The geochemistry of the Olivine Melilitetes and related rocks of Namaqualand Bushmanland, South Africa
Doctoral Thesis
1979
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University of Cape Town
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Abstract
Early Tertiary olivine melilitites and associated non-diamondiferous kimberlites occur in two pipe clusters in the Namaqualand and Bushmanland regions of South Africa. The more northerly cluster is centred about the town of Gamoep and the second is located south of Garies. The olivine melilitites are characterized by olivine phenocrysts which exhibit a broad range in composition on the scale of individual hand specimens. It is possible to distinguish four petrographically and chemically distinct olivine populations in both the northern and southern pipes studied: (a) Rare anhedral or subhedral olivines that display marked disequilibrium features with the surrounding matrix and which are characterized by having high iron and extremely low nickel contents (referred to as HILN olivines) relative to the remaining olivines in the same rock; (b) euhedral and often skeletal (hopper) olivines that are richer in Mg and Ni than HILN olivines in the same rock. Some of the euhedral olivines show abnormal zonation patterns ("unusual" hoppers). Hopper and HILN olivines contain fluid and carbonate inclusions which apparently record the separation of a vapour phase and an immiscible carbonate liquid during magma ascent: (c) large rounded olivines (megacrysts) up to 40 mm across. Individuals are chemically homogeneous, but megacrysts from the same pipe collectively define a trend of decreasing Mg with Ni (F092, 0.36% Ni to F075, 0.17% Ni). The most fayalitic megacrysts are depleted in Mg and Ni relative to the hopper olivines in the same rock. [d] In addition to these main populations, there also occur rare magnesium-rich (F091) anhedral olivines which show strained extinction and are believed to be xenocrysts.
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Includes bibliographical references.
Reference:
Moore, A. 1979. The geochemistry of the Olivine Melilitetes and related rocks of Namaqualand Bushmanland, South Africa. University of Cape Town.