1887

oa South African Journal of Geology - Crustal xenoliths in Venetia kimberlite pipes indicate a décollement at -10 km beneath the Central Zone of the Limpopo Belt, South Africa

Volume 101 Number 4
  • ISSN : 1012-0750
  • E-ISSN: 1996-8590

 

Abstract

The results of P-wave velocity and petrographic studies of lithospheric nodules from the Yenetia kimberlite pipes indicate that at this locality, supracrustal lithologies related to the Central Zone of the Limpopo Belt, southern Africa, extend to seismic, gravity, and geoelectrical discontinuities at -10 km. Below these discontinuities to -80 km, the lithosphere is mafic to ultramafic. Textures indicate that amphibolite extending from -10 km to -32 km is restitic, resulting from partial melting of primary amphibolite to create large volumes of tonalitic-trondhjemitic-granodioritic magmas. Heat to produce this partial melting may have been introduced into the lithosphere by intrusion of mafic to ultramafic magma, now represented by the zone of cumulate rocks at between -42 and -57 km depth. No age measurements have been made on the restitic amphibolite. However, comparison of the emplacement ages of tonalitic-trondhjemitic-granodioritic rocks in the Limpopo Belt and adjacent Zimbabwe and Kaapvaal Cratons, which on the basis of their REE compositions could be the products of such melting, suggests that partial melting occurred during the Archaean (>-2.6 Ga ago). By implication, the present lithospheric structure dates from that time. However, in marked contrast to the marginal zones of the Limpopo Belt and the adjacent cratons, tonalitic and trondhjemitic rocks are volumetrically minor in the Central Zone and partial melting of arnphibolite could not have produced the widespread -2.57 Ga granitic rocks of the Central Zone. Seismic and gravity data suggest that rocks of the Zimbabwe Craton extend beneath the Central Zone, and hence the products of partial melting of amphibolite beneath the Yenetia kimberlite pipes, should be found among those rocks. REE compositions of the voluminous -2.7 to -2.6 Ga igneous enderbite and chamoenderbite of the Northern Marginal Zone and the Razi granite are consistent with being that product. If true, major high-grade metamorphism and tectonism in the Central Zone of the Limpopo Belt between -2.03 and 1.9 Ga are not reflected in rocks below -10 km, indicating that the rocks of the Central Zone were tectonically emplaced into their present position after this high-grade metamorphism. Thus, the discontinuities at -10 km mark the base of Central Zone lithologies and reflect an important décollement.


Die gevolge van P-golfsnelheid en petrografiese studies van lithosferiese nodules uit die Venetia-kimberlietpype dui daarop dat by hierdie lokaliteit, bokors litologiee wat verwant is aan die Sentrale Sane van die Limpopogordel, suider-Afrika, uitbrei na seismiese, gravitasie, en geoelektriese diskontinui'teite tot by -10 km. Die litosfeer is mafies tot ultramafies onder hierdie diskontinuïteite tot -80 km. Teksture dui daarop dat amfiboliet wat van -10 km tot -32 km strek restities is as gevolg van gedeeltelike smelting van primêre amfiboliet om groot volumes tonalities-trondhjemities-granodioritiese magmas te vorm. Hirte om hierdie gedeeltelike smelting te lewer kon in die litosfeer ingebring word deur intrusie van mafies tot ultramafiese magma wat nou verteenwoordig word deur die sane van kumulaatgesteentes tussen -42 en -57 km diepte. Geen ouderdomsmetings is op die restitiese amfiboliet gedoen nie. Die vergelyking van die inplasingsouderdomme van tonalities-trondhjemities-granodioritiese gesteentes in die Limpopogordel en naasliggende Zimbabwe- en Kaapvaalkratons. wat op die basis van hulle SAE-samestellings die produkte van sodanige smelting kan wees, dui egter moontlik daarop dat gedeeltelike smelting gedurende die Argeïkum (>-2.6 Ga gelede) plaasgevind het. Dit impliseer dat die huidige Iitosferiese structuur uit daardie tyd dateer. Tonalitiese en trondhjemitiese gesteentes is, in skelle kontras tot die marginale sanes van die Limpopogordel en die naasliggende kratons, volumetries in die minderheid in die Sentrale Sone en gedeeltelike smelting van amfiboliet kon nie die wydverspreide -2.57 Ga granitiese gesteentes van die Sentrale Sone gevorm het nie. Seismiese en gravitasiedata dui aan dat gesteentes van die Zimbabwekraton tot onder die Sentrale Sone voorkom, en dus behoort die produkte van gedeeltelike smelting van amfiboliet onder die Venetia-kimberlietpype geassosieer met daardie gesteentes gevind te word. SAE-samestellings van die volumineuse -2.7 tot -2.6 Ga stollingsenderbiet en charno-end-erbiet van die Noordelike Marginale Sane en die Razi-graniet beantwoord aan die vereistes om daardie produk te wees. Indien dit waar is, word die vernaamste hoegraadse metamorfisme en tektonisme in die Sentral Sone van die Limpopogordel tussen -2.03 en 1. 9 Ga nie in gesteentes onder -10 km gereflekteer nie, wat daarop dui dat gesteentes van die Sentral Sane tektonies ingeplaas is in hulle huidige posisie na hierdie hoëgraadse metamorfisme. Die diskontinuiteit by -10 km vorm dus die basis van die litologieë van die Sentral Sane en weerspieël 'n belangrike afstroping.

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1998-12-01
2020-09-25

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