Geologisch-mineralogische Untersuchungen am Donkerhoek-Granit, Karibib-Distrikt, Südwestafrika

Abstract

Abstract provided by author

The type area of the Donkerhoek Granite of about 2100 km, situated at the fringe of the Namib Desert in South West Africa, was mapped to a scale of 1:50 000. The Donkerhoek Granite was studied petrographically and petrologically, and the relationship of the granite to the fold structures of the surrounding metasediments of the Damara Group was investigated. This work forms part of a research project of the "Sonderforschungsbereich 48" financed by the Deutsche Forschungsgemeinschaft. Under this project representative parts of orogenic belts of different ages are being studied in order to advance the understanding of the evolution of the continental crust

It was the aim of the present investigation, to establish the mechanism and the structural history of the intrusion of the Donkerhoek Granite into the eugeosynclinal frame of the late-Precambrian Damaran Orogenic belt

The field evidence and the results of the mineralogical investigation characterize the Donkerhoek Granite as a light gray, medium grained, homogeneous biotite-muscovite monzogranite, which intruded the Damara Group metasediments post-tectonically at the height of the regional metamorphism. The Donkerhoek Granite penetrates the basal portion of the Khomas Formation (upper portion of the Damara Group) in irregular bodies forming parts of a large NE-SW striking granite complex. Discordant contacts, considerable deformation and disturbance of the country rock and the development of various, intrusive contact features indicate an active, forceful emplacement of the Donkerhoek Granite. In the investigated area, the sediment fill of the eugeosyncline consists to a minor part of quartzitic metasediments of the Nosib Group in the north of the area and to a major part of an extensive sequence of high-grade metamorphic, pelitic quartz-mica schists of the Damara Group in the south. The NW-dipping metasediments form a south-easterly overturned Synclinorium. The Donkerhoek Granite intruded into the overturned limb of this Synclinorium

The metasediments of the Damara Group have been deformed by the first and the third folding phase of the Damaran orogeny, The supra structure of the southern part of the fold belt ends at the northern boundary of the Damara Group against the Nosib Group

The position of the Donkerhoek Granite within the deep Synclinorium formed by the Damara metasediments assigns the granite to the supra structure. The northwestern margin of the investigated area meets the infra structure, which shows different structural style and forms the central part of the fold belt

A compilation of the observations and deductions suggests the following history of intrusion: The initial granitic melt of the Donkerhoek Granite probably originated by anatexis from eugeosynclinal sediments, supposedly of the Damara Group, at a level only a few kilometers below the present surface. The formation of the magma was initiated by regional metamorphism after the termination of the main phases of folding

The wet melt ascended under plastic deformation from its place of origin as a result of gravity inversion, supposedly after the termination of Orogenic compression. This primarily "passive" ascent with mechanical assimilation of parts of the penetrated rocks passes into an "active" emplacement towards the end of the intrusion in a zone of relatively brittle rocks, accompanied by processes like stoping and sheeting, which caused the various contact developments. The position of the Donkerhoek Granite in the structural setting of the Damaran Orogenic belt suggests an ascent along a NE-SW striking thrust fault within the southeasterly overturned limb of the Synclinorium mentioned above. The thrust fault may have been the results of pre-granitic movements between the infra and supra structure

The Donkerhoek Granite reached its present level of exposure after a relatively short ascent of only a few kilometers at an original depth of 14 - 15 km as deduced from the p-t conditions under which the surrounding Khomas schists have been metamorphosed. The occurence of primary muscovite in the granite and melting experiments support this deduction. The petrological findings of a granite with short ascent and characterized by a low-melting composition (648°C at 4 kbar H2O pressure solidus), which supplies much melt over a small temperature interval, are in agreement with the position of the Donkerhoek Granite in the immediate vicinity of the central belt with highest-grade metamorphism

The deformations, disturbances and dislocations of the country rock prove that the emplacement of the Donkerhoek Granite was forceful and "active". The magma maintained its mobility until the end of the intrusion and then crystrallized uniformly. Fracture zones with feather joints within the country rock at the granite contact, which possibly originated during the cooling of. the granite, confirm the relative brittleness of the country rock. The presently exposed roof portion of the Donkerhoek Granite suggests an accumulation of "drop-like" melt bodies to form the granite complex. The Donkerhoek Granite is not surrounded by a contact aureole and, besides an occasional weak foliation at the contact, does not show any mineral lineation nor mineral deformation, neither macroscopically nor in thin section

The emplacement of the Donkerhoek Granite ended with the intrusion of innumerable inhomogeneous layered pegmatites. The pegmatites were supplied by the Donkerhoek Granite and cut through its roof portions and the country rock in a dense, characteristic network of dikes and plugs. Detailed mapping revealed the presence of up to 30 percent, by volume, of pegmatite in the Khomas Formation between the granite bodies. The pegmatite intrusion occurred in part actively (producing space) with deformation of the country rock and in part passively into opening fissures. Most contacts of the pegmatites are discordant and traverse the country rock irregularly

A fission track age determination carried out on a garnet yielded an age of 434 + 55 m. y. for the pegmatites associated with the Donkerhoek Granite. It is supported by a K/Ar age determination of 420 ± 20 m. y. on biotites of a Donkerhoek Granite sample. A comparison of the Donkerhoek Granite with the Sorris-Sorris Granite situated at the northern margin of the eugeosynclinal belt revealed similarities between these two granites

The Achas Granite stock at the northwestern margin of the investigated area is briefly discussed and compared with the Donkerhoek Granite. The Achas Granite seems also to be post-tectonically intruded, but before the Donkerhoek Granite emplacement. The most obvious differences include contact features, completely developed granite tectonics, the mineral composition and the petrological properties (higher solidus temperature). Geological and mineralogical findings tend to assign the Achas Granite to the infra structure of the central belt

The post-orogenic geological features comprise a small outcrop of (?) glaciofluvial Karroo deposits. Karroo dolerite dikes and young, prominent, N-S striking faults

The sand, debris, calcrete and calcareous terrace deposits are of Tertiary to Recent age