Die Grenze der zentralen Granitzone südwestlich Otjimbingwe (Karibib-Distrikt, Südwestafrika)

Abstract

Summary provided by author:

An area of about 500 km was mapped with the aid of aerial photographs. The granitic rocks could be subdivided while mapping into Donkerhoek Granite, granodiorite and migmatite. A close relationship was observed between the intrusions and the tectonic structure

According to the tectonic structures, the open B1 folds and solitary isoclinal folds, the area could be assigned to"isotectonic zone I" in the scheme proposed by PORADA & WITTIG (1975). A transverse, NE/ SW tectonic zone observed in the south is interpreted as a dextral shear zone

The Donkerhoek Granite is the most extensive of the granitic rocks in the area. The outcrops can be divided into four zones according to the nature of the intrusive contact and the general structural setting. The zones are described and interpreted individually. Zone I comprises that part of the Donkerhoek Granite which belongs to the central granite zone. Zones II and III include slab-like granite bodies in contact with migmatite or Khomas Schist. They are generally concordantly intruded into the tectonic structures. The largest such bodies occur in anticlines. Only a few, smaller bodies occur in synclines. Zone IV lies within the transverse tectonic zone. The drop-shaped granite intrusions have discordant contacts

Thin-sections were examined from 21 granite samples, which were found to have the modes of monzogranites

The mesonorm was determined on the basis of chemical analyses of six selected samples of Donkerhoek granite. The crystallization sequence was determined from plots of the component ratios in the system Qz-Ab-Or-An-H2O. The sequence alkali feldspar - plagioclase - quartz was confirmed by the results of melting experiments

Two large granodiorite occurrences are described. One lies concordantly in the country rock like the granite massifs of this region, and reproduces the form of the tectonic structures. The other occurrence is in the farm Mooirivier in the southern transverse zone. Smaller granodiorite bodies, which "swim" in the Donkerhoek Granite and are disrupted by it, occur in the Davetsaub-Rivier in the Otjimbingwe reserve

The granodiorite of the investigated area is compared with those of the surrounding region. Strong similarities to the Achas-Granite (PAUPEL 1974, JACOB 1974) were established and the two rocks are correlated

Migmatites are widely distributed in the area investigated. In accordance with FAUPEL (1974), an area in the west is interpreted as the roof portion of the Donkerhoek Granite

The migmatite occurrence on Davetsaub farm was intensively investigated. Leucocratic material and the disintegration of the tectonic fabric increase from S to N toward the central granite zone. This observation was used to divide the migmatite into three types, which are described individually

The microscopic investigation of the leucosome showed that the mineral content is not uniform. Granitic, granodioritic and trondhjemitic leucosome compositions were determined and investigated separately. The results of the observations and analyses were compared in order to explain the genetic relationships of the granitic rocks to one another and to the host rocks and migmatites. Two possible modes of formation are discussed: (1) Production of the leucosome by in situ anatexis (2) Injection into the country rock by and during intrusion (sheeting)

It is concluded that the granitic and granodioritic leucosomes are the result of injection into the country rock. This case is supported by the occurrence of granitic and granodioritic intrusions in the neighbourhood, which also show connections with the leucosome. Furthermore, the intrusive bodies and the corresponding leucosome have the same chemical composition

On the other hand, the trondhjemitic leucosome is attributed to a partial anatexis of the Khomas Schists. The necessary heat supply is provided by the intruding melts of the Donkerhoek Granite and/or the granodiorite

The relationships between the Donkerhoek Granite and the granodiorite were also investigated. The granodiorite may be older than the Donkerhoek Granite. The similarity of the contact forms suggests that the intrusions took place in similar conditions

The main difference in origin between the Donkerhoek Granite and the granodiorite may be the source of the magma. It is assumed that the Donkerhoek Granite melt was derived from the pre-Damara basement rocks. It is considered possible that the granodiorite was produced by melting psammitic Khomas Schists

In conclusion it is again shown that the geology of the area is strongly influenced by the interplay of tectonics and intrusion