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Precise U-Pb zircon ages for early Damaran magmatism in the Summas Mountains and Welwitschia Inlier, northern Damara Belt, Namibia

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... The Ugab Subgroup consists mainly of peritidal carbonates intercalated with siliciclastics. Its relationship to the Ombombo Subgroup of the Otavi Platform is still not fully understood, but a U-Pb zircon age of 746 ± 2 Ma from an ash-flow tuff in the Naauwpoort Formation (Hoffman et al., 1996) suggests that the Ugab Subgroup must be at least 15 myr younger than the upper Devede Formation, which contains a tuff dated (U-Pb zircon) at 760 ± 1 Ma (Halverson et al., 2005). The youngest possible age for the top of the Ugab Subgroup can be inferred from a maximum age constraint of 717.4 ± 0.2 Ma for the onset of the Sturtian glaciation (Macdonald et al., 2010(Macdonald et al., , 2018MacLennan et al., 2018). ...
... The Naauwpoort Formation in the Summas Mountains consists mainly of peralkaline to alkaline rhyolitic tuffs, with minor mafic volcanics (Miller, 1980). A U-Pb zircon date of 746 ± 2 Ma exists for an ash-flow tuff collected approximately 1000 m stratigraphically below the contact with the Ugab Subgroup (Hoffman et al., 1996). At the core of the inlier, proposed to be a cauldron-subsidence structure (Guj, 1974), the cumulative thickness of the rhyolitic ash-flow tuffs exceeds 6 km and the base is not visible (Miller, 1974(Miller, , 1980. ...
... In the North Dome, U2 is dominated by coarse-grained conglomerate. A maximum age constraint of 743 ± 10 Ma for the upper part of U2 from U-Pb ages on detrital zircons (Nascimento et al., 2017;Hoffman and Halverson, 2018) is consistent with the 746 ± 2 Ma date from the Nauuwpoort Formation (Hoffman et al., 1996). The top of U2 is defined by a flooding surface and U3 marks a transition to carbonate-dominated deposition. ...
... The sedimentary successions of the Damara, Kaoko, and Gariep belts have been interpreted as deposited in a continental rift to passive continental margin environment fringing the Angola-Congo and Kalahari cratons (e.g., Porada, 1989;Frimmel et al., 2002;Paciullo et al., 2007;Frimmel and Miller, 2009;Nascimento et al., 2016). Alkaline volcanic rocks in the Damara, Kaoko and Gariep belts are interpreted as syn-rift magmatism (Fig. 2;e.g., Miller, 1980;Hoffman et al., 1996;Nascimento et al., 2016). According to Frimmel et al. (2001), intracontinental rifting began between ca. ...
... The basal unit is the Nosib Group, interpreted as a record of the initial rifting phase (e.g., Porada, 1983;Borg, 2000;Miller, 2008;Nascimento et al., 2016). In the NMZ and NZ of Miller ( , 2008; Fig. 1B), this group includes felsic and mafic volcanic rocks of the Naauwpoort Formation which represent bimodal magmatism attributed to the syn-rift (e.g., Miller, 1980;Hoffman et al., 1996;Jung et al., 2007;Nascimento et al., 2016) to transitional evolution stage of the basin (e.g., Nascimento et al., 2016). These igneous rocks yielded U-Pb (zircon) ages between 759 and 746 Ma (Hoffman et al., 1996;Nascimento et al., 2016) defining the approximate age for the basal succession in the Outjo Basin. ...
... In the NMZ and NZ of Miller ( , 2008; Fig. 1B), this group includes felsic and mafic volcanic rocks of the Naauwpoort Formation which represent bimodal magmatism attributed to the syn-rift (e.g., Miller, 1980;Hoffman et al., 1996;Jung et al., 2007;Nascimento et al., 2016) to transitional evolution stage of the basin (e.g., Nascimento et al., 2016). These igneous rocks yielded U-Pb (zircon) ages between 759 and 746 Ma (Hoffman et al., 1996;Nascimento et al., 2016) defining the approximate age for the basal succession in the Outjo Basin. This stage was followed by open marine basin conditions (between 740 and 590 Ma; Milani et al., 2015) that resulted in the deposition of the Otavi Group sediments, as well as the correlated Swakop, Zerrissene, Hakos, and Witvlei groups. ...
Article
The Damara Orogen is composed of the Damara, Kaoko and Gariep belts developed during the Neoproterozoic Pan-African Orogeny. The Damara Belt contains Neoproterozoic siliciclastic and carbonate successions of the Damara Supergroup that record rift to proto-ocean depositional phases during the Rodinia supercontinent break up. There are two conflicting interpretations of the geotectonic framework of the Damara Supergroup basin: i) as one major basin, composed of the Outjo and Khomas basins, related to rifting in the Angola-Congo-Kalahari paleocontinent or, ii) as two independent passive margin basins, one related to the Angola-Congo and the other to the Kalahari proto-cratons. Detrital zircon provenance studies linked to field geology were used to solve this controversy. U-Pb zircon age data were analyzed in order to characterize depositional ages and provenance of the sediments and evolution of the succession in the northern part of the Outjo Basin. The basal Nabis Formation (Nosib Group) and the base of the Chuos Formation were deposited between ca. 870 Ma and 760 Ma. The upper Chuos, Berg Aukas, Gauss, Auros and lower Brak River formations formed between ca. 760 Ma and 635 Ma. It also includes the time span recorded by the unconformity between the Auros and lower Brak River formations. The Ghaub, upper Brak River, Karibib and Kuiseb formations were deposited between 663 Ma and 590 Ma. The geochronological data indicate that the main source areas are related to: i) the Angola-Congo Craton, ii) rift-related intrabasinal igneous rocks of the Naauwpoort Formation, iii) an intrabasinal basement structural high (Abbabis High), and iv) the Coastal Terrane of the Kaoko Belt. The Kalahari Craton units apparently did not constitute a main source area for the studied succession. This is possibly due to the position of the succession in the northern part of the Outjo Basin, at the southern margin of the Congo Craton. Comparison of the obtained geochronological data with those from the literature shows that the Abbabis High forms part of the Kalahari proto-craton and that Angola-Congo and Kalahari cratons were part of the same paleocontinent in Rodinia times.
