Die metamorphe Entwicklung im Damara Orogen, Südwest Afrika/Namibia, abgeleitet aus K/Ar-Datierungen an detritischen Hellglimmern aus Molassesedimenten der Nama Group

Abstract

Abstract and summary provided by author:

(1) Muscovites from the Kuibis Subgroup and the underlying pre-Damara Sinclair Sequence have K/Ar ages of 1000-1100 Ma and thus evidence that these sediments originated from a very old basement overprinted by the Kibarian orogeny. The initial deposition of sediment from the rising Damara geosyncline toward the east and south may already be possible in the uppermost Kuibis Subgroup, as it is suggested by the K/Ar age of 624 Ma found there. Sediment supply from the Damara Orogen is also observed in the Urusis Formation of the lower Schwarzrand Subgroup, but these series can not be regarded as molasse sequences since a continuous connection to the Damara Orogen is not substantiated

(2) In the upper Nama Group, i. e., in the upper Schwarzrand and Fish River subgroups, K/Ar ages of 570-670 Ma were measured on detrital white micas in the fractions between 250-100 µm. These values are interpreted as the cooling age of an initial metamorphic alteration in the Damara Orogen because based, for example, on sedimentological criteria, particularly paleocurrent directions, the upper Schwarzrand and Fish River subgroups are viewed as molasse sequences of the Damara Orogen. Cooling ages of the younger metamorphism, which reached their highest temperatures about 530 Ma ago during the Damara Orogeny, were not detected in the Nama Group because detrital K/Ar ages of over 600 Ma were measured even into the uppermost Fish River Subgroup. Hence, the sediments of the upper Nama Group were deposited during a molasse stage following an initial Orogenic cycle

An exact reconstruction of the source area of the molasse sequences of the Nama Group was not achieved. However, the sediments of the Fish River Subgroup most probably originated out of the Central Zone of the Damara Orogen and were transported onto the Southern Platform after the closing of an ocean in the Southern Zone (the Khomas Trough). The sediments of the Schwarzrand Subgroup probably came for the most part from the southern margin (Southern Margin Zone) of the Damara Orogen; it cannot be definitely said how much, if any, sediment from the Central Zone was already deposited here. The radiometric K/Ar datings of detrital white micas from these sediments, however, do document an early metamorphic event in the Damara Orogen whose thermal high point was apparently about 660 Ma ago

Grain size fractions of 2 µm as well as 2-1; 1-0, 63 and 0, 63 µm were studied to detect possible thermal alterations of the Nama sediments, whereby the following was determined:

(1) X-ray studies on illite crystallinity of argillaceous sediments from the Nama Group yielded relative peak width at half height values which lie within the range of diagenesis up to the beginning anchizone. Higher illite crystallinity values were measured on the fractions of 2 um in sandstones and siltstones which due to accessory detritus do not correspond to the true alteration of the Nama sediments. The temperatures derived from the illite crystallinities lie far below the closing temperature of the K/Ar system for white mica and thus exclude a postsedimentary alteration of the detrital white mica

(2) Determinations of the illite polytypes in the fine fractions predominantly yielded 2M polytypes; 1M polytypes were only, if at all, ascertained in very small quantities and usually lay below the detectability limit for X-ray. These observations are indicative of a very advanced conversion of the 1Md polytype into the 2M polytype inthe fine fractions and confirms a diagenetic to anchizonal alteration of the Nama sediments

(3) The K/Ar ages of the fine fractions (2 µm) lie between 530 and 500 Ma and clearly differ from the ages of the coarse detrital white mica. These observations suggest a thermal event which led at least to a partial reopening of the K/Ar systems in the fine fractions, but was not sufficient for reequilibrating the detrital white mica

(4) Radiometric datings on the individual fractions of 2-1; 1-0, 63 and 0, 63 µm showed a distinct dependency of the age values on grain size, marked by decreasing K/Ar ages from the coarser to the finer fractions. This is explained by the relatively higher amount of detrital components in the coarser fractions. The K/Ar ages of the total 2 µm fraction thus represent mixed ages which alone have no geological significance but do suggest that there were still thermal alterations in the Nama Group after 530 Ma

X-ray determinations of the polytypes of coarse white mica in the fraction between 250 and 100 um exclusively yielded 2M polytypes and thus confirm a detrital origin for these minerals. Thereby, the K/Ar ages of the detrital white micas can be interpreted as cooling ages of metamorphic alterations which originated from the source areas of the Nama sediments

The time of sedimentation of the upper " Nama Group can be narrowed down to between 570 and 530-500 Ma based on detrital white mica datings and the K/Ar ages of the fine fractions

Marginal areas, particularly in the northern part of the Nama Group, were also included in the Orogenic movements of the tectono-metamorphic phase in the Damara Orogen about 530 Ma ago. These movements led to a syncrystalline cleavage in the northern Nama which can be traced up into the upper Fish River Subgroup. Molasse sediments of the younger Damara phase around 530 Ma may possibly have overlain the Nama Group exposed today and have contributed to the formation of cleavage with an overburden of 5-6 km thickness. However, this material was completely eroded since the younger metamophic alteration is not detected in the detrital components of the Nama Group

Hence, there were at least two metamorphic events in the evolution of the Damara Orogen of which the first is documented in the molasse sediments of the Nama Group following the uplift and erosion of the orogeny