Fundamental Minerals

Sialic Minerals

Feldspar

Feldspathoid

Quartz

Femic Minerals

Accessory Minerals

Very Rare Minerals

Secondary Minerals

Texture

Alteration Products

Glass and related Texture

Volcanic Rocks

Komatiites

Lamprophyres

komatiite (Zimbawe)

Belingwe greenstone belt komatiite

The Belingwe greenstone belt is located in the southern part of Zimbabwe Craton, north of the Limpopo orogenic belt and directly east of the Great Dyke (Fig. 1). The Belingwe Greenstone Belt comprises the Lower Greenstones (2.9 Ga Mtshingwe Group), which consists of three sedimentary formations whose deposition environment ranges from probable beach to deeper water conditions, and the unconformably overlying Upper Greenstones (2.7 Ga Ngezi Group). It is bordered on the east and west by 3.5 and 2.9Ga gneissic complexes. The northern and southern borders of the Belingwe greenstone belt are intruded by 2.6Ga batholiths (Martin et al., 1993).

Fig.1: Geological map of Belingwe greenstone belt modified after Nisbet et al. (1977).

The Belingwe Greenstone Belt comprises (Fig.2):

Basement Gneiss Complex: The basement, or rocks upon which the greenstone belt rests, consists of many different units (Quartzo-feldspathic gneissic rocks, metapelitic gneiss, and mafic intrusions.

Ngezi Group

Manjeri Formation: a thin clastic sedimentary succession (maximum thickness 250m) deposited unconformably on an older felsic basement.
The Reliance and Zeederbergs Formations: volcanic succession of basalt, komatiitic basalt and komatiite, and thin intercalated tuffaceous layers. Chauvel et al. (1993) obtained an age of 2692 Ma for komatiitic basalts in the Reliance Formation by Pb-Pb dating. There is strong evidence that the volcanic successions of the Reliance and Zeederbergs Formation erupted in shallow submarine conditions onto submerged continental crust.
Cheshire Formation: the uppermost stratigraphic unit of the Belingwe greenstone belt, comprises shallow-marine carbonates.

Mtshingwe Group

Mtshingwe Group consisting of mafic, ultramafic, intermediate, and felsic volcanic rocks, pyroclastic deposits, and a variety of sedimentary.

The metamorphic grade of the upper greenstones is astonishingly low, with abundant fresh olivine, clinopyroxene, feldspar, and rare orthopyroxene, in the mafic and ultramafic rocks (Martin, 1978). Where encountered, serpentinization is attributed to weathering, and metamorphic mineral growth in the sedimentary rocks is restricted to the growth of sericite and chlorite (Martin, 1978).

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Fig.2: Interpretative stratigraphic column of Belingwe greenstone belt. Modified after Kusky, T. M., & Kidd, W. S. (1992).

Komatiites flow

Komatiites in the Belingwe greenstone belt are restricted to the Reliance Formation. Typical komatiite flows consist of upper spinifex-textured layers (A zone) and lower layers of olivine cumulate (B zone). A zone, which are one third thick of the flow are composed mainly of random olivine spinifex and oriented, or bladed olivine spinifex. B zones that are two thirds thick of the flow are divided into two zones: thin skeletal and hopper olivine layer (B1 zone) at the upper olivine cumulate zone, and euhedral olivine layer at lower cumulate zone.

Spinifex zone (A zone): In spinifex zone, random spinifex textured olivines gradually become longer down to the flow, ranging from 1cm to 5cm in length. And suddenly the length of spinifex textured olivines become much longer (maximum length of them is 80cm), and oriented vertically to the flow, then spinifex textured olivines again become smaller and randomly oriented, continuing to B1 zone. Random spinifex zones contain spinifex olivine and clinopyroxene and Cr-spinel as igneous relicts. They also contain, As secondary phases, chlorite, magnetite, sulfide (mainly pyrite) serpentine. Most of the olivines remain original igneous texture, only rims and cracks of them are serpentinized.

Upper cumulate zone (B1 zone): Upper cumulate zone (B1 zone) contains euhedral and mainly skeletal and hopper olivines. Plumose clinopyroxene dominates the groundmass. Renner et al. (1993) and Silva et al. (1997) suggest from texture and chemical composition of B1 zone, that the B1 zones of komatiite flows are the very last liquid to solidify in komatiite flows.

Sample kindly given to me by Kevin Walsh

Bibliography

• Asafov, E. V., Sobolev, A. V., Gurenko, A. A., Arndt, N. T., Batanova, V. G., Portnyagin, M. V., ... & Krasheninnikov, S. P. (2018). Belingwe komatiites (2.7 Ga) originate from a plume with moderate water content, as inferred from inclusions in olivine. Chemical Geology, 478, 39-59.
• Nisbet, E. G., Bickle, M. J., & Martin, A. (1977). The mafic and ultramafic lavas of the Belingwe greenstone belt, Rhodesia. Journal of Petrology, 18(4), 521-566.
• Nisbet, E. G., Arndt, N. T., Bickle, M. J., Cameron, W. E., Chauvel, C., Cheadle, M., & Roedder, E. (1987). Uniquely fresh 2.7 Ga komatiites from the Belingwe greenstone belt, Zimbabwe. Geology, 15(12), 1147-1150.
• Kusky, T. M., & Kidd, W. S. (1992). Remnants of an Archean oceanic plateau, Belingwe greenstone belt, Zimbabwe. Geology, 20(1), 43-46.