... Stratigraphy can be correlated across the entire region, and between zones of distinctly different tectonic setting, these being the Northern Foreland, Northern Zone and Ugab Zone (Fig. 1). Basal strata consist of rift-related siliciclastics of the Nosib Group, including Naauwpoort Formation felsic volcanics of $755 Ma and $747 Ma age (Fig. 3;Hoffmann, 1994;Hoffman et al., 1996;DeKock et al., 2000;Nascimento et al., 2016). Post-Nosib Group stratigraphy of <740 Ma age, includes Sturtian glaciogenic deposits of the Chuos Formation, and at higher stratigraphic levels Marinoan glaciogenic deposits of the Ghaub Formation, which are overlain by Karibib Formation cap carbonates (Hoffman et al., 1994(Hoffman et al., , 1998Hoffmann et al., 2004). ...
... Stratigraphy is modified after Hoffmann (1989) with latest corrections from Hoffmann et al. (2004), Halverson et al. (2002) and geochronology of felsic extrusives (e.g. Hoffman et al., 1996). The most recent and accepted stratigraphy is presented (Swart, 1992;Hoffmann et al., 2004;Hoffmann, 1994Hoffmann, , 1997Hoffmann, , 1989Hoffman et al., 1996Hoffman et al., , 1998Halverson et al., 2002;DeKock et al., 2000;Nascimento et al., 2016;Hoffmann and Schreiber, 1998;Becker et al., 2004). ...
... Hoffman et al., 1996). The most recent and accepted stratigraphy is presented (Swart, 1992;Hoffmann et al., 2004;Hoffmann, 1994Hoffmann, , 1997Hoffmann, , 1989Hoffman et al., 1996Hoffman et al., , 1998Halverson et al., 2002;DeKock et al., 2000;Nascimento et al., 2016;Hoffmann and Schreiber, 1998;Becker et al., 2004). Grey -Published age range for different episodes of granite magmatism. ...
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Collision, high-angle contraction, crustal thickening and heating at 555–516 Ma, primed the Damara Belt ready for crustal collapse, which was triggered by a transition to ENE–WSW contraction along the length of the belt in response to orogenic events in east Gondwana at 516–505 Ma. Along-orogen shortening reworked and thickened the high-grade core of the belt, increasing gravitational instability, and establishing an NW–SE extension direction across the belt that was conducive to reactivation of pre-existing structures and eventual collapse. This extension direction persisted, and the switch to vertical σ1 and collapse was signalled by decompression melting at ∼502 Ma and subsequent rapid cooling. Collapse was focused on the high-grade core of the belt that was exhumed as a ∼170 km wide, semi-coherent massif-type metamorphic core complex with steep extensional shear zones and faults in the marginal flanks. During exhumation the core complex was reactivated by oblique-slip extensional shear zones that responded to external transient stress fields. Reactivation by middle- to lower-amphibolite facies dextral-normal shear zones at ∼500–495 Ma and ∼495–490 Ma, involved E–W to ENE–WSW shortening consistent with accretionary events in the west Gondwana margin during the Pampean Orogeny. Reactivation by greenschist facies sinistral-normal shear zones at ∼485 Ma, involved N–S shortening consistent with accretionary events in the south Gondwana margin during the Famatinian Orogen. Early stages of exhumation involved decompression melting, flattening folds and ductile ultramylonite zones within carbonate that formed by NW–SE extension. Late stage exhumation in the brittle field from ∼480 Ma onwards, involved a stress-switch to radial extension directions dominated by NE–SW. This stage involved flat-lying breccia, inclined faults, vertical fractures, and oxidizing fluids partitioned into the top of the lower-levels of the massif. Ongoing exhumation of the core complex drove localized NW–SE shortening within the flanking margins and hanging-wall, and produced low-strain reverse structures that straddle the ductile to brittle transition. The pressure difference between exhumed massif (4.8–5.5 kbar) and hanging-wall margins (3.9–4.2 kbar), indicate that ∼3.2–4.6 km of crust was stripped from above the core complex.
... More recently, based on correlations with Zaire and the Congo, the initiation of rifting was placed at around 880 Ma (Miller, 2013). Hoffman et al. (1996) gave a minimum age of 756 ± 2 Ma (obtained from zircon dating) for the Nosib Group terrigenous rift sedimentation. This longlasting rifting dissected Rodinia and opened the Damara Ocean in a roughly NE-SW direction, and the Adamastor Ocean, a part of it represented by the Kaokoveld, aligned N-S (all directions are with respect to the present continental orientation) (Fig. 1). ...
... A study by Kendall et al. (2009) highlights the problems of correlation when radiometric age dates are scarce or missing. Hoffman et al. (1996) published ages of 746 ± 2 Ma and 747 ± 2 Ma for the Naauwport rhyolites, which are said to represent a maximum age for the Chuos Formation. Nascimento et al. (2016) obtained a somewhat older minimum age of 757 ± 5 Ma for the base of the Chuos Formation. ...
... Their measurements employed an ion microprobe using SHRIMP II and RG on a sample from a Naauwport dacite sill intercalated with the basal Chuos Formation. This age is very close to the minimum age of 756 ± 2 Ma given by Hoffman et al. (1996) for the lower Nosib Group terrigenous rift sedimentation. Therefore, the age of the Chuos Formation apparently is not synchronous everywhere, and differs greatly from the Sturtian ages for Australia (see below). ...
