Browsing by Author "Le Roex, Anton P"

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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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    Geochemistry and petrogenesis of kimberlite intrusions from the eastern lobe the Du Toitspan kimberlite pipe, South Africa
    (University of Cape Town, 2020) Ramokgaba, Lesego; Le Roex, Anton P; Robey, Jock V A
    The Du Toitspan kimberlite pipe located on the outskirts of Kimberley South Africa, is one four Cretaceous aged major kimberlite pipes from the well-known Kimberley cluster, the type locality for archetypal group I kimberlites. Twenty-seven samples representative of various kimberlite intrusions from the eastern lobe of the Du Toitspan kimberlite pipe have been analysed for their whole-rock geochemistry and mineral chemistry (olivine and phlogopite) with the aim of developing semi-quantitative models that constrain their petrogenesis and characterise their respective source region(s). Investigated intrusions include; D13-phlogopite kimberlite, D14-monticellite kimberlite, D17-serpentinized phlogopite kimberlite, and several narrow (<1m) calcite kimberlite dykes ranging in texture from aphanitic to macrocrystic. The aphanitic calcite dykes were further sub-divided into; a phlogopite-rich calcite kimberlite, a perovskite-rich calcite kimberlite, opaque-rich calcite kimberlites and serpentine calcit e kimberlites.