Photo
Skeletal olivine crystals (Serpentinized) in a groundmass of skeletal Pyroxenes. Belingwe greenstone belt. PPL image, 2x (Field of view = 7mm)	Skeletal olivine crystals in a groundmass of skeletal Pyroxenes. Belingwe greenstone belt. XPL image, 2x (Field of view = 7mm)	Skeletal olivine crystals (Serpentinized) in a groundmass of skeletal Pyroxenes. Belingwe greenstone belt. PPL image, 2x (Field of view = 7mm)
Skeletal olivine crystals in a groundmass of skeletal Pyroxenes. Belingwe greenstone belt. XPL image, 2x (Field of view = 7mm)	Skeletal olivine crystals (Serpentinized) in a groundmass of skeletal Pyroxenes. Belingwe greenstone belt. PPL image, 2x (Field of view = 7mm)	Skeletal olivine crystals in a groundmass of skeletal Pyroxenes. Belingwe greenstone belt. XPL image, 2x (Field of view = 7mm)
skeletal Pyroxenes. Belingwe greenstone belt. PPL image, 2x (Field of view = 7mm)	Skeletal olivine crystals (Serpentinized) in a groundmass of skeletal Pyroxenes. Belingwe greenstone belt. PPL image, 2x (Field of view = 7mm)	Skeletal olivine crystals in a groundmass of skeletal Pyroxenes. Belingwe greenstone belt. XPL image, 2x (Field of view = 7mm)
Skeletal olivine crystals (Serpentinized) in a groundmass of skeletal Pyroxenes. Belingwe greenstone belt. PPL image, 2x (Field of view = 7mm)	Skeletal olivine crystals in a groundmass of skeletal Pyroxenes. Belingwe greenstone belt. XPL image, 2x (Field of view = 7mm)	Skeletal olivine crystals (Serpentinized) in a groundmass of skeletal Pyroxenes. Belingwe greenstone belt. PPL image, 2x (Field of view = 7mm)
Skeletal olivine crystals in a groundmass of skeletal Pyroxenes. Belingwe greenstone belt. XPL image, 2x (Field of view = 7mm)	Skeletal olivine crystals (Serpentinized) in a groundmass of skeletal Pyroxenes. Belingwe greenstone belt. PPL image, 2x (Field of view = 7mm)	Skeletal olivine crystals in a groundmass of skeletal Pyroxenes. Belingwe greenstone belt. XPL image, 2x (Field of view = 7mm)
Skeletal olivine crystals (Serpentinized) in a groundmass of skeletal Pyroxenes. Belingwe greenstone belt. PPL image, 2x (Field of view = 7mm)	Skeletal olivine crystals in a groundmass of skeletal Pyroxenes. Belingwe greenstone belt. XPL image, 2x (Field of view = 7mm)	Skeletal olivine crystals (Serpentinized) in a groundmass of skeletal Pyroxenes. Belingwe greenstone belt. PPL image, 2x (Field of view = 7mm)
Skeletal olivine crystals in a groundmass of skeletal Pyroxenes. Belingwe greenstone belt. XPL image, 2x (Field of view = 7mm)	Skeletal olivine crystals (Serpentinized) in a groundmass of skeletal Pyroxenes. Belingwe greenstone belt. PPL image, 2x (Field of view = 7mm)	Skeletal olivine crystals in a groundmass of skeletal Pyroxenes. Belingwe greenstone belt. XPL image, 2x (Field of view = 7mm)
Skeletal olivine crystals (Serpentinized) in a groundmass of skeletal Pyroxenes. Belingwe greenstone belt. PPL image, 2x (Field of view = 7mm)	Skeletal olivine crystals in a groundmass of skeletal Pyroxenes. Belingwe greenstone belt. XPL image, 2x (Field of view = 7mm)	Skeletal olivine crystals (Serpentinized) in a groundmass of skeletal Pyroxenes. Belingwe greenstone belt. PPL image, 2x (Field of view = 7mm)
Skeletal olivine crystals in a groundmass of skeletal Pyroxenes. Belingwe greenstone belt. XPL image, 2x (Field of view = 7mm)	Skeletal olivine crystals (Serpentinized) in a groundmass of skeletal Pyroxenes. Belingwe greenstone belt. PPL image, 2x (Field of view = 7mm)	Skeletal olivine crystals (Serpentinized) in a groundmass of skeletal Pyroxenes. Belingwe greenstone belt. PPL image, 2x (Field of view = 7mm)
Skeletal olivine crystals (Serpentinized) in a groundmass of skeletal Pyroxenes. Belingwe greenstone belt. PPL image, 2x (Field of view = 7mm)	Skeletal olivine crystals in a groundmass of skeletal Pyroxenes. Belingwe greenstone belt. XPL image, 2x (Field of view = 7mm)	Skeletal olivine crystals (Serpentinized) in a groundmass of skeletal Pyroxenes. Belingwe greenstone belt. PPL image, 2x (Field of view = 7mm)
Skeletal olivine crystals (Serpentinized) in a groundmass of skeletal Pyroxenes. Belingwe greenstone belt. PPL image, 2x (Field of view = 7mm)	Skeletal olivine crystals (Serpentinized) in a groundmass of skeletal Pyroxenes. Belingwe greenstone belt. PPL image, 2x (Field of view = 7mm)	Skeletal pyroxen between olivine crystals. Belingwe greenstone belt. PPL image, 10x (Field of view = 2mm)
Skeletal pyroxen between olivine crystals. Belingwe greenstone belt. PPL image, 10x (Field of view = 2mm)	Skeletal pyroxen between olivine crystals. Belingwe greenstone belt. XPL image, 10x (Field of view = 2mm)	Skeletal pyroxen between olivine crystals. Belingwe greenstone belt. PPL image, 10x (Field of view = 2mm)
Skeletal pyroxen between olivine crystals. Belingwe greenstone belt. PPL image, 10x (Field of view = 2mm)	Skeletal pyroxen between olivine crystals. Belingwe greenstone belt. XPL image, 10x (Field of view = 2mm)