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

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.