Petrology and geochemistry of early Precambrian graywackes from the fig tree group, South Africa

Kent C. Condie, John E. Macke, Thomas O. Reimer

Research output: Contribution to journalArticleResearchpeer-review

Abstract

Graywackes of the Fig Tree Group in South Africa were derived from a diverse source area composed principally of chert, volcanics, granitic-metamorphic rocks, and some ultramafics. Chert and quarts pseudomorphs after shard-like fragments and well-formed plagioclase and K-feldspar crystals in some graywackes indicate that some detritus was derived from contemporary volcanism. Immature textures indicate mild source-area weathering and rapid erosion, deposition, and burial. An unusually large amount of dolomite in the graywackes was probably deposited by carbonate-rich pore fluids during diagenesis. A relative Sr depletion (compared to K, Rb, Ca, and Ba) in graywackes of the lower Fig Tree Group (Sheba Formation) is interpreted in terms of an abundance of Sr-depleted igneous rocks in their source area. Large amounts of Ni in all Fig Tree graywackes and shales appear to have been derived from ultramafic source rocks. A progressive stratigraphic increase in granitic components in the graywackes suggests progressive unroofing of a granitic metamorphic terrane which was initially covered by a thick sequence of Onverwacht or Onverwacht-like volcanic rocks. This source appears to have been located in central Swaziland. The relatively abundant granitic detritus in the graywackes indirectly records one or more pre-Fig Tree (3.4-4.0 b.y.) granite-forming events in southern Africa.

LanguageEnglish
Pages2759-2776
Number of pages18
JournalBulletin of the Geological Society of America
Volume81
Issue number9
DOIs
Publication statusPublished - 1970
Externally publishedYes

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petrology
Precambrian
geochemistry
chert
detritus
volcanic rock
unroofing
ultramafic rock
igneous rock
metamorphic rock
source rock
diagenesis
feldspar
dolomite
terrane
volcanism
plagioclase
weathering
granite
texture

Cite this

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title = "Petrology and geochemistry of early Precambrian graywackes from the fig tree group, South Africa",
abstract = "Graywackes of the Fig Tree Group in South Africa were derived from a diverse source area composed principally of chert, volcanics, granitic-metamorphic rocks, and some ultramafics. Chert and quarts pseudomorphs after shard-like fragments and well-formed plagioclase and K-feldspar crystals in some graywackes indicate that some detritus was derived from contemporary volcanism. Immature textures indicate mild source-area weathering and rapid erosion, deposition, and burial. An unusually large amount of dolomite in the graywackes was probably deposited by carbonate-rich pore fluids during diagenesis. A relative Sr depletion (compared to K, Rb, Ca, and Ba) in graywackes of the lower Fig Tree Group (Sheba Formation) is interpreted in terms of an abundance of Sr-depleted igneous rocks in their source area. Large amounts of Ni in all Fig Tree graywackes and shales appear to have been derived from ultramafic source rocks. A progressive stratigraphic increase in granitic components in the graywackes suggests progressive unroofing of a granitic metamorphic terrane which was initially covered by a thick sequence of Onverwacht or Onverwacht-like volcanic rocks. This source appears to have been located in central Swaziland. The relatively abundant granitic detritus in the graywackes indirectly records one or more pre-Fig Tree (3.4-4.0 b.y.) granite-forming events in southern Africa.",
author = "Condie, {Kent C.} and Macke, {John E.} and Reimer, {Thomas O.}",
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Petrology and geochemistry of early Precambrian graywackes from the fig tree group, South Africa. / Condie, Kent C.; Macke, John E.; Reimer, Thomas O.

In: Bulletin of the Geological Society of America, Vol. 81, No. 9, 1970, p. 2759-2776.

Research output: Contribution to journalArticleResearchpeer-review

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