Abstract

Kimberlite and nephelinitic magmas have sampled the upper mantle and crust beneath the ancient cratons en route to the surface from their points of origin within the upper mantle. Study of the xenolith suites at most localities favours the hypothesis that the Moho is a chemical break, with lower crustal granulite facies rocks overlying harzburgitic or dunitic upper mantle rocks. The model for Kansas, USA, however, favours the hypothesis that a broad zone of granulite-facies rocks straddles the Moho. Bulk chemical analyses of garnet-pyroxene granulites from Matsoku, Lesotho and Lashaine, Tanzania, show them to be similar to gabbroic rocks, but certain other features (high K/Rb, low Rb/Sr) suggest that they may arise by ‘basification’ of rock of intermediate composition during high-grade metamorphism with associated anatexis. These lower crustal granulites are not chemically similar to upper mantle eclogites.

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