. (1979) Garnet granulite and associated xenoliths in minette and kimberlitic diatremes of the Colorado Plateau. Geolog. 7, 483-7.Google Scholar
Ellis, D.J., and Green, D.H. (1980) An experimental study of the effects of Ca upon garnet-clinopyroxene Fe-Mg exchange equilibria. Contrib. Mineral. Petrol.. 71, 13-22.Google Scholar
Essene, E.J. (1982) Geologic thermometry and barometry. In Characterisation of metamorphism through mineral equilibria (Ferry, J.M., ed.). Rev. Mineral. 10, 153-206.Google Scholar
Evensen, N.M., Hamilton, P.J., and O'Nions, R.K. (1978) Rare Earth abundances in chondritic meteorites. Geochim. Cosmochim. Acta. 42, 1199-211.Google Scholar
Fairhead, J.D. (1986) Geophysical controls on sedimentation within the African rift systems. In Sedimentation in the African Rifts (Frostick, L.E. et al., eds). Geol. Soc. Lond. Spec. Publ. 25, 19-27.Google Scholar
Fairhead, J.D. and Reeves, C.V. (1977) Teleseismic delay times, bouguer anomalies and inferred thickness of the African lithosphere. Earth Planet Sci. Lett. 36, 63-76.Google Scholar
Ganguly, J. (1979) Garnet and dinopyroxene solid solutions and geothermometry based on Fe-Mg distribution coefficients. Geochim. Cosmochim. Acta. 43, 1021-9.Google Scholar
Green, D.H., and Ringwood, A.E. (1967) An experimental investigation of the gabbro to eclogite transition and its petrological applications. Ibid. 31, 767-833.Google Scholar
Griffin, W.L., Carswell, D.A., and Nixon, P.H. (1979) Lower crustal granulites and eclogites from Lesotho, S. Africa. In The mantle sample; Inclusions in kimberlites and other volcanics (Boyd, F.R., ed.). Proc. Int. Kimberlite Conference. Am. Geophys. Union, 2,59-86.Google Scholar
Irving, A.J. (1974) Geochemical and high pressure experimental studies of garnet pyroxenite and pyroxene granulite xenoliths from the Delegate Basaltic Pipes, Australia. J. Petrol. 15, 1-40.Google Scholar
Ito, K., and Kennedy, G.C. (1971) An experimental study of the basalt-garnet granulite-eclogite transition. In The Structure and Physical Properties of the Earth's Crust (Heacock, I, ed.). Geophysical Monograph Series 14, 303-14.Google Scholar
Jones, A.P., Smith, J.V., Dawson, J.B., and Hensen, E.C. (1983) Metamorphism, partial melting and Kmetasomatism of garnet-scapolite-kyanite granulite xenoliths from Lashaine, Tanzania. J. Geol. 91, 143-65.Google Scholar
Kay, R.W., and Kay, S.M. (1981) The nature of the lower continental crust. Inferences from geophysics, surface geology and crustal xenoliths. Rev. Geophys. Space Phys. 19, 271-97.Google Scholar
Lindsley, D.H., Grovel J. E , and Davidson, P.M. (1981) The thermodynamics of the Mg2Si2O-CaMgSi2O6 join: A review and an improved model. In Thermodynamics of Minerals and Melts (Newton, R.C. Navrotsky, A., and Wood, B.J., eds). Advances in Physical Geochemistry. Springer-Verlag, 304 pp.Google Scholar
Lloyd, F.E. (1981) Upper-mantle metasomatism beneath a continental rift: clinopyroxenes in alkali mafic lavas and nodules from South Western Uganda. Mineral. Mag. 44, 315-23.Google Scholar
Lovering, J., and White, A. (1969) Granulitic and eclogitic inclusions from basic pipes at Delegate, Australia. Contrib. Mineral. Petrol. 21, 9-52.Google Scholar
McGetchin, T.R., and Silver, L.T. (1972) A crustal and upper mantle model for the Colorado Plateau based on observations of crystalline rock fragments in the Moses Rock Dike. J. Geophys. Res. 77, 7022-37.Google Scholar
Mehnart, K.R. (1975) The Ivrea Zone: A model of the deep crust. Neues Jahrb. Mineral. 178, 156-99.Google Scholar