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The Neoproterozoic Cryogenian (‘Marinoan’) Ghaub Formation of northwestern Namibia represents an important founding pillar of the Snowball Earth hypothesis and its derivative, the Panglacial Earth hypothesis. These hypotheses assume oceans and continents covered by thick ice, even in the tropics, which caused a very distinct drop in eustatic sea-level. Over time, strongly increased CO2 contents of the atmosphere led to sudden ice melting, very substantial sea-level rise, and strong weathering on the continents associated with the deposition of cap carbonates in the newly ice-free oceans. The ongoing controversy about Snowball-type glaciations in Namibia and elsewhere is reviewed, and other hypotheses (Slushball Earth, Waterbelt Earth, Jormungand state of the Earth, Thin Ice state of the Earth, Zipper-Rift Earth, High-Obliquity Earth) are discussed. We prefer the term ‘Waterbelt Earth’ instead of the originally proposed ‘Waterbelt state’ because of the clearer contrast with ‘Snowball Earth’. Because a great deal of information related to Cryogenian glaciations comes from the Ghaub Formation of northwestern Namibia, these hypotheses should be tested independently based on a time-equivalent depositional system. This analogue was found in the carbonate-dominated successions of the Otavi Mountainland (OML), northeastern Namibia, and is highly comparable with the successions in the well-investigated northwest of the country. An extreme eustatic sea-level drop caused by a global glaciation of oceans and continents and imposed on a carbonate platform or ramp such as the one in the OML would have led either to glacial cover or widespread subaerial exposure and extensive erosion, including deeply incised valleys. The presence of such features would strongly support the Snowball Earth hypotheses if tectonic effects did not play a major role. During the postglacial transgression, distinct reworking of the carbonate platform/ramp surface would have occurred, leaving behind lag deposits, as well as infills of incised valleys with fluvial, reworked glacial, and marine deposits. The main objective of our research was to weigh and investigate the strengths and weaknesses of the proposed Snowball Earth model of glacially induced large-amplitude sea-level changes during Ghaub time and to compare different models to obtain a rough estimate of the amount of glaciation. The study area in the OML includes two different, age-equivalent facies realms: platform sedimentation in the Southern area without diamictites, and slope deposits, including Ghaub diamictites, in the Northern area. The southern, continuously shallow-marine area shows a shallowing-upward succession from the pre-glacial lower Auros Formation, often varve-like laminated shales formed below wave base, to metre-high columnar stromatolites and microbial mat-related carbonates with intervals of vertical tubes (degassing features) of the upper Auros Formation, overlain by cap carbonates of the Maieberg Formation. The columnar stromatolites and the microbial tubestone lithotypes were clearly deposited in the euphotic zone. Indications for tidal conditions or subaerial exposure were not recorded in this platform succession without unconformities. Neither dropstones, nor incised channels, nor transgressive lag deposits were observed. The facies changes from below storm wave base to the photic zone and finally a shallow subtidal zone is explained by a prolonged, modest sea-level fall, partly counterbalanced by subsidence, followed by a slow transgression. In contrast, coarse-grained sedimentary rocks (e.g., oolites, debrites) characterise the time-equivalent successions in the Northern area. Starting with laminated shales at the base, similar to the Southern area, the overlying redeposited oolites and breccias of the Auros Formation show distinct lateral and vertical inhomogeneities and thickness changes, which indicate long-lasting synsedimentary tectonism. The same phenomenon is observed in the overlying diamictites of the Ghaub Formation. Their variable clast content indicates erosion of a strongly uplifted local source area covered by a thick carbonate succession, which was downstripped to the crystalline basement. The prograding diamictite succession with repeatedly intercalated silt-stringers is interpreted as periglacial debris flows into a marine environment. Sparse striated clasts in the diamictites and very rare dropstones (much less common than in northwestern Namibia) are indicators of glaciations somewhere in the area. However, compared with other glacial sequences, e.g. Quaternary periglacial sediments at the forefront of continental ice, dropstones and striated clasts would be expected to be much more common and more uniformly distributed if the entire area was covered by melting continental ice, as proposed in the Snowball/Panglacial Earth scenario. In the Southern area, dropstones would be expected to occur on the flooded platforms/ramps as well even when diamictites are absent. Both the relatively moderate sea-level change and the less common, irregular distribution of locally concentrated glacial rainouts provide strong evidence against the presence of a thick, laterally continuous ice cover over oceans and continents extending to equatorial areas. The oceans possibly corresponded to the scenario of a Waterbelt Earth or High-Obliquity Earth; evidence of open oceanic water exists, which would have enabled the continued evolution of biota. Glacial ice was present on tropical continents, but its occurrences may have been regional in patches, sourced from mountainous areas, and ice streams would have reached the oceans only locally, unrelated to a thick continental ice cover.
... Paleomagnetic data from the~1110 Ma Umkondo large igneous province (LIP) suggests that the Kalahari craton was separated from Laurentia by > 30°latitude at the time (Hanson et al., 2004;Swanson-Hyell et al., 2015), and so Kalahari may not have joined Rodinia until after~1100 Ma. Rift-related magmatism suggests that the Kalahari craton broke away from Rodinia~800 to 750 Ma (Frimmel et al., 1996;Hoffman et al., 1996;Hoffman et al., 2004;Halverson et al., 2005;Johnson et al., 2007). ...
... After the initial collision with Laurentia, Amazonia collided with Baltica~1050-960 Ma (Bogdanova et al., 2009). Amazonia broke away from Rodinia by~750 Ma (Frimmel et al., 1996;Hoffman et al., 1996;Hoffman et al., 2004;Halverson et al., 2005;Johnson et al., 2007). ...
... In addition, sedimentary evolution of the Upper Espinhaco Basin, including the 1190-900 Ma passive margin or intraplate foredeep basin Sopa-Bumadinho and overlying formations, suggests a direct connection between the Congo-Sao Francisco craton and Rodinia (Chemale Jr. et al., 2012). Despite its potential role during Rodinia assembly, most authors agree that the Congo-Sao Francisco craton was not part of the supercontinent at 750 Ma (Frimmel et al., 1996;Hoffman et al., 1996Hoffman et al., , 2004Halverson et al., 2005;Johnson et al., 2007). ...
