Die Altersstellung von Rotgneisen im Rostock Deckenkomplex am Südrand des Damara Orogens, Namibia, abgeleitet aus U/Pb- und Rb/Sr-Isotopenuntersuchungen, und ihre Bedeutung für die Basement-Cover-Beziehungen in der Damara Südrandzone

Abstract

Summary provided by author:

Local intrusive relations between the granitic orthogneisses and bimodal volcanics suggests a subvolcanic emplacement for the whole sequence. Chemical characteristics of the igneous rocks suggest an origin related to continental extension

The igneous rocks are overlain by alluvial fan deposits interfingering with rudaceous and areneous rocks of a fluvial system and argillaceous and carbonaceous rocks including evaporitic horizons of a sabhka playa facies. These sediments correlate with the Kamtsas Formation and the Duruchaus Formation of the basal Damara Sequence (Nosib Group). The presence of plutonic and volcanic components in mud-supported conglomerates autochthonously overlying the red orthogneisses and associated volcanic rocks implies the direct deposition upon the underlying magmatic rocks related to tectonic instability in the beginning of subsidense of continental graben structures

Prominent dolomitic marbles (Corona Formation) associated with extraformational mud-supported conglomerates (Rostock Conglomerate) grade into an interbedded sequence of alternating rudites and graphite schists or to several pelitic schists of the black shale facies (Blaukrans Formation). These units indicate a possible marine transgression in the initial stage of a more general subsidense. A correlation with the basal stratigraphic units of the marine parts of the Damara Sequence (Swakop Group) appears feasible. However, due to a missing stratigraphic discordance between Nosib Group Sediments and these units, deposition in prograding limnic brackish basins and therefore a correlation with the Nosib Group cannot be excluded

Components of the stratigraphieally higher conglomerates, as welt as decreasing thickness of beds in the turblditic sequence away from the massifs show that the magmatic rocks in the area of the Rostock massifs formed a swell within a southern graben structure of the Damaran system. From there, sediments were supplied in a southeastern to southwestern direction to a basin between the swell and the Southern Foreland

The structures in the Rostock area are dominated by folding and thrusting. All discovered lithological units comprise structural features of two compressional phases, D1 and D2, which both exhibit 3 homoaxial strain increments. During the course of a prograde deformation path, the first phase, D1, has resulted in repeated NE-directed folding and thrust tectonics, culminating in telescoping of primary adjoining lithologies. Younger D2 structures comprise SE-directed folding with subsequent thrusting culminating in imbricating of individual lithological horizons. However, thrusting did not lead to a far-reaching nappe emplacement

The tectonic evolution is associated with polyphase mineral growth. The rocks in the Rostock area have undergone regional dynamothermal metamorphism. Synkinematic mineral assemblages in pelitic and mafic rocks indicate P-T conditions of the upper greenschist facies up to albite-epidote amphibolite facies. Furthermore, static post-tectonic metamorphism under similar conditions is a distinct feature in the rocks

The same development of both D1 and D2 structures in all units, including the Damara Sequence, give clear evidence for an initiation of deformation after the deposition ol the Damara Sequence. Postulated major influence of pre-Damaran tectono-metamorphic processes (Kibaran orogenesis) on the basal gneisses and the associated volcano-sedimentary suite can be excluded

Regional comparisons give evidence for great significance of the NE-directed D1 structures in the structural development in the western parts of the Southern Margin Zone and probably in the whole western Damaran Belt. Generated in a first Orogenic cycle, these structures are assumed to have evolved from the closure of a proto-South Atlantic Ocean prolonged parallel to the African west-coast at 730 -580 Ma

Strain increments of the younger, SE-directed D2 compression phase are comparable to SE-directed tectonic movements in the Southern Margin Zone, that appear to be related to continent collision of the Kalahari Craton and the Kongo Craton at 540 Ma before present

Local normal downthrust faults to NW are related to late- to postorogenic extension

Based on the results of the geological investigations the presence of magmatic rocks in the basal parts may be attributed to the formation of an initial continental rift. Therefore dating of these rocks should confirm a corresponding model for the geodynamic evolution

Rb-Sr ages and U-Pb zircon ages have been obtained for several granite gneisses and porphyry gneisses of the associated volcono-sedimentary unit. The isotopic systems of the rocks give relevant information about the time of formation, despite alteration caused by Damaran metamorphism or later uplift and erosion. Rb-Sr whole-rock data of granitic gneisses give an age of 1049 ± 28 Ma with an initial 87Sr/86Sr value of 0. 7061 ± 0. 0002, while their U-Pb zircon data indicate a maximum age around 1200 Ma for the time of crystallisation. U-Pb zircon data of porphyritic members of the metavolcanic unit define an age around 1100 Ma for the time of formation. Rb-Sr whole rock data of mafic members within this unit provide no information about the time of formation, probably due to metamorphic effects. Rb-Sr mineral systems and Rb-Sr small-scale whole-rock systems were influenced by complete resetting during Damaran metamorphism around 530-490 Ma. In general, a period of granitoid emplacement and volcanic activity between 1200-1050 Ma and a Damaran tectono-metamorphic overprinting are indicated by the results

These rocks are consequently considered to be equivalent to the Sinclair Sequence and related intrusives, that are widely distributed along the Southern Margin of the Damara Belt. In conclusion the emplacement of the magmatic - sedimentary sequence of the Rostock massifs is related to induced rifting, complementary to the Kibaran orogeny. Rift related magmatic rocks of the Sinclair Sequence formed the basement for the deposition of the basal sediments of the Damara Sequence in the southern graben of the Damara rift