Alkaline Rocks and Carbonatites of the World

Setup during HiTech AlkCarb: an online database of alkaline rock and carbonatite occurrences

Namaqualand-Bushmanland Plugs

stripes

Occurrence number: 
151-00-038
Country: 
South Africa
Location: 
Longitude: 18.03, Latitude: -30.17
Carbonatite: 
No

Over 270 olivine melilitite and kimberlite plugs and sediment-filled pipes extend over 200 km from Aggeneys to west of Bitterfontein in Bushmanland and Namaqualand. A southerly group northwest of Bitterfontein and south of Garies is somewhat isolated from the main concentration of plugs and pipes and consists essentially of olivine melilite plugs with only one kimberlite pipe, and the Sandkopsdrif carbonatite complex (No. 151-00-037) (Moore and Verwoerd, 1985). Nearly 60 olivine melilite plugs are known. They are poorly exposed, typically forming rubble strewn mounds and low hills or are revealed simply as a collection of boulders. They range in diameter from a few tens to about 400 m. Contacts with country rock gneisses are sharp with little metamorphism; columnar jointing is occasionally developed at pipe margins. The most northerly pipe, Hoedkop, is unusual in displaying a coarse, approximately vertical banding adjacent and parallel to the margin which reflects the presence of phlogopite-rich streaks. Similar flow structures are to be seen in boulders at other plugs. Olivine is invariably the major phase in these rocks, usually forming phenocrysts and Moore and Verwoerd (1985) were able to distinguish a number of textural types. Other phenocryst phases include titanomagnetite, very rare picroilmenite (with up to 12% MgO) which is invariably mantled by titanomagnetite and this in turn by perovskite, and extremely rare clinopyroxenes. Perovskite and titanomagnetite are ubiquitous as microphenocrysts in a matrix of titaniferous clinopyroxene, nepheline, a second generation of perovskite and occasional phlogopite. Melilite occurring in some, but not all, rocks, generally forms slender laths, occasionally as much as 4 mm in length. It is absent from coarse rocks in which it is considered to have been replaced by clinopyroxene and nepheline. The melilite-free rocks are olivine nephelinites. Micropegmatitic stringers occur in many of the melilitites and nephelinites and comprise principally clinopyroxene and zeolites, but nepheline, apatite, titanomagnetite, perovskite and pyrite may be present, and calcite may form irregular patches. Spinel peridotite mantle xenoliths are found in some of the northern pipes and garnet lherzolite and spinel-garnet lherzolite have been found in the Hoedkop plug. A detailed discussion of the petrographic types and chemistry of olivine in the melilitites will be found in Moore and Erlank (1979) and of perovskite textural relationships in Moore (1981); melilite compositions are given in Moore (1983), and Boctor and Yoder (1986). Complex ilmenite xenocrysts ('macrocrysts') are described in some detail by Haggerty et al. (1985) with implications for oxidation states in the source regions. A range of mineral and rock analyses are presented by McIver (1981) and there are mineralogical data and a discussion of the relationship between melilite-bearing rocks and kimberlites in Boctor and Yoder (1986).

Age: 
Available dates are tabulated in Verwoerd et al. (1990) and range from 54 to 77 Ma.
References: 

BOCTOR, N.Z. and YODER, H.S. 1986. Petrology of some melilite-bearing rocks from Cape Province, Republic of South Africa: relationship to kimberlite. American Journal of Science, 286: 513-39.HAGGERTY, S.E., MOORE, A.E. and ERLANK, A.J. 1985. Macrocryst Fe-Ti oxides in olivine melilitites from Namaqualand-Bushmanland, South Africa. Contributions to Mineralogy and Petrology, 91: 163-70.KRONER, A. 1973. Comments on "Is the African Plate stationary". Nature, Physical Science, 243: 29-30.MCIVER, J.R. 1981. Aspects of ultrabasic and basic alkaline intrusive rocks from Bitterfontein, South Africa. Contributions to Mineralogy and Petrology, 78: 1-11.MOORE, A.E. 1976. Controls of post-Gondwanaland alkaline volcanism in Southern Africa. Earth and Planetary Science Letters, 31: 291-6.MOORE, A.E. 1981. Unusual perovskite textural relationships in olivine melilitites from Namaqualand-Bushmanland, South Africa. Mineralogical Magazine, 44: 147-50.MOORE, A.E. 1983. A note on the occurrence of melilite in kimberlites and olivine melilitites. Mineralogical Magazine, 47: 404-6.MOORE, A.E. and ERLANK, A.J. 1979. Unusual olivine zoning - evidence for complex physico-chemical changes during the evolution of olivine melilitite and kimberlite magmas. Contributions to Mineralogy and Petrology, 70: 391-405.MOORE, A.E. and VERWOERD, W.J. 1985. The olivine melilitite-"kimberlite"-carbonatite suite of Namaqualand and Bushmanland, South Africa. Transactions of the Geological Society of South Africa, 88: 281-94.VERWOERD, W.J., VILJOEN, J.H.A. and VILJOEN, K.S. 1990. Olivine melilitites and associated intrusives of the southwestern Cape province. Guidebook Geocongress ’90, Geological Society of South Africa, PR3: 1-60.

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