The structural interpretation of the geology of the Shabani area Rhodesia

Abstract

The major structure of the Shabani area is a north-south trending syncline, involving "Greenstones" which rest unconformably on various schists and gneisses of pre-Bulawayan ago. These basement gneisses have been divided into: migmatites, banded gneisses, granitic gneisses andgneissic granite, a sub-division that has proved extremely useful in evaluating the mutual relationships within the gneisses and between the gneisses and the later formations. The Basement Rocks; The oldest rocks found within the area are the migmatites and banded gneisses and, where more granitized, the granitic gneisses. Although no lithological distinction may be seen, it is believed that the strongly banded gneisses and the migmatites together consist of at least two structurally distinct generations of rocks; an earlier group having an east-south-east, west-north-west trend and a later group having a predominantly north-south trend. It is suggested that a second period of deformation caused the refolding of the already deformed first series of gneisses. This situation is most clearly demonstrated in the area to the north-west of the "Great Dyke", A gradation into only partially migmatized rocks is found within both of these groups, the original rocks were evidently iron rich sandstones, greenstones and a more calcareous sediment. A west-north-west, east-south-east trending batholith, into which the earlier migmatites grade both laterally and marginally, is responsible for the formation of the earlier migmatites. This granite batholith must have been at least partially fluid during its emplacement, since during the growth of microcline phonocrysts within the rock, small euhedral crystals of plagioclase, sphene and muscovite, attached themselves onto the growing crystal faces of the phenocryst. The Lower Sedimentary and Volcanic Greenstones : Over-lying the basement gneisses (structurally higher), mainly in the south of the area, is a series of sedimentary and volcanic greenstones that have been thermally metamorphosed by the intrusion of later granitic rocks. Near the top of this sedimentary sequence, opposite the Ngesi Antimony Claims, at the Ngesi River bridge on the Shabani-Belingwe road, a silt of subaqueously deposited tuff occurs. Within this deposit, graded bedding, flame structures, ripple marks and accretionary volcanic lapilli may be observed, demonstrating the sedimentary nature of this horizon, and in this locality its slightly overturned attitude. After the deposition of this series, the intrusion of concordant ultrabasic sills occurred, the presence of suchsills seems characteristic of these deposits. The Middle Sedimentary and Volcanic Greenstone Group: Resting unconformably upon the Lower Sedimentary and Volcanic Greenstones, which had already been folded about north-east, south-west trending axial planes, are the Middle Sedimentary and Volcanic Greenstone rocks, composed of volcanic lavas,tuffs and sub-aqueous sediments, This sequence begins as a predominantly sedimentary series of ironstones and conglomerates, volcanic materials increasing in amount at higher stratigraphic levels. The Middle Sedimentary and folded Volcanic rocks overstep the folded Lower Sedimentary and Volcanic Group onto the gneisses of the basement. The Middle Sedimentary and Volcanic Greenstone Group compose the most widespread lithological unit of the greenstone rocks and are preserved in a tight synform, trending north-south and slightly overturned towards the east. Throughout this large structure, graded bedding, ripple marks, flame structures and pillow lavas, all demonstrate that the rocks arc the "right way up", and that the structure is an upward "facing" syncline. The Upper Sedimentary Group i In the south of the area, the synclinal core of the Middle Sedimentary and Volcanic Greenstone Group is occupied by rocks of a distinctly different lithology; conglomerates, grits, siltstones and mudstones. The strike of this younger series is on the whole parallel to that of the Middle Sedimentary and Volcanic Greenstone Group, the dip vortical, but there are many local variations in strike and many minor, internal unconformities. The outcrop pattern of this Upper Sedimentary Group is suggestive of one that has been modified by the slumping of still unconsolidated sediments during the latter stages of synclinal infilling. Later Tectonic Events: Following the deposition of the Upper Sedimentary Group, the major synclinal structure suffered an up doming about a west-north-west, east-south-east axis, which was accompanied in its later stages of development by the intrusion of the Younger Granite. The latest major tectonic event was the emplacement of the Great Dyke, parallel to the north-south axis of the Middle Sedimentary and Volcanic Greenstone syncline. Later, normal both sinistrai faulting both sinistral and dextral wrench faulting, trending west-north-west, east-south-east, has affected the whole area.