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Kariba porphyritic biotite granitoid gneiss

The Kariba porphyritic biotite granitoid gneiss is coarse-grained, with a weak gneissic fabric, and contains large K-feldspar phenocrysts, and numerous decimetric slab-like mafic xenoliths. It contains small garnets rimmed and replaced by biotite. The zircons dated have oscillatory zoning in the centre (of igneous origin), but the margins have been affected by metasomatic alteration zones, and in some cases have discrete overgrowth rims. The zircons have yielded ages of 1962.9 ± 8.5 Ma and 2.1-2.17 Ga.

Walwyn Conglomerate

There are some excellent exposures (Figure 4) of an unusual course, polymict, clast-supported sedimentary deposit with well-rounded boulders of a number of different lithotypes including granitic and other felsic rocks, basaltic greenstone, chert and limestone. Boulders are up to a metre in diameter (Stidolph, 1977) but most range from 5 to 30 cm. This wedge-shaped outcrop is up to 300 m thick and extends for 2.3 km. Very good examples of pressure solution can also be seen. The origin of this deposit is up for debate.

 

Palaeoproterozoic (ca 2.07 Ga) granitoid , with complex (inherited) 3.4 and 2.7 Ga zircons

In this outcrop, there is a deformed granite with anastomosing subhorizontal fractures, which is weathered with brownish iron oxide staining. It has yielded a very complex set of zircons, with inherited cores as old as 3.4 Ga, with 2.7 Ga overgrowths, and a final overgrowth at 2.07 Ga, which is possibly the age of intrusion of the granite, which was subsequently deformed (Master et al., 2013a,b).

Manjeri Formation Type Section

This exposure is described in detail by Martin (1978) and Bickle et al. (1975) and only a summary is given here.
 
The sedimentary succession here is 130m thick with the lower part dominated by argillite and quartzite and ironstone and the other 80m by siltstone and argillite.

Kariba sillimanite quartzite Kariba Heights

The Kariba sillimanite quartzite Kariba Heights, Sample ZMB13/11 is an aluminous quartzite consisting mainly of quartz and sillimanite. The zircons have yielded concordant ages of 2.018 Ga, 2.172, 2.220 and 2.70 Ga (Master et al., 2015). The maximum age of the quartzite is 2.018 Ga, the age of the youngest concordant detrital zircon, while the other zircons reflect a provenance from older crust dated at 2.17, 2.22, and 2.70 Ga.

Felsic volcanic / clastic rocks

Stidolph (1977) assigned these outcrops to the lower unit of the Shamvaian Group which consists of volcaniclastic sediments and pyroclastic rocks devoid of granitic clasts. These rocks form an extensive belt stretching across much of the south-eastern limb of the Shamva Belt.

Mixtures of sedimentary and pyroclastic deposits can be seen over scattered outcrops at this locality, indicating reworking of fine pyroclastic material and agglomerates. Some unusual textures of debateable origin are present.

 

Pink paragneiss (meta-arkose) of the Malaputese Group

At this outcrop locality, about 10 km further NW from the previous stop, there are numerous roadside outcrops of strongly recrystallized pink paragneiss of the Malaputese Group. The gneisses are interpreted as meta-arkoses, and some show relict cross bedding. They have yielded a detrital zircon population that ranges in age from 2254 ± 18 to 2796 ± 17 Ma, with a strong age peak at ca. 2.7 Ga (Master et al., 2013a,b).

Reliance Formation Type Section

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).

The Kariba Dam Observation Point

The steps ascending to the interpretive centre (beneath the crochet work) at the Observation Point, and those beyond, reveal banded Kariba Quartzite, with apparent biotite gneiss interleaved in either bedded or thrust relationship.  It is in this situation where weathering is deep and where the dip corresponds with the hill slope, landslip took place threatening to disrupt the tailraces on the south bank.  Extensive drainage, rock bolting and slope sculpture has aleviated this threat.

Felsic volcanics

Many of extrusive rocks within the Shamvaian  are porphyritic and have been interpreted as crystal tuffs with plagioclase phenocrysts, but on fresh surfaces these are difficult to see. The rocks range in composition from rhyolite to dacite and some may be intrusive.

 

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