The geology of the Nuanesti Igneous Province

Abstract

The Nuanetsi Igneous Province is situated in the south-eastern corner of Southern Rhodesia and marks the intersection of the Limpopo lineament with the volcanic monocline of the Lebombo. The latter is part of a zone of intense Karroo and post-Karroo volcanicity which lies along the boundary between the uplifted central portion of southern Africa and the depressed Mozambique geosynclinal area to the east. With interruptions, due to the overlap of Cretaceous sediments, the volcanic zone can be traced from Natal to the Zambezi. In the Nuanetsi area a number of late-Karroo ring complexes are found, cutting a thick (ca. 25000 ft.) succession of Karroo lavas consisting predominantly of basalts and rhyolites. A brief description is given of the gneissose Basement complex rocks lying unconformably beneath the Karroo. By plotting Basement trends from aerial photographs it has proved possible to link areas which had previously been mapped by Sohnge (1945) in the Messina area, immediately to the south-west of the province, and by Swift et (1953) at the northern end of the province. A general picture has thus emerged of the extent and trend of the Limpopo orogenic belt, a basement orogeny provisionally dated at 2000 My (Holmes & Cahen 1955). The belt can be divided into three longitudinal zones which have an immense significance in the interpretation of subsequent geological events. The central zone is characterized by the great diversity of the trends shown by individual beds. Folds tend to run oblique to, or even normal to, the overall east-north-east trend of the belt. Flanking the central zone on each side is a zone in which strikes are uniformly parallel to the elongation of the belt. The central zone is about 30 miles wide and the transition to the marginal zones is comparatively sharp, taking place in a matter of 2 to 3 miles. The whole belt is well over 100 miles wide. During the Karroo period the marginal zones were the sites of sedimentary troughs and, later, of intense volcanicity and faulting. The central zone, in contrast, remained as a comparatively stable, positive, unit, termed the Messina Block, and was covered only by a thin veneer of sediments and lavas. The centre line of this zone was marked, during the culmination of the volcanism, by the intrusion of 7 ring complexes lying on a straight east-north-east line. Details of Karroo sedimentation are then given and particular attention is paid to the variations in thickness reflecting the underlying basement structure. Following this, the distribution and general nature of the volcanics and their associated dyke rocks are discussed. Small amounts of nephelinite and allied rocks were erupted at a very early stage and were succeeded by great thicknesses of limburgitic lavas, passing upwards into normal tholeiites. The lower part of the basaltic succession contains numerous intrusions of picritic rocks, many with strong alkaline affinities. The later lavas were of rhyodacitic and rhyolitic composition and were to a large extent erupted as ignimbrites. A brief account is given of the late-Karroo intrusive rocks, viz. the rocks of the ring complexes and the regionally developed Main Granophyre. The latter is an immense sill with an inferred extent of at least 100 miles along the strike. In the south-western part of the province the sill is preserved in the Nuanetsi syncline, a deep and extensive volcano-tectonic fold. The discordance of the sill suggests that, though it has itself a synclinal structure, the folding affecting it is less intense than that affecting its country rocks. Hence it is concluded that the intrusion took place towards the close of deformation of the syncline. All the ring complexes contain granitic or granophyric rocks and many in addition contain earlier intrusions of gabbro. Nordmarkites are occasionally present, falling between the gabbros and granites in time. One complex, Marangudzi, includes a post-granite intrusion of nepheline syenite. The section on tectonics deals first with the descriptive aspects of the structural geology and covers the main tectonic units of the province and surrounding areas. The principal fold structures affecting the Karroo rocks are the Lebombo monocline and its extension northwards in the Mateke—Sabi monocline. The Tuli syncline and Nuanetsi syncline jointly form an approximately 75 east-west structure lying on the north side of the Messina Block and meeting the Lebombo at the southern end of the province. South of the Messina Block lies the Northern Transvaal fault-zone which again contains considerable thicknesses of Karroo rocks. Structural features are summarized on the basis of trend and