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Garnet-olivine reaction in the upper mantle: evidence from peridotite xenoliths in the Letseng-la-Terae kimberlites, Lesotho

Published online by Cambridge University Press:  03 November 2011

N. P. Lock  and
J. B. Dawson
N. P. Lock
Affiliation:
Department of Geology, University of Sheffield, Sheffield S1 3JD, England.
J. B. Dawson
Affiliation:
Department of Geology, University of Sheffield, Sheffield S1 3JD, England.
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Abstract

Garnet-bearing xenoliths from Letseng-la-Terae display a range of textures from coarse to granuloblastic. Equilibration temperatures and pressures of primary phases are in the ranges 950-1400°C and 27-50 kb, respectively. Deformed lherzolites equilibrated throughout this temperature range but coarse xenoliths are restricted to low temperature equilibration.

All garnets display coronas developed during the reaction:

In some rocks, reaction has completely eliminated garnet.

In rocks where garnet is disrupted, the corona minerals are strung out in the fluidal texture indicating that reaction occurred before deformation. Rocks transitional to, and of granuloblastic texture, contain garnet and aluminous spinel; in addition ‘pools’ of minerals originating by dynamic separation of corona fragments are observed.

Chemical comparison between the corona minerals and minerals in a garnet-spinel rock and two spinel granuloblastites, suggests that these spinel-bearing rocks may be derived from normal garnet peridotite by a complex sequence of reaction, followed by deformation, annealing and chemical homogenisation. The conclusion that reaction and deformation took place at high levels in the upper mantle is contrary to some earlier hypotheses of shearing within the low velocity zone in response to continental plate movement, but is consistent with mantle diapir models.

Keywords

Lherzolitecoronadeformationannealinghomogenisationmantlediapir
Type
Research Article
Information
Earth and Environmental Science Transactions of The Royal Society of Edinburgh , Volume 71 , Issue 1 , 1980 , pp. 47 - 53
Copyright
Copyright © Royal Society of Edinburgh 1980