Article
It has been long observed that the amalgamation of supercontinents, including Rodinia, is coeval with peaks of UPb ages of global detrital zircons. However, our new compilation of global geochemical, mineralogical, and ore geologic records shows that the assembly of Rodinia stands out from others, in terms of whole-rock trace element geochemistry, as well as records of mineralogy and ore deposits. During the assembly of Rodinia, Nb, Y, and Zr concentrations were enriched in igneous rocks, with prolific formation of zircon and minerals bearing Th, Nb or Y, and REE-bearing ore deposits. At the same time, many types of ore deposits are relatively poorly represented in Rodinin terranes, including deposits of orogenic gold, porphyry copper, and volcanic hosted massive sulfide deposits, with a corresponding paucity of many minerals (e.g., minerals bearing Au, Sb, Ni) associated with these deposits. We interpret these records as indicating the prevalence of ‘non-arc’ magmatism and a relative lack of subduction-related arc magma preserved in the surviving pieces of the Rodinia supercontinent, distinct from other episodes of supercontinent assembly. We further attribute the prevalence of ‘non-arc’ magmatism to enhanced asthenosphere–lithosphere interactions in the Mesoproterozoic, and speculate that the lack of ‘arc-collisional’ magma may be related to enhanced erosion of Rodinia orogenic belts.
... In this contribution, we focus on the pre-collisional evolution and position of the Kaoko-Dom Feliciano-Gariep orogenic system, which is exposed along the coasts of the South Atlantic Ocean (Fig. 2) and originated from the Neoproterozoic orogenic activity at the present-day western edges of the Congo and Kalahari cratons (Porada 1979(Porada , 1989. The rifting along the Congo and Kalahari margins preceding the orogenic evolution started at ca. 830 Ma and ended at ca. 680-660 Ma, as established by dating of volcanic/magmatic activity in the basinal parts of the former rift system (Hoffman et al. 1996;Frimmel et al. 1996Frimmel et al. , 2001Borg et al. 2003;Halverson et al. 2005;Konopásek et al. 2008Konopásek et al. , 2014 and through the studies of the sedimentological record in the adjacent continental platform (Hoffman and Halverson 2008). We present an extensive data set of absolute ages, detrital zircon age data, and geochemistry of syn-rifting igneous and sedimentary activity from tectonic units in the Kaoko and Dom Feliciano belts, which in our interpretation once represented an axial part of a rift system at the edge of the Congo and Kalahari cratons. ...
... This sedimentation was accompanied by ca. Hoffman et al. (1996);13-Frimmel et al. (1996) and Borg et al. (2003); 14- Leite et al. (1998) and Gubert et al. (2016) in the Gariep Belt (Fig. 16c). The igneous rocks show tholeiitic or even mildly alkaline chemistry, with no depletion in HFSE relative to the adjacent LILE. ...
Article
Early Neoproterozoic metaigneous rocks occur in the central part of the Kaoko–Dom Feliciano–Gariep orogenic system along the coasts of the southern Atlantic Ocean. In the Coastal Terrane (Kaoko Belt, Namibia), the bimodal character of the ca. 820–785 Ma magmatic suite and associated sedimentation sourced in the neighbouring pre-Neoproterozoic crust are taken as evidence that the Coastal Terrane formed as the shallow part of a developing back arc/rift. The arc-like chemistry of the bimodal magmas is interpreted as inherited from crustal and/or lithospheric mantle sources that have retained geochemical signature acquired during an older (Mesoproterozoic) subduction-related episode. In contrast, the mantle contribution was small in ca. 800–770 Ma plutonic suites in the Punta del Este Terrane (Dom Feliciano Belt, Uruguay) and in southern Brazil; still, the arc-like geochemistry of the prevalent felsic rocks seems inherited from their crustal sources. The within-plate geochemistry of a subsequent, ca. 740–710 Ma syn-sedimentary volcanism reflects the ongoing crustal stretching and sedimentation on top of the Congo and Kalahari cratons. The Punta del Este–Coastal Terrane is interpreted as an axial part of a Neoproterozoic “Adamastor Rift”. Its opening started in a back-arc position of a long-lasting subduction system at the edge of a continent that fragmented into the Nico Pérez–Luís Alves Terrane and the Congo and Kalahari cratons. The continent had to be facing an open ocean and consequently could not be located in the interior of the Rodinia. Nevertheless, the early opening of the Adamastor Rift coincided with the lifetime of the circum-Rodinia subduction system.
... The youngest zircon grains in this population could reflect 760 Ma volcanic activity north of the Ugab Region in the Otavi Platform (Halverson et al., 2005) and slightly younger syn-rift ca. 746 Ma magmatism along the southern margin of the Congo Craton and western margin of the Kalahari Craton (Hoffman et al., 1996). Slightly older ca. ...
... Of the two scenarios that have been proposed, east-directed subduction (e.g., Basei et al., 2005;Gray, 2007, 2008) has more recently been favored (Siegesmund et al., 2018), and suggests that the Damara Sequence was deposited in a back-arc setting. Although polarity still remains unconstrained, west-directed subduction is denoted here (Fig. 10;Porada, 1979;Masberg et al., 2005) (Hoffman et al., 1996) Adamastor Ocean stratigraphic accumulation of the Zerrissene Group is more typical of a remnant ocean basin setting than the alternative back-arc setting, given the short time duration and low preservation potential of back arc basins (Saunders and Tarney, 1991;Ingersoll, 2011). ...
... Initial rifting in the late-Neoproterozoic (ca. 780-750 Ma;Hoffman et al., 1996;Jung et al., 2007) was succeeded by marine sedimentation in the segmented Khomas ocean basin between the two cratons, documented by the thick, mixed siliciclasticcarbonate succession of the Damara Supergroup. Subsequent convergence and subduction of the Kalahari Craton below the Congo Craton led to the closure of the Khomas sea from ca. 580-570 Ma onwards (Miller, 2008). ...
