Introduction: The Zimbabwe Craton and controversies over Archaean granite-greenstone terrains
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Abstract:
The Belingwe Greenstone Belt contains a wide variety of Archaean rocks which provide evidence of major significance in interpreting the strati graphic history of the Zimbabwe Craton, and, more generally, in understanding the nature of the Archaean Earth. The stratigraphic sequences in the belt are well preserved, and the interpretation of the Belingwe rocks provides the underpinning of the present stratigraphic evolution of the Zimbabwe Craton. The sequence includes old basement, consisting of granitoid gneisses ca. 3.5 Ga old, overlain by three older greenstone sequences and a well developed younger (2.7 Ga) greenstone sequence. At the base of the 2.7 Ga sequence is a well exposed unconformity, demonstrating that the lavas and sediments of the younger greenstones were laid down on a mature terrain of eroded granitoid and greenstones. The belt contains a wide variety of unusual rocks, including an extensive and locally very fresh suite of komatiites and komatiitic basalts, as well as carbonates which contain stromatolites. These rocks demonstrate that some greenstone belts formed on continental crust. The variety of sedimentary and volcanic environments probably reflects interaction of a range of tectonic processes as diverse as those operating today.Keywords:
Greenstone belt
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Ultramafic rock
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Supercontinent
Dharwar Craton
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The Red Lake greenstone belt is Archaean in age (older than 2.5 billion years) and is located in the Superior province of the Canadian Precambrian Shield. It is a fairly typical greenstone belt, being composed of a complex assemblage of lavas, sediments, and intrusives. The belt is completely surrounded, and therefore is isolated from other greenstone belts, by granitic batholiths and acid paragneiss. Generally speaking, greenstones are more dense than the surrounding granitic rocks and they therefore give positive gravity effects, the amplitudes of which give some indication of their shape and overall thickness.At Red Lake, the greenstone belt is approximately 35 mi long by 18 mi wide. Gravity readings taken across the width of the belt indicate that the greenstones taper sharply in depth to a maximum thickness of approximately 25 000 ft. These results appear to confirm, as most geologists feel intuitively, that greenstone belts are basin-shaped and are underlain by granitic batholiths and gneiss.
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Abstract Major Archaean greenstone gold deposits are located in distinctive structural settings, and Zimbabwe is a good example with over 90% of its gold production being derived from structurally controlled veisns, shear zones and probably epigenetic replacement BIF hosted deposits. The structural framework of the Zimbabwe craton is dominated by the evolution of the c. 2700 Ma Limpopo belt. Strongly developed, major shear zones were initiated at this time. These major shear zones focused strain, which in the Midlands greenstone belt, evolved from pure N-S compression, to lateral extrusion of the main Rhodesdale gneiss block westwards. The structural features seen in the gold deposits of the Midlands greenstone belt are consistent with such a model of NNW-SSE compression linked to lateral extrusion of the Rhodesdale gneiss block westwards. Initiation of structures during NNW-SSE compression resulted in early folding such as the Kadoma anticline, together with initiation of the major mineralized shear systems as thrust faults. Simple shear evolved to transpressive shear with development of oblique dextral and sinistral mineralized shear systems. Rotation of the principal stress direction clockwise to an ENE-WSW orientation led to evolution of dominantly dextral transpressive shear on the major Lily, Munyati and Rhodesdale boundary shear systems, probably in response to the lateral extrusion of the Rhodesdale gneiss block westwards. This even is linked to waning gold mineralization and accounts for the major dextral offsets along the Lily fault and the Munyati shear close to Battlefields.
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