Submitted by Andrew du Toit on Thu, 30/10/2014 - 15:03 | Last updated: Tue, 04/11/2014 - 10:52
Grid Ref (WGS84 Lat/Long in decimal degrees)
Take Borrowdale Road north out of Harare and turn right at the Museums and National Monuments sign.
Chinamora Igneous Complex
The Chinamora Igneous Complex, some 1 500 sq km in extent, is an area of granitoid rocks north of Harare which is almost completely enveloped by ancient volcanic and metasedimentary rocks of the Harare-Shamva Greenstone Belt. The clear distinction between these two terrains, as one drives north towards Domboshawa, is a change from heavy red-brown loams to pale yellow-grey sandy soils. The plutonic complex is roughly elliptical in shape and it comprises a variety of gneisses, migmatites and granites of different age and composition. The dominant, centrally placed late-stage (2600 million year old) intrusive rock is referred to as the Chinamora Porphyritic Granite, exposed as prominent dome-shaped hills, steep-sided dwalas or bornhardts and as castle koppies. The relationship between this massive granite and the enclosing gneisses suggests that it was emplaced from a central source and spread flatly outwards in sheet form. It is not envisaged that it is a single updomed intrusion or diaphiric batholith.
The country north of Domboshawa reflects granite geomorphological expressions spectacularly. The rock exposures are controlled by erosional down grading from the old African Landsurface within the Post-African cycle. The presence of the massive rock outcrops conform to the rectangular disposition of joint and fault lines which cross the granite complex whilst the variable spacing between joint planes dictates whale-back, dwala or castle koppie expression. Domboshawa Hill itself is only sparsely jointed but granite outcrops surrounding it are castlated due to the closer proximity of joints, which may be near vertical or horizontally displayed.
As one ascends Domboshawa Hill from the car park you are aware of a gneissic foliation in the granite. This displays a slightly convoluted mineral alignment, which in places encloses fragments of older, usually dark-coloured rock referred to as xenoliths. Over most of the remaining exposure the rock is coarse-grained with a development of large rectangular feldspar crystals called phenocrysts. These give the granite its porphyritic texture. The granite is cut in places by linear reefs of very coarse quartz-feldspar pegmatite up to 30 cm in width. Close spaced joints bound a north-north-easterly, near vertical face below what is called caterpillar rock. The low domed shape of Domboshawa, leading up to the trigonometrical beacon at 1638 metres above sea level, is due to exfoliation, a process akin to the peeling of an onion. Vertical joints and open exfoliation planes occur in the vicinity of Domboshawa Cave and combine to give this its location. The interconnection of these planes provide the passage way for smoke to 'the chimney' above the cave, so important an aspect of the rain making tradition.
One is aware of small-scale ridges and valleys in the massive granite as the beacon is approached. These are drainage features caused by erosion from rainfall runoff. The more sharply defined valleys are often preferentially placed along local joint lines or pegmatite bodies but others are purely erosional, sometimes with an incised channel bounded by a resistant rim. It has been surmised that these irregular rims were protected by lichen growth, but they may represent a locus for silicification. In places, especially on the local watersheds, surface weathering results in a pan-like hollow or gnamma which may retain water after rainfall. Such hollows may coalesce to form larger ponds. Subsequent chemical weathering can result in a cracked, decomposing granite surface. All of these features create micro-environments in which soil and humus can accumulate. These support the varied flower, grass, tree, aloe and lichen growth to be seen on the hill. Outwash in front of Domboshawa Cave concentrates various microliths of quartz crystal, dolerite and even glass. These materials are exotic to Domboshawa, although the first two are associated with faults and intrusions in the region, and they have been brought in by Man.
A 360-degree view from the beacon on Domboshawa shows a number of topographic points to be of concordant height. These include the Great Dyke and Mount Hampden in the west, the Iron Mask and Shashi ranges to the north-west and north, Ngomokurira and other bornhardts in the Chinamora Complex extending north and north-east to the Shamva ranges, Dombo re Mbudzi and the whale-backs of the Mangwendi terrain and the planation across the northern suburbs of Harare. These levels represent remnants of the African, and in some places older, landsurfaces. Uplift during the Miocene Period created a new base level to which the Post-African surface eroded and which has advanced up the valleys of the Mazowe, Pote and Umwindsi rivers to exhume the present landscape. The two surfaces link in a nick point defined either as a scarp face or as a transitional zone. The shaping of our landscape has been cyclical and the preserved landsurfaces can be recognised elsewhere in Africa, correlating to the tectonic and stratigraphic history of the subcontinent.
National Monument - pay on arrival
BAGLOW, N. 1992. Bindura. Published geological map. Scale, 1 : 100 000. To accompany Zim. geol. Surv., Bull. No. 97. BALDOCK, J.W. 1991. The geology of the Harare Greenstone Belt and surrounding granitic terrain. Zim. geol. Surv., Bull. No. 94, 213 pp. (With published map). LISTER, L.A. 1973. The microgeomorphology of granite hills in northeastern Rhodesia. pp. 157-161 in L.A. Lister Editor), Symposium on Granites, gneisses and Related Rocks, Salisbury, 1971. Geol. Soc. S. Afr., Spec. Publ. No. 3, 509 pp. LISTER, L.A. 1987. The erosion surfaces of Zimbabwe. Zim. geol. Surv., Bull. No. 90, 163 pp.