Geochronology of Mesoproterozoic hybrid intrusions in the Konkiep Terrane, Namibia, from passive to active continental margin in the Namaqua-Natal Wilson Cycle (original) (raw)
Related papers
2007
Mapping in the Karas Mountains area of southern Namibia has shown the presence of a major lithotectonic and geophysical boundary at the northern margin of the Namaqua-Natal Metamorphic Province. The area is subdivided into the Northern Volcanic Zone largely comprising metamorphosed mafic, intermediate and felsic volcanic and intrusive rocks, and the Southern Sedimentary Zone, dominated by pelitic rocks, metamorphosed at amphibolite to granulite facies, and intruded by syntectonic granites. The intervening Lord Hill Boundary Zone is characterized by anastomosing mylonite belts with dips and lineations plunging steeply to the south, within less-deformed metalavas and intrusions similar in character to those of the Northern Volcanic Zone. The intrusive and metavolcanic rocks of the Northern Volcanic and Lord Hill Boundary Zones have geological characteristics of present-day volcanic arcs. Texturally well-preserved felsic metavolcanic rocks of the Lord Hill Boundary Zone have been dated by SHRIMP U-Pb isotope analysis of zircons, and gave an age of 1286 ± 7 Ma. The age and tectonic setting are similar to those of the Areachap Group in South Africa, and point to an extensive Mesoproterozoic volcanic arc at the southern edge of the Kalahari Craton.
Journal of African Earth Sciences, 2006
Juvenile crust formation within the Namaqua-Natal Belt occurred during two principal periods at 1.4 and 2.2 Ga with little evidence for significant contributions from older crustal sources. Palaeoproterozoic lavas and associated calc-alkaline granitoids are preserved in the Richtersveld sub-province and, to a much lesser extent, in the Bushmanland sub-province. Development of crust within the Aggeneys and Okiep terranes of the Bushmanland sub-province during the Mesoproterozoic involved significant reworking of pre-existing Palaeoproterozoic lithosphere, whereas the Garies terrane (Bushmanland sub-province) and the Gordonia and Natal subprovinces show little or no reworking of older protoliths.
A provenance study using petrography, geochemistry and U-Pb zircon dating was carried out on selected siliciclastic units of the Nama Group, a major lithostratigraphic unit in southwestern Gondwana which crops out in southern Namibia and adjacent north-western South Africa. Petrographic and geochemical results from the Nama Group indicate a homogenous recycled upper crust composition characterized mainly by metamorphic and granitic sources, with minor input from mafic rock sources that have not undergone significant weathering. Previous works based on facies and palaeocurrent analyses, and silicified volcanic ash beds and chromian spinel bearing sandstones of the Nama Basin points to a syn-tectonic volcanic island arc source located in the adjacent Damara Belt. U-Pb detrital zircon geochronology of the Nama Group rocks displays major peaks at Neoproterozoic (Pan-African orogeny) and Mesoproterozoic (Namaqua-Natal orogeny) indicating a foreland geotectonic setting for Nama deposition, confirming facies and palaeocurrents analyses. A paleocurrent shift from the north to the west in the upper "molassic" Nama Group is associated with a switch to an influx of Neoproterozoic-Cambrian detrital zircons (76%). These ages probably indicate exhumation (after 531 ± 9 Ma) of a felsic volcanic arc (Arachania) root, which is presently attached to the Río de la Plata Craton. The provenance of the Nama foreland basin suggests that continent-continent collision of the Kalahari/Congo Cratons and the Cuchilla Dionisio Pelotas Terrane (Arachania Arc) with the Río de la Plata Craton most likely occurred due to strike-slip accretion related to a component of N-S shortening in the period between 530 and 495 Ma.
Journal of Volcanology and Geothermal Research, 2004
The Erongo subvolcanic center in Namibia is the largest composite, bimodal complex in the Mesozoic Etendeka igneous province of Namibia. This study of 40 Ar/ 39 Ar and high spatial resolution U^Pb zircon dating demonstrates that emplacement of the various igneous units at Erongo took place within a time span equivalent to or shorter than geochronologically resolvable age differences (ca. 2 Ma), and at the peak of regional flood-basalt activity in the Etendeka^Parana ¤ province. The Erongo complex comprises a series of felsic volcanic and intrusive units that overlie or intrude basaltic lavas attributed to the Etendeka Group. The stratigraphically oldest rhyodacite (Erongorus unit) yielded U^Pb zircon ages of 131.8 þ 1.1 Ma (1c). The overlying Ombu rhyodacite was previously dated at 135.0 þ 1.6 Ma and our new ages for a resurgent intrusion of the same magma type (Ombu granodiorite) in the vent area are 132.6 þ 1.1 Ma ( 40 Ar/ 39 Ar) and 132.3 þ 1.9 Ma (U^Pb zircon) respectively. The youngest felsic magmas at Erongo are the high-silica Ekuta rhyolite and compositionally equivalent Erongo granite. These units yielded U^Pb zircon ages of 131.9 þ 2.9 and 130.3 þ 1.4 Ma, respectively. Concordant 40 Ar/ 39 Ar biotite ages from two samples of the granite average 132.2 þ 0.8 Ma. The final stage of magmatism at Erongo involved emplacement of basic alkaline plugs and dikes. Phlogopite and kaersutite from foidite plugs of this series yielded 40 Ar/ 39 Ar ages of 130.8 þ 1.0 and 132.0 þ 1.0 Ma. In a regional context, these age results indicate that silicic magmatism in the largest complexes of the Damaraland (Erongo, Brandberg, Paresis, Messum) began simultaneously with the peak of flood-basalt effusion at about 132 Ma throughout the Etendeka province and ceased by about 130 Ma. The silicic magmas are hybrid, with varying contributions of crustal and mantle-derived melts, and the age constraints suggest that crustal melting was caused by a short-lived thermal pulse related to the main flood-basalt event. Basic magmatism in the Damaraland complexes continued sporadically thereafter to about 123 Ma, but lacked the power to create further crustal melting. ß
Geochemistry and tectonic setting of magmatic units in the Pan-African Gariep Belt, Namibia
Chemical Geology, 1996
Major-, trace-, rare-.earth-element, and Rb-Sr and Sm-Nd isotope data of the various low-grade metamorphosed magmatic units in the Pan-African Gariep Belt, southwestern Namibia, presented in this paper, provide the basis for the reconstruction of the evolution of the late Proterozoic Adamastor ocean between the South American and Kalahari Cratons. The Gariep Belt is subdivided into two major zones, a para-autochthonous, predominantly sedimentary rift and passive continental margin succession (Port Nolloth Zone, PNZ) and the allochthonous, predominantly marie Marmora Terrane. The latter has been thrust in southeasterly direction over the former. All of the magmatic activity in the PNZ can be related to lithospheric stretching. The earliest magmatic activity is expressed as bimodal volcanism with continental within-plate affinity along an embryonic rift graben (Rosh Pinah Formation) and 4s found within the older sediment package of the para-antochthonous external zone of the orogen. After ,,, 24 Ma, this was followed by the intrusion of mafic, tholeiitic dykes into the basement and the lower parts of the PNZ, heralding the opening of the Gariepian basin at 717 Ma.