Article
The ~545 Ma-old syn-collisional Otjimbingwe alkaline complex is composed of pyroxene-amphibole-biotite-bearing, mildly nepheline-normative to quartz-normative rocks ranging in composition from monzogabbro to monzonite, syenite and granite. The alkaline rocks have moderate to high SiO2 (50.5–73.0 wt%) and Na2O + K2O (5.1–11.5 wt%) and moderate to low MgO (6.6–0.2 wt%) concentrations. All samples have high large ion lithophile element (LILE: Ba up to 4600 ppm) and high-field-strength element contents (HFSE; Zr: 155–1328 ppm; Nb: 16–110 ppm; Ta: 1.4–7.1 ppm and Hf: 4–24 ppm) and have strongly fractionated LREE patterns ((La/Yb)N = 14–51). The most primitive members lack significant negative Eu anomalies. Mantle-normalized multi-element diagrams show depletion in Ba, Rb, Nb (Ta), P and Ti. The alkaline rocks have moderate radiogenic initial ⁸⁷Sr/⁸⁶Sr ratios (0.7061–0.7087) and unradiogenic initial ɛNd values (−3.9 to −6.1). This isotope signature, associated with high LREE/HFSE ratios indicates that the parental melts were generated in enriched portions of the shallow lithospheric mantle, which was probably affected by previous subduction zone processes. In addition, correlations between Sr and Nd isotopes indicate that some of these variations result from combined crustal assimilation and fractional crystallization (AFC) processes. A new model of flat subduction is presented that explains most of the unsolved problems in the orogenic evolution of the Damara orogen, namely (i) the absence of early intrusive rocks with a clear subduction zone setting, (ii) the absence of high-pressure rocks such as blueschists and eclogites, (iii) the unusual distribution of igneous rocks with a clear predominance of granite and granodiorite and (iv) the need for a asthenospheric window during a classical subduction to explain the high T/moderate P granulite facies conditions in the overriding plate.
... The Neoproterozoic Damara Sequence is composed of the fluvial Nosib group and calcareous, glacio-marine Swakop Group. The lower Nosib Group included the the Etusis formation and Khan formation, while the upper Swakop Group included the Rössing formation, Chuos formation, Karibib formation, and Kuiseb formation [8,[22][23][24][25][26][27]. The Salem-type granites in the Gaudeanmus area outcrop in the northwestern and southeastern part of the study area. ...
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The Gaudeanmus area is located at the southern Central Zone of the Damara orogenic belt in south-western Africa. In this paper, we investigate the whole rock major and trace element compositions and Sr–Nd–Pb isotopic compositions of the biotite granite, and determine the age of the samples utilising U–Pb zircon dating methods. Our results provide an LA–collector inductively plasma mass spectrometer (ICP–MS) zircon U–Pb age for the biotite granite of 540 ± 4 Ma (i.e., earliest Cambrian). The biotite granites show the characteristics of metaluminous compositions belonging to high-K calc–alkaline to shoshonite series. The granites contain high alkali and rare earth elements (REE), are enriched in large-ion lithophile elements (Rb, K, Pb), and depleted in high field-strength elements (Nb, Ta, Ti). The REE patterns are characterised by enrichment of LREEs relative to HREEs and medium negative Eu anomalies in the chondrite-normalised REE diagram. These rocks have high initial 87Sr/86Sr ratios (0.71400–0.71768); low εNd(t) value (−12.0 to −7.1); Sm–Nd isotope crust model ages ranging from 1711 to 2235 Ma; and large variations in 206Pb/204Pb (18.0851–19.2757), 207Pb/204Pb (15.6258–15.7269), and 208Pb/204Pb ratios (38.7437–40.5607). Such geochemical signatures indicate that the biotite granite rocks derive mainly from partial melting of ancient crustal rocks resembling the local basement meta-sedimentary rocks. However, minor mantle-derived materials may have also been involved in the formation of these rocks. Combining with regional tectonic evolution, we consider that the biotite granite intrusions in the Gaudeanmus area formed in a transitional tectonic regime that went from compressional to extensional tectonics.
... Some previous studies have suggested that the Rodinia supercontinent rifting began at ca. 780 Ma, which led to the final breakup of the supercontinent between 750 and 725 Ma [63,64]. The mantle plume activity at ca. 780-745 Ma has been reported in northern Namibia [65], North America [66,67], northwestern India [68], Australia [69], India [70] and South China [71] (Figure 10). As discussed above, the ca. ...
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The Xingdi No.2 intrusion in the Kuluktag Block of northeastern Tarim Craton, which intrudes into the Palaeoproterozoic basement with an exposed area of ca. 12 km2, is an orthopyroxene-rich mafic-ultramafic intrusion in a continental rift setting. It consists of gabbros, pyroxenites, and peridotites, and exhibits a crystallization sequence of the principal rock-forming minerals from olivine, orthopyroxene, clinopyroxene, to plagioclase. The gabbros show a concordant SHRIMP U-Pb zircon age of 752 ± 5.4 Ma. In addition, the olivine grains have forsterite content values of 78–85 mole% and mostly contain low NiO, MnO, CaO, and Cr2O3. The rocks are relatively enriched in large ion lithophile elements and LREE, and depleted in HSFE, have non-radiogenic Pb, low εNd (t) values (−2.8 to −23), initial 87Sr/86Sr ratios (0.7059–0.7130). It could be concluded that the rocks represent an analogue of siliceous high magnesium basaltic magma originated by the partial melting of a hydrous and enriched subcontinental lithospheric mantle and contaminated by the continental crust.
