Magmatic metasomatism and formation of the Merensky reef, Bushveld Complex

Abstract

The rare earth element (REE) contents of pyroxenes and other minerals from the Merensky reef and stratigraphically adjacent rocks of the Atok section, Bushveld Complex, have been determined with the ion microprobe. Merensky reef clinopyroxene and orthopyroxene contain much higher and more variable concentrations of the REE than their cumulus counterparts in rocks several meters below the reef. Chondrite-normalized Merensky clinopyroxene Ce contents vary from ≈10 to 90 for Ce and from ≈4 to 17 for Yb. They also possess deep, negative Eu anomalies, the Eu anomalics being deeper for crystals having high REE contents and relatively shallow for pyroxenes with low REE contents. Similar compositional characteristics are displayed by Cl-rich apatite, which is an accessory phase in the rocks. Interstitial pyroxene in cumulates above and below the reef also tends to have elevated REE contents and in general is not in equilibrium with coexisting cumulus minerals. The melt from which the cumulus minerals crystallized falls within the compositional range of continental basalts; that from which Merensky and postcumulus pyroxenes crystallized is inferred to be much more highly enriched in REE than any normal tholeiitic or alkalic basalt. Despite their highly evolved nature in terms of the REE, the Merensky reef pyroxenes are not evolved in terms of major elements. The decoupling of incompatible trace and major elements is best explained by a metasomatic process. It is speculated that metasomatism involved upward percolation of hydrated silicate melt through and its reaction with the crystalline cumulate pile. The fact that the rocks enriched in the platinum group elements are also those that show evidence for metasomatism suggests that these elements were also metasomatically redistributed.