Eclogites and the Metasomatism of Eclogites from the Jagersfontein Kimberlite: Punctuated Transport and Implications for Alkali Magmatism

• Published in: Geochimica et cosmochimica acta Vol. 62; no. 7; pp. 1207 - 1231
• Main Authors: Pyle, Joseph M., Haggerty, Stephen E.
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 01-04-1998
• ISSN: 0016-7037; 1872-9533
• DOI: 10.1016/S0016-7037(98)00040-4

The first detailed petrographic and electron microbeam study of eclogite xenoliths and associated metasomatism from the Jagersfontein Kimberlite are presented with the objectives of determining depths of origin and of establishing possible links to cratonic magmatism. Jagersfontein eclogites are primarily bimineralic garnet + clinopyroxene with wide variations in color, size, and modal abundances, and with initial equilibration in the diamond stability field; five of seventy-nine eclogites derive from P > 8 Gpa. Phlogopite, amphibole, and calcite are the dominant metasomatic minerals, with accessory spinel, feldspar, and serpentine. Garnets are py
39 to py
77 in composition, and zoned garnets are observed as overgrowth rims on early garnet, adjacent to phlogopite, and as relicts in spinel-bearing subassemblages. Phlogopite and amphibole are enriched in Ti, and serpentine contains noticeable Al and Fe. Pleonaste has limited magnetite and minor Cr; and a strong correlation exists between MgO and Cr
2O
3 contents in spinel and in coexisting garnet. Pyroxene is compositionally diverse, and three distinct varieties exist: (1) primary omphacite (jd
7-jd
55) with distinct high-Al
[VI] and low-Al
[VI] groups; (2) secondary symplectitic diopsidic omphacite (jd
2-jd
25) with slightly greater Al
[IV] content than primary omphacite; and (3) spinel-associated pyroxene with little or no jadeite component (max 0.96 wt% Na
2O), variable Ca/(Ca + Mg + Fe) (0.03–0.47), and high Tschermak content (up to ∼0.3 cations Al
[IV] pfu). Feldspar compositions are variable and controlled by associated minerals (garnet, pyroxene, phlogopite) and composition of reacting phases. Pressure-T estimates for metasomatism are <2 Gpa and <900 C, in the field of graphite stability. From mass balance considerations, the metasomatic melts contained K, Ti, Cr, H
2O, and CO
2, but also Na in amphibole and Nb in rutile. Garnet:pyroxene ratios control metasomatic end-product minerals. Hydrous metasomatism of garnet-rich eclogite approaches lamproite; and clinopyroxene-rich eclogite produces a MARID-type rock (mica, amphibole, rutile, ilmenite, diopside). These observations confirm earlier results, that if carbonate exceeds water, amphibole > phlogopite; and for advanced C-H-O metasomatism of eclogites, conjugate silicate (melilitic to phonolitic) and carbonatitic (sovite) melts, similar to the field settings of carbonatites, are produced.