Eclogites with Oceanic Crustal and Mantle Signatures from the Bellsbank Kimberlite, South Africa, Part I: Mineralogy, Petrography, and Whole Rock Chemistry 论文

摘要

Three groups of eclogite xenoliths have been identified from the DeBruyn and Martin Mine of the Bellsbank kimberlite, South Africa. These eclogites are divided into the three groups on the basis of petrography, clinopyroxene and garnet mineral chemistry, and rare earth element (REE) contents of the whole rocks, clinopyroxenes, and garnets. Abundances of the REE and garnet-clinopyroxene Kd's are consistent with a petrogenesis by fractional crystallization for Group A eclogites, but not Groups and We suggest, on the basis of bulk-rock major-element and REE analyses and reconstructed REE patterns, that eclogites from Groups and are the metamorphosed products of ancient subducted oceanic crust. Group eclogites have major element chemistry similar to Archean basalts, and Group eclogites have high $$Al_{2}O_{3}$$ abundances. Metasomatism has affected all groups of eclogites, producing phlogopite, feldspar, amphibole, and a breakdown of primary clinopyroxene. On the basis of large ion lithophile (LIL) elements, this metasomatism can be traced to the host kimberlite. Our study demonstrates that petrogenesis by fractional crystallization cannot account for all eclogite xenolith compositions found in kimberlite.