... 1). The Damara Belt extends from the Atlantic coast near Swakopmund inland towards Botswana and represents a typical "Wilson Cycle" between 750 Ma to 440 Ma, which culminated in the collision of the Kalahari and Congo cratons (Martin and Porada, 1977;Hoffman et al., 1996;Prave, 1996;De Kock et al., 2000;Gray et al, 2006;Miller, 2009). On the basis of structure, stratigraphy, igneous activity and metamorphic grade, Miller (1983Miller ( , 2008Miller ( , and 2009) divided the Damara Belt into a number of tectonostratigraphic zones bound by major lineaments and faults (Fig. 1). ...
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The geochemical evolution of Nb-Ta-Sn oxides from pegmatites of the Cape Cross–Uis pegmatite belt, Namibia - Warrick C. Fuchsloch, Paul A.M. Nex, Judith A. Kinnaird
... In the Kaoko, Damara, and Gariep belts of southwestern Africa, the time-lapse ca. 840-728 Ma was characterized by alkaline magmatism associated with extension and rifting, related to the break-up of the Rodinia supercontinent (e.g., Hoffman et al., 1996;Frimmel et al., 2001;Jacobs et al., 2008;Konopásek et al., 2008Konopásek et al., , 2014Konopásek et al., , 2018. The Loma Marcelo Orthogneiss has alkaline geochemical characteristics that can be related to continental rifting (A 1 -type magmatism), also has mantle affinity (εNd 777 = +1.65). ...
Article
This study describes the geology, geochemistry, and LA-ICP-MS U–Pb geochronology of igneous rocks that crop out in the Sauce Chico Inlier (SCI) and constitute the Neoproterozoic basement of the Ventania System, Argentina. Magmatism registered in the SCI has developed in two phases. The first phase, of Tonian age, is represented by rift-related calc-alkaline and alkaline granites with ages of 783.8 ± 3.7 Ma (εNd(t) = −7.20) and 776.5 ± 4.7 Ma (εNd(t) = +1.65), respectively. This phase would be related to the break-up of the Rodinia supercontinent and subsequent opening of the Adamastor Ocean. In the Dom Feliciano Belt of southern Brazil and eastern Uruguay, the Tonian magmatism is represented in the Cerro Olivo Complex of the Punta del Este Terrane and in basement inliers of the Pelotas Batholith. The second phase, of Ediacaran age, started with the intrusion of syn-orogenic calc-alkaline granites (620.8 ± 5.8 Ma and 620.3 ± 2.5 Ma; εNd(t) = −9.94/−9.18) and continued with the extrusion of post-orogenic alkaline (577.3 ± 3.9 Ma; εNd(t) = −6.29) and calc-alkaline (543.6 ± 4.0 Ma; εNd(t) = −3.38) acid volcanic rocks. This phase would be related to the closure of the Adamastor Ocean, generation of a magmatic arc along the western margin of the Kalahari Craton, and collision between this and the Río de la Plata Craton. A post-collisional magmatism would have developed due to orogenic collapse (< 580 Ma). With the exception of the Tonian alkaline granite, in the remaining SCI igneous rocks, the juvenile component becomes more important as the crystallization age decreases. The available Nd model ages are between 1.80 and 1.14 Ga and suggest a mixing of older crust with juvenile material. The tectonic evolution of the Ventania System basement is consistent with that observed in the Dom Feliciano Belt and its African counterparts. We present here the first evidence of Tonian magmatism in the Ventania System basement. The results presented here also confirm those obtained previously by other authors. Zircon cores from the SCI igneous rocks have U–Pb inherited ages between ca. 1200 and 900 Ma that could be indicative of a lineage with the Gariep Belt and its Namaqua basement in southwestern Africa.
... Combined with the synchronous felsic magmatism and typical rift deposition in this region, this suggests that the Neoproterozoic continental rifting once occurred in the YZB. In addition, the widely distributed Neoproterozoic anorogenic magmatism also occurred in other Rodinia blocks, such as Australia [10,116,130,131], Laurentia [2,14,132], India [133,134], South Africa [5,135], and Tarim [13,136]. All these observations indicate that the Neoproterozoic anorogenic magmatism (including LTQ basic rocks) and continental rifting in the YZB belong to a series of global rifting magmatic events related to the breakup of the Rodinia supercontinent. ...
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Neoproterozoic intraplate magmatic rocks are widespread in the Yangtze Block (YZB). The contrasting interpretations on their petrogenesis and tectonic evolution induce stimulating discussions on the coeval tectonic setting, including the two competing models of rift-related (R-model) and arc-related (A-model). Their main evidence is dominantly from felsic magmatic rocks. In contrast, the less evolved basic rocks are more suitable for tectonic setting discrimination. Here we study the Longtanqing basic intrusions (LTQ) that are exposed to the central part of the N–S trending Kangdian rift in the western YZB, by detailed geochemical and geochronological investigations. Zircon U–Pb dating of the two diabases from LTQ yield identical ages within error of 777 ± 17 Ma and 780 ± 5.3 Ma, respectively. LTQ rocks are characterized by low SiO2 (49.83–50.71 wt %), high MgO (5.91–6.53 wt %), and Cr (140–150 ppm) contents, supporting the significant mantle affinity. They also display dual geochemical characteristics, including a series of features of continental within-plate basalts (WPB, Ti/V = 37.3–47.5, Zr/Y = 3.4–3.8, Ta/Hf = 0.19–0.23), and the typical signatures of island arc basalt (IAB), such as highly depleted in HFSE and HREE, and enriched in LREE and LILE. Most zircon εHf(t) values are positive (1.6–9.4) while the corresponding Hf depleted mantle model ages (TDM1) range from 1.0 Ga to 1.3 Ga. In combination with the occurrence of inherited zircons (991–1190 Ma), it is suggested that their sources are dominantly derived from the lithospheric mantle that was reconstructed in the late Mesoproterozoic. Thus, LTQ is mainly formed by partial melting of the enriched lithospheric mantle, and subsequently assimilated by a juvenile crust during upwelling. The melt compositions are controlled by different degrees of the crystal fractionation of the dominant clinopyroxene and plagioclase with minor amphibole under high fO2 conditions. Combined with previous geochronological and geochemical data in the YZB, our new results support the theory that the R-model can be responsible for the petrogenesis of Neoproterozoic magmatic rocks in South China.