Meyer, H.D.A., and Brookins, D.C. (1976) Sapphirine, sillimanite and garnet in granulite xenoliths from Stockdale Kimberlite, Kansas. Am. Mineral 61,1194- 202.Google Scholar
Newton, R.C. Smith, J.V., and Windley, B.F. (1980) Carbonic metamorphism, granulites and crustal growth. Natur. 288, 45-50.Google Scholar
Nixon, P.H., and Hornung, G. (1973) The carbonatite lavas and tuffs near Fort Portal, Western Uganda. Overseas Geol. Mineral. Resour. 41, 168-79.Google Scholar
O'Hara, M.J. (1977) Thermal history and excavation of Archaean gneisses from the base of the continental crust. J. Geol. Soc. Land. 134, 185-200.Google Scholar
Richardson, S.W., and Wilson, G. (1971) Garnet peridotite stability and occurrence in the crust and mantle. Contrib. Mineral. Petrol. 32, 46-68.Google Scholar
Okrusch, M., Schroder, B., and Schnutgen, A. (1979) Granulite facies metabasite ejecta in the Laacher-See area, Eifel, West Germany. Litho. 12, 251-70.Google Scholar
O'Reilly, S.Y., and Griffin, W.L. (1985) A xenolithderived geotherm for Southeastern Australia and its geophysical implications. Tectonophys. Ill , 41-63.Google Scholar
Pearce, T.H., Gorman, B.E., and Birkett, T.E. (1975) The TiO2-K2O-P2O5 diagram: A method of discriminating between oceanic and non-oceanic basalts. Earth Planet Sci. Lett. 24, 419-26.Google Scholar
Powell, R. (1978) The thermodynamics of pyroxene geotherms. Philos. Trans. R. Soc. London. A288, 457-69.Google Scholar
Pride, C, and Muecke, G.K. (1980) Rare Earth Element geochemistry of the Scourian complex, N.W.S.otland. Evidence for the granite-granulite link. Contrib. Mineral. Petrol. 73, 403-12.Google Scholar
Raheim, A., and Green, D.H. (1974) Experimental determination of the temperature and pressure dependence of the Fe-Mg partition coefficient of garnet and clinopyroxene. Ibid. 48, 179-203.Google Scholar
Rogers, N.W. (1977) Granulite xenoliths from Lesotho kimberlite pipes and the lower continental crust. Natur. 270, 681-4.Google Scholar
Rogers, N.W. (1980) Trace element analysis ofkimberlites and associated rocks and xenoliths. Ph.D. Thesis, Univ. of London.Google Scholar
Rogers, N.W. and Hawkesworth, C.J. (1982) Proterozoic age and cumulate origin for granulite xenoliths, Lesotho. Natur. 299, 409-13.Google Scholar
Saxena, S.K. (1979) Garnet-clinopyroxene geothermometer. Contrib. Mineral. Petrol. 70, 229-35.Google Scholar
Shaw, D.M. (1968) A review of K/Rb fractionation trends by covariance analysis. Geochim. Cosmochim. Ada. ,32, 573.Google Scholar
Tarney, J., and Windley, B.F. (1977) Chemistry, thermal gradients and evolution of the lower continental crust. J. Geol. Soc. Land. 134, 153-72.Google Scholar
Vollmer, R., and Norry, M.J. (1983a) Possible origin of K-rich volcanic rocks from Virunga, East Africa, by metasomatism of continental crustal material: Pb, Nd and Sr isotopic evidence. Earth Planet. Sci. Lett. 64, 374-86.Google Scholar
Vollmer, R., and Norry, M.J. (1983b) Unusual isotopic variations in Nyiragongo nephelinites. Natur. 301, 141-3.Google Scholar
von Knorring, O., and Du Bois, C.G.B. (1961) Carbonatitic lava from Fort Portal area in western Uganda. Ibid. 192, 1064-5.Google Scholar
Wass, S.Y., and Hollis, J.D. (1983) Crustal growth in south-eastern Australia—evidence from lower crustal eclogitic and granulitic xenoliths. J. Metamorphic Geol.. 1, 25-45.Google Scholar
Wohlenberg, J. (1976) The structure of the lithosphere beneath the East Africa Rift zones from the interpretation of Bouger anomalies. In Afar between continental and oceanic rifting 2 (Pilger, A., and Rossler, A., eds). Inter-Union Comm. Geodynamics. Sci. Report No. 16.Google Scholar

A correction has been issued for this article:

Errata