Contributions to Mineralogy and Petrology, 2012
Leucocratic granites of the Proterozoic Kaoko Belt, northern Namibia, now preserved as meta-granites, define a rock suite that is distinct from the surrounding granitoids based on their chemical and isotopic characteristics. Least evolved members of this *1.5-1.6-Ga-old leucogranite suite can be distinguished from ordinary calcalkaline granites that occur elsewhere in the Kaoko Belt by higher abundances of Zr, Y, and REE, more radiogenic initial e Nd values and unradiogenic initial 87 Sr/ 86 Sr. The leucogranites have high calculated zircon saturation temperatures (mostly [ 920°C for least fractionated samples), suggesting that they represent high-temperature melts originating from deep crustal levels. Isotope data (i.e., e Ndi : ?2.3 to-4.2) demonstrate that the granites formed from different sources and differentiated by a variety of processes including partial melting of mantle-derived meta-igneous rocks followed by crystal fractionation and interaction with older crustal material. Most fractionationcorrected Nd model ages (T DM) are between 1.7 and 1.8 Ga and only slightly older than the inferred intrusion age of ca. 1.6 Ga, indicating that the precursor rocks must have been dominated by juvenile material. Epsilon Hf values of zircon separated from two granite samples are positive (?11 and ?13), and Hf model ages (1.5 and 1.6 Ga) are similar to the U-Pb zircon ages, again supporting the dominance of juvenile material. In contrast, the Hf model ages of the respective whole rock samples are 2.3 and 2.4 Ga, demonstrating the involvement of older material in the generation of the granites. The last major tectonothermal event in the Kaoko Belt in the Proterozoic occurred at ca. 2.0 Ga and led to reworking of mostly 2.6-Ga-old rocks. However, the presence of 1.6 Ga ''postcollisional'' granites reflects addition of some juvenile mantle-derived material after the last major tectonic event. The results suggest that similar A-type leucogranites are potentially more abundant in crustal terranes but are masked by AFC processes. In the case of the Kaoko Belt, it is suggested that this rock suite indicates a yet unidentified period of mantle-derived crustal growth in the Proterozoic of South Western Africa.
Precambrian Research, 2018
The ∼2.23 Ga Hekpoort Formation (Transvaal sub-basin) and the ∼2.43 Ga Ongeluk Formation (GriqualandWest sub-basin) represent voluminous Paleoproterozoic Igneous events on the Kaapvaal craton of South Africa that predate the emplacement of the ∼2.055 Ga Bushveld Complex, and probably covered most of the craton at the time of their extrusion. In this contribution, we present field, petrological and geochemical studies of the Hekpoort Formation and compare it with the Ongeluk Formation. The Hekpoort Formation consists of a thick subaerial volcanic sequence in which volcanoclastic rocks occur mainly at the base. Rare, localized hyaloclastites and variolitic rocks record the presence of ponded water, while interbedded sedimentary rocks and paleoweathered flow tops suggest prolonged time-breaks in volcanic activity. The Hekpoort rocks underwent metamorphism up to greenschist facies but also episodes of metasomatism and silicification. Preserved primary magmatic minerals are clinopyroxene (pigeonite, augite and diopside), and rarely plagioclase (labradorite). Both the variable whole rock Mg# (evolving from 69 to 50) and the changes in clinopyroxene composition attest to magmatic fractionation. Lava units of both the Hekpoort and Ongeluk formations are mostly basalts, with silicification responsible for increased SiO2 contents. Lava units of both formations also display remarkably similar trace elements patterns, which is noteworthy for units separated by 200 million years, and unique among the Precambrian mafic magmatic units of the Kaapvaal craton that we evaluated. Similar to other Precambrian mafic magmatic units of the Kaapvaal craton, the Hekpoort Formation shows an arc-like trace element signature, mainly represented by negative Nb-Ta anomalies (in normalized trace element patterns). The Hekpoort (and Ongeluk), together with three other Paleoproterozoic mafic units of the creation older than 2.2 Ga, exhibit relatively high contents of Th and U, which sharply contrasts with Archean units. The data suggest that a subduction process marked the Archean-Proterozoic boundary on the Kaapvaal craton.