... The Mahanadi and Godavari, two NW-SE trending Mesozoic rifts, are the major rifts transecting the EGMB. Mahanadi Rift is presumed to be the extension of the Lambert graben of east Antartica (Federov et al., 1982;Hoffman et al., 1996). The formation of the Mahanadi Rift, generally regarded of the Precambrian origin, is accounted to the strike-slip motion along the series of Precambrian shear zones running sub-parallel to the northern contact boundary between EGMB and the Singhbhum craton (Chetty, 1995;Nash et al., 1996). ...
... The IFs older than Cryogenian have only been reported from Namibia and SE China. In Namibia, in the Congo craton, the glacial Chuos Formation with iron ores overlies the Naauwpoort volcanic rock dated at 746 ± 2 Ma (Hoffman et al., 1996;Condon & Bowring, 2011). This provides a robust maximum age constraint for the Chuos IF and diamictites which hence could be considered as a relevant chronostratigraphic counterpart of the Scandinavian IFs and glacial diamictites. ...
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Carbon and strontium isotope chemostratigraphy (178 δ13Ccarb and δ18O, and 81 87Sr/86Sr analyses of carbonate components in whole-rock samples) was applied to constrain apparent depositional ages of the carbonate protoliths of amphibolite-grade, calcite marbles occurring in siliciclastic sedimentary sequences within the Upper and Uppermost Allochthons in the North–Central Norwegian Caledonides. The Sr-rich marbles hosting banded iron formations occur only in the Uppermost Allochthon. The marbles show, over a distance of 350 km, rather similar least-altered 87Sr/86Sr (0.70645–0.70665) and δ13C (+6 to +8‰) values which are all consistent with a late Tonian (800–735 Ma) age. This sets up a maximum depositional age for the overlying iron formations and somewhat younger diamictites. The apparent maximum ages of the Scandinavian iron formations suggest their contemporaneous deposition with the oldest known Neoproterozoic iron formations reported from China (Shilu Formation) and Namibia (Chuos Formation). However, these maximum ages do not rule out the iron deposition and the diamictite accumulation in the early Cryogenian within a presumed Tonian–Cryogenian transition. Three other studied marble units in schistmarble sequences, spatially unrelated to iron formations, show different 87Sr/86Sr and δ13C values matching younger apparent depositional ages of 685–600 Ma (the Uppermost Allochthon), and 550 or 425–410 Ma (the Upper Allochthon). The schist-marble-iron formations sequences in several areas contain extrusive meta-igneous rocks, and rare glacial diamictites. In places, all are intruded by intermediate and mafic sills. The iron formations were originally formed outside Baltica and were subsequently thrust upon the Baltoscandian margin during the Scandian orogeny. The provenance of these iron formations represents an enigma, hinting towards a passive continental margin of an unknown, apparently missing microcontinent. The accumulation of the Scandinavian iron formations within a passive continental margin or a large back-arc basin, in places glacially influenced, represents an exception to other reported clastic, sediment-dominated, Neoproterozoic (Cryogenian) iron formations which all were formed in volcanically active continental rift settings.
... In addition, the ages determined in the present study are only slightly older than U-Pb and Pb-Pb zircon ages of 752 ± 6 Ma (Borg et al., 2003) and 741 ± 6 Ma (Frimmel et al., 1996) for syn-rift within-plate felsic volcanic rocks of the Rosh Pinah Formation in the Gariep Belt. Further north in southwestern African, early Neoproterozoic rifting and/ or bimodal igneous activity occurred in the Damara Belt in northern Namibia between 756 and 746 Ma (Hoffman et al., 1996) and in the Coastal Terrane of the Kaoko Belt in northwestern Namibia between 820 and 710 Ma (Konopásek et al., 2014(Konopásek et al., , 2018. Hence, early Tonian crustal thinning and rift-related extensional magmatism during initial Rodinia break-up was widespread on both sides of the present-day South Atlantic. ...
... 780 Ma coarse-grained fluvial clastic rocks (Martin, 1965;Guj, 1970;Fig. 6d,e) followed by igneous activity dated at ca. 755-745 Ma (Hoffman et al., 1996). In the Gariep Belt, the rifting-related sedimentation started after ca. ...
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Existing models of tectonic evolution of the Neoproterozoic orogenic system rimming the shores of the South Atlantic Ocean (the Araçuaí–Ribeira–Congo and Dom Feliciano–Kaoko–Gariep belts) interpret the belts as subduction-related orogens and emphasize the role of the “Adamastor Ocean” in their pre-collisional evolution. A critical problem in such an interpretation is the confined nature of the northern termination of the orogenic system, as well as a very short time span between the end of rifting and onset of convergence recognized in its southern part. In this contribution, we review the data for the pre- and synorogenic evolution of this system of orogens (here collectively called the South Atlantic Neoproterozoic Orogenic System) and show that the data speak against the presence of a large oceanic domain before the onset of its orogenic evolution. We propose a new and simple intracontinental model, suggesting that Neoproterozoic oceanic crust played only a minor role in the development of the South Atlantic Neoproterozoic Orogenic System and that its overall architecture and thermal evolution is the result of inversion of large-scale rift structures with a protracted, and probably episodic, extensional history. True oceanic crust probably developed only in the southern part of the rift system, but it must have been narrow, akin to the Red Sea–Gulf of Aden stage of the “Adamastor Rift” evolution just before the onset of convergent thickening.