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References

Bloomer, A. G. & Nixon, P. H. 1973. The geology of Letseng-Ia-Terae kimberlite pipes. In Nixon, P. H. (ed.) Lesotho Kimberlites, 20–36. Maseru: Lesotho National Development Corporation.Google Scholar
Boyd, F. R. & Nixon, P. H. 1972. Ultramafic nodules from the Thaba Putsoa kimberlite pipe. ANNU REP DIR GEOPHYS LAB CARNEGIE INST 71, 362–73.Google Scholar
Carswell, D. A. 1975. Primary and secondary phlogopites and clinopyroxenes in garnet lherzolite xenoliths. PHYS CHEM EARTH OXFORD 9, 417–30.CrossRefGoogle Scholar
Cox, K. G., Gurney, J. J. & Harte, B. 1973. Xenoliths from the Matsoku pipe. In Nixon, P. H. (ed.) Lesotho Kimberlites, 76–98. Maseru: Lesotho National Development Corporation.Google Scholar
Dawson, J. B.Gurney, J. J. & Lawless, P. J. 1975. Palaeogeothermal gradients derived from xenoliths in kimberlite. NATURE LONDON 257, 299–300.CrossRefGoogle Scholar
Dawson, J. B. & Stephens, W. E. 1976. Addendum: Statistical classification of garnets from kimberlite and associated xenoliths. J GEOL 84, 495–6.CrossRefGoogle Scholar
Ferguson, J.Ellis, D. J. & England, R. N. 1977. Unique spinel-garnet lherzolite inclusions in kimberlite from Australia. GEOLOGY 5, 278–80.2.0.CO;2>CrossRefGoogle Scholar
Green, D. H. & Ringwood, A. E. 1967. The stability fields of aluminous pyroxene peridotite and garnet peridotite and their relevance in upper mantle structures. EARTH PLANET SCI LETT 3, 151–60.CrossRefGoogle Scholar
Haggerty, S. E. 1975. The chemistry and genesis of opaque minerals in kimberlites. PHYS CHEM EARTH OXFORD 9, 295–307.CrossRefGoogle Scholar
Harte, B. 1977. Rock nomenclature with particular relation to deformation and recrystallisation textures in olivine bearing xenoliths. J GEOL 85, 279–88.CrossRefGoogle Scholar
Harte, B., Cox, K. G. & Gurney, J. J. 1975. Petrography and geological history of upper mantle xenoliths from the Matsoku kimberlite pipe. PHYS CHEM EARTH OXFORD 9, 617–46.Google Scholar
Jenkins, D. M. & Newton, R. C. 1979. Experimental determination of the spinel peridotite to garnet peridotite inversion at 900°C and 1000°C in the system CaO-MgO-Al2O3-SiO2, and at 900°C with natural garnet and olivine. CONTRIB MINERAL PETROL 68, 407–18.CrossRefGoogle Scholar
Kuno, H. 1967. Mafic and ultramafic nodules from Itinome-Gata, Japan. In Wyllie, P. J. (ed.) Ultramafic and related rocks, 337–42. New York: J. Wiley.Google Scholar
MacGregor, I. D. 1965. Stability fields of spinel and garnet peridotite in the synthetic system MgO-CaO-Al2O3-SiO2. ANNU REP DIR GEOPHYS LAB CARNEGIE INST 64, 126–34.Google Scholar
MacGregor, I. D. 1970. The effect of CaO, Cr2O3, Fe2O3 and A12O3 on the stability of spinel and garnet peridotites. PHYS EARTH PLANET INTER 3, 372–7.CrossRefGoogle Scholar
MacGregor, I. D. 1974. The system MgO-Al2O3-SiO2: Solubility of Al2O3 in enstatite for spinel and garnet peridotite compositions. AM MINERAL 59, 110–9.Google Scholar
Mercier, J. C. & Nicolas, A. 1975. Textures and fabrics of upper mantle peridotites as illustrated by xenoliths from basalts. J. PETROL 16, 454–87.CrossRefGoogle Scholar
Nixon, P. H. & Boyd, F. R. 1973. Petrogenesis of the granular and sheared ultrabasic nodule suite in kimberlites. In Nixon, P. H. (ed.) Lesotho Kimberlites, 48–56. Maseru: Lesotho National Development Corporation.Google Scholar
O'Hara, M. J., Richardson, S. W. & Wilson, G. 1971. Garnet peridotite stability and occurrence in crust and mantle. CONTRIB MINERAL PETROL 32, 48–68.CrossRefGoogle Scholar
Reid, A. M. & Dawson, J. B. 1972. Olivine-garnet reaction in peridotites from Tanzania. LITHOS 5, 115–24.CrossRefGoogle Scholar
Ringwood, A. E. 1975. Composition and Petrology of the Earth's Mantle. New York: McGraw-Hill.Google Scholar
Smith, D. 1977. The origin and interpretation of spinel-pyroxene clusters in peridotite. J GEOL 85, 476–82.CrossRefGoogle Scholar
Smith, J. V. & Dawson, J. B. 1975. Chemistry of Ti-poor spinels, ilmenites and rutiles from peridotite and eclogite xenoliths. PHYS CHEM EARTH OXFORD 9, 309–22.CrossRefGoogle Scholar
Stephens, W. E. & Dawson, J. B. 1977. Statistical comparison between pyroxenes from kimberlites and their associated xenoliths. J GEOL 85, 433–49.CrossRefGoogle Scholar
Wells, P. R. A. 1977. Pyroxene thermometry in simple and complex systems. CONTRIB MINERAL PETROL 62, 129–39.CrossRefGoogle Scholar
Wood, B. J. 1974. The solubility of alumina in orthopyroxene coexisting with garnet. CONTRIB MINERAL PETROL 46, 1–15.CrossRefGoogle Scholar
Wood, B. J. & Banno, S. 1973. Garnet-orthopyroxene and orthopyroxene-clinopyroxene relationships in simple and complex systems. CONTRIB MINERAL PETROL 42, 109–42.CrossRefGoogle Scholar