Primary magmatic amphibole in Archaean meta-pyroxenite from the Central Zone of the Limpopo Complex, South Africa

M. Keeditse, H. M. Rajesh, G. A. Belyanin, M. Fukuyama, T. Tsunogae

Research output: Contribution to journalArticle

3 Citations (Scopus)

Abstract

Mapping of a section along the Sand River in the high-grade metamorphic Central Zone of the Limpopo Complex, South Africa, delineated meta-peridotite, meta-pyroxenite and meta-gabbro occurring as conformable layers in the host supracaistal rocks. The ultramafic to mafic rocks form part of the Palaeoarchaean Messina layered intrusion, Modal mineralogy of the pyroxenite is typical of an amphibole websterite. Although metamorphic re-equilibration of the pyroxenite is obvious, no new metamorphic mineral had formed. Detailed evaluation of grain margins between the prominent minerals (amphibole, clinopyroxene, orthopyroxene and olivine) indicated two types of grain margins, with their mutual relation used to support the poly-metamorphic nature of overprint on the pyroxenite. Inclusion-host relations between the prominent minerals helped to infer the original igneous order of crystallization. Petrographic and mineral chemical analyses permitted distinction of two varieties of amphiboles, the earlier occurring as polygonal grains as well as inclusions in olivine and pyroxenes, and the later occurring along grain margins, often replacing clinopyroxene. Two varieties of magnetite were also recognized, an earlier Cr-rich variety occurring as inclusions and interstitial grains, and a later Cr-poor variety occurring together with serpentine. Application of geothermobarometers indicate metamorphic re-equilibration conditions (∼830 to 879°C; 5.3 to 6.1 kbars). Comparison with compositional characteristics of primary and secondary/metamorphic amphiboles reported from Archaean ultramafic rocks occurring elsewhere, Alaskan-type layered intrusions, and metasomatised rocks from the vicinity of the study area, helped to place constraints on the nature of amphiboles reported here. The identification of primary magmatic amphiboles in the meta-pyroxenite implies likely formation from a hydrous parental magma in the Archaean mantle.

Original languageEnglish
Pages (from-to)607-622
Number of pages16
JournalSouth African Journal of Geology
Volume119
Issue number4
DOIs
Publication statusPublished - Sep 1 2016
Externally publishedYes

Fingerprint

pyroxenite
amphibole
Archean
layered intrusion
mineral
clinopyroxene
olivine
ultramafic rock
mafic rock
orthopyroxene
gabbro
peridotite
host rock
Africa
magnetite
mineralogy
crystallization
magma
mantle
sand

All Science Journal Classification (ASJC) codes

  • Geology

Cite this

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title = "Primary magmatic amphibole in Archaean meta-pyroxenite from the Central Zone of the Limpopo Complex, South Africa",
abstract = "Mapping of a section along the Sand River in the high-grade metamorphic Central Zone of the Limpopo Complex, South Africa, delineated meta-peridotite, meta-pyroxenite and meta-gabbro occurring as conformable layers in the host supracaistal rocks. The ultramafic to mafic rocks form part of the Palaeoarchaean Messina layered intrusion, Modal mineralogy of the pyroxenite is typical of an amphibole websterite. Although metamorphic re-equilibration of the pyroxenite is obvious, no new metamorphic mineral had formed. Detailed evaluation of grain margins between the prominent minerals (amphibole, clinopyroxene, orthopyroxene and olivine) indicated two types of grain margins, with their mutual relation used to support the poly-metamorphic nature of overprint on the pyroxenite. Inclusion-host relations between the prominent minerals helped to infer the original igneous order of crystallization. Petrographic and mineral chemical analyses permitted distinction of two varieties of amphiboles, the earlier occurring as polygonal grains as well as inclusions in olivine and pyroxenes, and the later occurring along grain margins, often replacing clinopyroxene. Two varieties of magnetite were also recognized, an earlier Cr-rich variety occurring as inclusions and interstitial grains, and a later Cr-poor variety occurring together with serpentine. Application of geothermobarometers indicate metamorphic re-equilibration conditions (∼830 to 879°C; 5.3 to 6.1 kbars). Comparison with compositional characteristics of primary and secondary/metamorphic amphiboles reported from Archaean ultramafic rocks occurring elsewhere, Alaskan-type layered intrusions, and metasomatised rocks from the vicinity of the study area, helped to place constraints on the nature of amphiboles reported here. The identification of primary magmatic amphiboles in the meta-pyroxenite implies likely formation from a hydrous parental magma in the Archaean mantle.",
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Primary magmatic amphibole in Archaean meta-pyroxenite from the Central Zone of the Limpopo Complex, South Africa. / Keeditse, M.; Rajesh, H. M.; Belyanin, G. A.; Fukuyama, M.; Tsunogae, T.

In: South African Journal of Geology, Vol. 119, No. 4, 01.09.2016, p. 607-622.

Research output: Contribution to journalArticle

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