Browsing by Subject "Phlogopite"

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    Eclogite xenoliths from the Premier kimberlite, South Africa: Geochemical evidence for subduction origin
    (2006) Dludla, Siyanda; Le Roex, Anton P; Gurney, John J
    A suite of mantle eclogite hosted within the Premier kimberlite on the Kaapvaal craton can be classified on the basis of Na2O content in garnets as group I type, although textures are ambiguous. No accessory phases of note occur, but rutile and phlogopite are found in a few samples. Clinopyroxenes show variable light rare element (LREE) enrichment (La/Ybn = 2–48), and the garnets are strongly LREE depleted relative to chondrites (La/Ybn = <0.04). Four pyroxenite samples include both garnet clinopyroxenite and garnet orthopyroxenite; clinopyroxenes in these samples are strongly LREE enriched (La/Ybn = 57–65). Calculated equilibration temperatures of the eclogites range from 999 ± 32 to 1168 ± 14° C with an average temperature of 1102 ± 37° C, assuming a pressure of 50 kbar. Relative to a shield geotherm of 40mW/m2, these temperatures suggest a sampling depth of 135 to 165 km. A single, calcium-rich sample gives an equilibration temperature of 1296 ± 32° C at the same assumed pressure. Calculated equilibrium temperatures and pressures for the garnet pyroxenites are 887 to 987° C and 26 to 39 kbar (clinopyroxenite) and 1135 to 1156° C and 48 ± 2 kbar (orthopyroxenite). Reconstituted bulk rock compositions of the eclogites indicate the presence of low- and high-MgO groups. The MgO-poor eclogites (8 to10.5 weight % MgO) have jadeite-rich clinopyroxenes and except for lower silica contents are similar to mid-ocean ridge basalts in major element composition, with slight negative Euanomalies (Eu/Eu*=0.83 to 0.96), indicative of (low-P) plagioclase fractionation. The MgO-rich eclogites (13.6 to 18 weight % MgO) are similar in composition to oceanic gabbro. In combination the geochemical data suggest that the Premier eclogite suite represents a fragment of a once composite oceanic crustal section; the protolith to the low-MgO eclogites was recycled oceanic crustal layer two metabasalt, which experienced silicic melt loss during subduction; the protolith to the high-MgO suite was oceanic crustal layer three cumulate gabbro/pyroxenite.
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    Fine-grained pyroxenites from the Gansfontein kimberlite, South Africa: Evidence for megacryst magma-mantle interaction
    (Geological Society of South Africa, 2004) Doyle, P M; Bell, D R; Le Roex, A P
    The Gansfontein kimberlite contains a suite of fine-grained xenoliths dominated by orthopyroxene, and containing ilmenite, phlogopite, and occasional garnet, with minor quantities of olivine and sulphide. Lamellar intergrowths of orthopyroxene and ilmenite were observed in one sample. The fine grained orthopyroxenite assemblages were observed as discrete xenoliths as a vein in lherzolite, and as a zone margin surrounding a megacrystic dunite. The minerals are characterized by intra- and inter-grain chemical heterogeneity, but are on the whole compositionally similar to those in the abundant, highly evolved Cr-poor megacryst suite at Gansfontein. However, they differ to varying degrees from megacrysts in the concentration of minor elements such as Cr, Al and Ti. Mineral compositions in a pyroxenite vein in lherzolite are higher Cr and Mg#, and lower in Fe3+ than the discrete fine-grained pyroxenites, indicating chemical interaction with peridotite. A single zircon-bearing mica-clinopyroxenite has mineral compositions similar to MARID xenoliths. Fine-grained orthopyroxenites, recognized previously from the Weltevreden and Mzongwana kimberlites and interpreted as rapidly crystallized magmas, are here suggested to result from a reaction between megacryst magma and solid mantle peridotite. Mica-clinopyroxenite may represent the liquid end-product of this reaction. Chemical and modal differences of orthopyroxenites from megacrysts result from reaction with peridotitic components, lack of buffering by typical megacryst mineral assemblages, and possibly shallower origins. Textures and fine-scale chemical disequilibrium indicate that reaction postdates some episodes of megacryst formation and was probably underway when the xenoliths were sampled by ascending kimberlite. Orthopyroxene-garnet thermobarometry indicates an origin of one Gansfontein pyroxenite at ∼1215°C and ∼3.3 GPA, similar to the locus of megacryst crystallization under East... [ABSTRACT FROM AUTHOR]