CHROMIUM ASSEMBLAGE OF MINERALS IN MICACEOUS KIMBERLITES OF YAKUTIAN PROVINCE
S.I. Kostrovitsky and D. de Bruin*
Institute of Geochemistry, Siberian Branch of the RAS,
1a ul. Favorskogo, Irkutsk, 664033, Russia
* Geological Survey of South Africa,
private box X112, Pretoria, South Africa, 0001
Keywords: Micaceous kimberlite, high-Cr assemblage, zonal garnets, kosmochlor, phenocryst origin
Pages: 521-535
Subsection: PETROLOGY, GEOCHEMISTRY AND MINERALOGY
Abstract
Study was given to the composition of garnet and its inclusions from micaceous Cr-diopside-containing kimberlites making up a few pipes of the Daldyn field in the Yakutian kimberlite province. The garnets of almandine-pyrope composition containing up to 10% knorringite end-member are of lherzolite and, partly, wehrlite parageneses. The inclusions (kosmochlor + Cr-spinel + phlogopite) belong to high-Mg high-Cr assemblage and are characterized by considerable variations of composition reflecting nonequilibrium conditions of crystallization in a wide range of pressures and temperatures. Different types of garnet zoning suggest a complicate mechanism of its formation: garnet crystallization under decreased pressure with simultaneous recrystallization of the garnet matrix in dislocation microzones. A common explanation of zoning as a result of garnet recrystallization alone is also possible. Genesis of the assemblage is discussed. Alternative explanations are proposed which relate the origin of high-Cr assemblage of minerals to (1) crystallization from a kimberlite melt and (2) metasomatic influence of the kimberlite melt on garnet macrocrysts.
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