age-relations and a structural interpretation based on this is suggested. It is concluded that the Northern Transvaal fault-zone and the Tuli syncline owe their location to the zonal structure of the Limpopo orogenic belt in the underlying basement. The Nuanetsi syncline, on the other hand, runs somewhat obliquely across the basement structure and overlaps Limpopo-trend (east-north-east) structural features, such as faults and the line of ring complexes, in both time and space. Hence the fold cannot be envisaged as localized by basement structure in the same way as the Tuli syncline. The uplifted shield area of southern Africa has, however, a deep indentation in its edge, represented by the change of direction of the Lebombo monocline as it swings into the Mateke-Sabi monocline. The Nuanetsi syncline lies in the shield area and meets the shield-margin at the point of the re-entrant, almost bisecting the angle. The syncline, moreover, is filled with volcanic rocks which were largely erupted from fissures parallel to the synclinal axis. Hence the fold can be envisaged as a tensional feature, localized as the result of the concentration of stress at the point of the re-entrant in the margin of the shield. Such stress appears to have been caused by essentially lateral tectonic movements affecting the shield. A very close analogy may be found in the Kangerdlugssuaq area of East Greenland (Wager 1947). A general tectonic synthesis, involving an assessment of vertical and lateral tectonics, cannot be attempted until more information is available on antithetic fault-zones such as that of the Northern Transvaal. Such zones are of wide occurrence in Africa and consist essentially of groups of parallel, apparently normal, faults all of which throw and hade in one direction and tend to offset the dip of the beds. Interpretation in terms of pure vertical tectonics fails to account for the antithetic nature of the faulting; interpretation in terms of pure lateral tectonics, however, does not explain the uniformity of hade. It is considered that a possible interpretation may be made by postulating a crust held relatively rigidly in position in its upper levels, overlying a mobile substratum which can flow laterally. Oblique shear planes transecting the whole crust could be formed by such a mechanism, rather in the way in which an incompetent formation is sheared in a folded sedimentary sequence. The petrography and geochemistry of the Karroo igneous rocks of the province and surrounding areas are then discussed. The basalts can be shown to form a continuous series from limburgites, carrying normative olivine, to oversaturated quartz-bearing types. This is termed the Tholeiitic Series, the limburgites and olivine-basalts being designated high-magnesia tholeiites, and the oversaturated types, low-magnesia tholeiites. The latter correspond with the original tholeiites of Kennedy (1933). A continuous series can also be established for the acid rocks and includes both extrusives and intrusives, the two types being chemically indistinguishable. It is considered that the rhyolites of the Lebombo and Nuanetsi areas represent primary acid magma and are not the differentiation products of Karroo basaltic magma. The Tholeiitic Series, too, is difficult to interpret in terms of conventional crystallization differentiation, mainly because the observed chemical variation cannot be explained by reference to the observed phenocrysts. It is therefore considered possible that the major variations in the Lebombo—Nuanetsi zone are due to a thermal disturbance causing the production of a large range of primary magmas by refusion or partial refusion of the substructure at differing levels. This leads to the concept of the Karroo volcanic cycle, an essentially thermal event consisting of a waxing phase during which geo-isotherms rise, a culmination when the geothermal gradient has reached its steepest inclination, and a waning phase during which thermal normality is regained. Deep-seated rocks of alkaline affinities are erupted at the beginning and the end of the cycle. Rhyolites correspond with the culmination. It may be said that the waxing phase represents the ideal environment for rock-variation by melting processes, whereas the waning phase is the environment of crystallization differentiation. The waning phase of the Karroo volcanic cycle is represented by the basalt—trachyte—phonolite association found overlying the Karroo rhyolites in the Lebombo and in the Lupata area on the Zambezi. The cycle therefore includes rocks of Cretaceous age and hence transects the normal stratigraphic time-divisions. Finally, some problems of carbonatite genesis are discussed in terms of the Karroo volcanic cycle.