Reliance Formation Type Section
Submitted by Andrew du Toit on Thu, 06/11/2014 - 09:55 | Last updated: Thu, 06/11/2014 - 09:56
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The Buchwa Road from Zvishavane is followed for 20km to the Railway Road, which turns off east along the railway line. Some 3.8km along this road and beyond Brest Siding (barely visible) the access road crosses the railway line and briefly follows it on the south side. This road is followed in an easterly and then SE direction on the north side of the Rupemba Ridge for 5km (do not take a more prominent south turn) where it passes on to the Type Section at its mid-point with the basal units to the south.
Reliance Formation, Ngezi Group
The petrography of the Type Section rocks (Martin 1978) and their chemistry (Nisbet et al., 1977) are described elsewhere and detailed information on some of the more magnesian rocks are given by Nisbet et al (1987). This description presents results obtained from a core drill hole which penetrated ultramafic flows 5km SW of Zvishavane.
The accompanying map provides numbers of thin section descriptions in Martin (1978), while the stratigraphic column gives brief thin section description numbers from Nisbet et al., 1977, as well as the MgO content of the flows.
At the base of the Type Section (and depending on the river level) two sheared sulphidic iron-formations of the Manjeri Formation occur. These rocks strike just north of east as a result of a major ESE trending fault. All dips are very steep. The first exposure is a homogeneous, basaltic greenstone consisting of untwined feldspar intergrown with semi-radiating tremolite (MgO = 8%).
In the succeeding 60m, several outcrops of massive and sheared komatiitic basalt (up to 12% MgO) occur, which are assumed to be parts of different flows, although no flow contacts are visible. Some thin sections show “hollow” and “solid” megacrysts of pigeonite set in a groundmass of felted tremolite, whereas others exhibit tremolite needles intergrown with small amounts of interstitial feldspar.
Above these basal units is a 90m thick differentiated flow or sill, the serpentinised lower cumulate zones of which form the only prominent topographic features within the Reliance Formation. The cumulate base contains equant serpentinised olivine set in a felted mass of antigorite (MgO up to 40%) but higher within the green serpentinite zone the intercumulus mineral is augite. Above the dunites and wehrlite is a narrow band of clinopyroxenite succeeded by a quartz gabbro of partly saussuritized feldspar and ragged clinopyroxene, with interstitial quartz increasing towards the top of the unit.
Exposures of the quartz gabbro and the overlying massive komatiitic basalt (spinifex textured in places) are intermittent and small. It is possible that these represent individual, thin flows above the differentiated unit or they may be part of it, representing a drilled upper zone.
Minor, partly sheared tuffs (MgO = 12%), some of which show graded bedding, are succeeded by further massive and pillowed komatiitic basalt flows. Typically the pillowed and massive flows consist of even aggregates of radiating tremolite with a wavy extinction pattern as an even groundmass cut by laths of chlorite probably after pyroxene (MgO = 11-16%). Some 40m south of the access track a well exposed differentiated flow is exposed, showing most of the spinifex elements described from Munro Township by Pyke et al. (1973), except that clinopyroxene is the dominant mineral, rather than olivine. The base of this flow is enriched with cumulate olivine now wholly altered to antigorite within a tremolite groundmass. Above this the mineralogy is similar, but widely spaced relict skeletal olivine and pyroxene is present, which gives way to a very narrow zone of parallel olivine spinifex. This zone is followed by spectacular clinopyroxene needles which have been deformed, probably by top loading of the partly consolidated flow. (No hammers at this outcrop, please). A section cut normal to these needles shows “hollow” basal sections of clinopyroxene contained by feathery tremolite which also fills the interior of the clinopyroxene. Minor feldspar and some sphene are also present. The random spinifex above has similar minerology and texture in thin section.
Above the main spinifex zone other massive flows occur, and north of the road further narrow flows and zones of clinopyroxene can be seen. It appears that the cumulate bases of the flows tend to be better exposed but there does appear, within the limits of exposure, to be an enrichment of MgO upwards in the sequence.
Near the upper part of the type section several thin flows, bounded by white sheared magnesite (?) occur. Each flow has a bluish basal zone typical of olivine cumulates in the Reliance Formation and a greenish upper zone which was originally more pyroxene rich. Pillows have been interpreted from some of these flows and Nisbet et al. (1977) record larger olivine phenocrysts from pillow centres. However the pillows are difficult to see because of the crumbling nature of the outcrops. The olivines are mostly altered to antigorite and show a variety of skeletal crystal forms described in Nisbet et al. (1977) within a plumose tremolitic groundmass.
The top of the Type Section is roughly half way through the Reliance stratigraphy. Exposure of the komatiites, which form the bulk of the upper part of the stratigraphic column, is generally very poor. However, some small outcrops of olivine spinifex textured flows exist to the SW of Zvishavane and these are described in detail by Nisbet et al. (1987). It is worth noting here that many of the komatiites show fresh relict olivine and it is likely that much of the alteration to serpentinite is due to surface processes, rather than metamorphism.
Chauvel, C., Dupré, B., Todt, W., Arndt, N.T. and Hofmann, A.W. (1983) Pd and Nd isotopic correlation in Archaean and Proterozoic greenstone belts. Eos (American Geophysical Union Transactions), 64, 330. Hall, R. (1983) B.Sc. Dissertation Project, University of Zimbabwe. Martin, A. (1978) The geology of the Belingwe-Shabani schist belt, Geological Survey of Rhodesia, Bulletin, 83. Nisbet, E.G., Bickle, M.J. and Martin, A. (1977) The mafic and ultramafic lavas of the Belingwe greenstone belt, Rhodesia, Journal of Petrology, 18, 521-66. Orpen, J.L., Bickle, M.J., Nisbet, E.G. and Martin, A. (1985) Belingwe Peak (1:100,000), Geological Survey of Zimbabwe. Scholey, S.P. (1989) M.Phil., Ph.D. Transfer Report, University of Southampton, U.K.