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A approximately 1.4 Ga alkaline mafic sill from the Carletonville area; connection to the Pilanesberg alkaline province?

F. Humbert, M. A. Elburg, A. Agangi, G. Belyanin, J. Akoh, A. J. B. Smith, Y. M. Chou and N. J. Beukes
A approximately 1.4 Ga alkaline mafic sill from the Carletonville area; connection to the Pilanesberg alkaline province?
South African Journal of Geology (November 2020) 123 (4): 597-614


Numerous Mesoproterozoic alkaline intrusions belonging to the Pilanesberg Alkaline Province are present within the Transvaal sub-basin of the Kaapvaal Craton. The Pilanesberg Complex is the best-known example; it represents one of the world's largest alkaline complexes, and is associated with a northwest-southeast trending dyke swarm that extends from Botswana to the southwest of Johannesburg. This paper documents the results of a petrological and geochemical study of a thin mafic sill (here referred to as an alkaline igneous body, AIB), which intrudes the ca. 2200 Ma Silverton Formation close to the southernmost part of the Pilanesberg dyke swarm. The AIB has only been observed in cores from a borehole drilled close to Carletonville. It is hypocrystalline, containing randomly oriented elongated skeletal kaersutite crystals and 6 to 8 mm varioles mainly composed of radially oriented acicular plagioclase. These two textures are related to undercooling, probably linked to the limited thickness (70 cm) of the AIB coupled with a probable shallow emplacement depth. Ar-Ar dating of the kaersutite gives an age of ca. 1400 Ma, similar to the age of Pilanesberg Complex. However, the AIB is an alkaline basaltic andesite and is thus notably less differentiated than the Pilanesberg Complex and some of its associated dykes, such as the Maanhaarrand dyke, for which we provide whole-rock geochemical data. Literature data indicate that the Pilanesberg dyke swarm also contains mafic hypabyssal rocks suggesting a link between the dyke swarm and the AIB. The AIB is characterized by strongly negative epsilon Nd and epsilon Hf, that cannot be related to crustal contamination, as shown by positive Ti and P anomalies, and the absence of negative Nb-Ta anomalies in mantle-normalised trace element diagrams. The AIB magma is interpreted to have been derived from a long-lived enriched, probably lithospheric mantle reservoir. The AIB thus provides important information on the magma source of the Pilanesberg Alkaline Province.

ISSN: 1012-0750
EISSN: 1996-8590
Serial Title: South African Journal of Geology
Serial Volume: 123
Serial Issue: 4
Title: A approximately 1.4 Ga alkaline mafic sill from the Carletonville area; connection to the Pilanesberg alkaline province?
Affiliation: University of Johannesburg, Department of Geology, DSI-NRF Centre of Excellence for Integrated Mineral and Energy Resource Analysis, Johannesburg, South Africa
Pages: 597-614
Published: 20201110
Text Language: English
Publisher: Bureau for Scientific Publications, Pretoria, South Africa
References: 67
Accession Number: 2021-006757
Categories: Geochemistry of rocks, soils, and sedimentsIgneous and metamorphic petrologyGeochronology
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. strat. cols., 2 tables, geol. sketch maps
S26°22'00" - S26°22'00", E27°23'60" - E27°23'60"
Secondary Affiliation: Southern University of Science and Technology, Department of Ocean Science and Engineering, CHN, China
Country of Publication: South Africa
Secondary Affiliation: GeoRef, Copyright 2021, American Geosciences Institute. Abstract, Copyright, Geological Society of South Africa. Reference includes data from GeoScienceWorld, Alexandria, VA, United States
Update Code: 202105
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