... In Zambia, the Mwashya Conglomerate forms a deep-marine olistostrome, whose clasts were derived from progressive erosion of the Roan platform (Wendorff, 2005a,b). Concomitant extrusions of pyroclastic rocks and lavas were associated with the closing stages of ~765-746 Ma Naauwpoort/ Devede-Mwashya volcanism (Hoffman et al., 1996;Key et al., 2001;Halverson et al., 2005) (Fig. 11). ...
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The origin of the Mwashya Conglomerate at the base of the Mwashya Subgroup in the Lufilian Belt is uncertain since it is considered as either a tectonic or as a sedimentary breccia. At Tenke Fungurume Mining District (TFMD) in the Democratic Republic of the Congo, the Mwashya Conglomerate is marked by an iron-bearing polymictic conglomerate embedded between the Kansuki and Kamoya formations. In this paper, the Kansuki-Mwashya platform succession at TFMD was investigated to shed light on the origin of this conglomerate, the depositional evolution and the tectonostratigraphic framework of the platform. Lithofacies analysis revealed that the Mwashya Conglomerate is a periglacial olistostrome, which was formed around ~765–745 Ma. A pre-Sturtian age for this conglomerate is supported by the Kamoya Formation, which is here interpreted as a post-glacial cap carbonate sequence. The Kansuki-Mwashya platform succession consists of a protected coastal lagoon adjacent to a tidal flat environment, both bordered by a barrier shoal. This paper concludes that the Kansuki-Mwashya platform succession was driven by rifting pulses, occurring gravity flows on instable slope, superimposed upon the ~750–717 Ma long-lasting Sturtian glacial period.
... As no major disconformity, which could have truncated the Naauwpoort volcanics, is evident between units 2 and 3 ( Fig. 5), we follow Schreiber (2006) in placing all four pre-glacial units within the Ugab Subgroup, designated U1-U4. In the Summas Mountains, the Ugab Subgroup overlies the Naauwpoort volcanics, dated at 746 ± 2 Ma at the top (Hoffman et al. 1996;Hoffman & Halverson, 2008), but the base of the Ugab Subgroup could be regionally diachronous. The litho-and chemostratigraphy of the Ugab Subgroup in the Vrede domes is currently under investigation by one of us (KGL) as part of a M.Sc. ...
... The NE-trending Damara Orogen formed during the Pan-African tectono-thermal event. Age-dating of volcanic units within the Nosib Group indicates a span of activity between 750 Ma and 440 Ma (De Kock et al., 2000;Hoffman et al., 1996). The orogen represents a triple point between the Congo, Kalahari and Rio de la Plata cratons that amalgamated during the assembly of Gondwana (Gray et al., 2006;Martin and Porada, 1977;Miller, , 2008Miller and Frimmel, 2009). ...
Article
The Pan African aged Damara Orogen in Namibia is host to the Cape Cross-Uis pegmatite belt, one of several NE-trending pegmatite belts which host Li, Nb, Ta and Sn mineralisation. Field mapping and structural analysis of thirty seven pegmatite bodies has shown that the pegmatites intrude along crustal weaknesses such as fold axes and bedding planes, predominantly following the approximate NE orientated regional structural fabric. The lack of deformation together with cross-cutting relationships mapped, suggest that pegmatites were emplaced during post-tectonic extension that resulted from end-orogeny crustal relaxation. Based on mineralogy, geochemistry and ore mineralogy, three groups of pegmatites are distinguished within the Cape Cross-Uis belt. (1) The most common are the unzoned Nb-Ta-Sn type with mineralised alteration areas in the form of greisenised and albitised zones that occur sporadically in the pegmatites in various morphologies. (2) The garnet-tourmaline, crudely zoned pegmatites are slightly less common. They are granite-hosted and differ from other pegmatite types in terms of their low Rb, Sr, Nb, Ta, Sn, Cs (< 30 ppm) but higher REE, U, Th, and Y values. (3) The zoned lithium-bearing pegmatites are rare but the most complex. They are subdivided into two groups: The Li-Nb-Ta-Sn spodumene-bearing pegmatites of the Karlowa swarm and the Li-Nb-Ta-Sn-Be petalite-bearing pegmatites of the Strathmore swarm. They are highly fractionated with typical values of Nb (206 ppm), Ta (185 ppm) and Sn (10,016 ppm). There is no spatial distribution or regional zonation of the pegmatites type relative to granite outcrops within the belt. Contrasting geochemical fractionation patterns and the lower concentrations of REE in pegmatites than in granites suggests that pegmatites resulted from varying degrees of partial melting of a muscovite-ilmenite-bearing source. The Sn, Nb and Ta in pegmatites is likely to have originated from this source rather than from assimilation of metasedimentary host rocks as clear fractionation trends from fine-grained pegmatite margins to alteration zones show that fractionation was the dominant process in which Nb, Ta, Cs and Sn were enriched.
... Moreover, the northward transportation patterns reported from the Paleozoic sediments of Yemen (Dabbagh and Rogers, 1983) could even suggest more remote sources such as the Damara-Kuunga orogeny (Fig. 1;e.g. De Kock et al., 2000;Gray et al., 2008;Gray et al., 2006;Hoffman et al., 1996;Jung et al., 2001;Morag et al., 2011b;Ostendorf et al., 2014 and references therein). ...
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Studying the provenance of the Cambro-Ordovician sedimentary sequence of Northern Gondwana has great implications on our understanding of the continental scale erosion and sedimentation that took place in the aftermath of the Pan African Orogeny. In this study, heavy minerals, clays and K-feldspars were studied using isotope-geochemistry of Sr, Nd and Pb in order to identify the provenance of these sediments in the section of Eilat, southern Israel. Geochemical measurements suggest that these sediments are a mixture of sediments derived from several sources of distinct geological and geochemical nature; the adjacent juvenile Arabian Nubian Shield, alongside remote, ancient remobilized terrains. These results complement previous studies that focused primarily on heavy minerals and zircons in particular. Furthermore, these results demonstrate the strength of this methodology in the field of provenance, and can be applied for comparison with future studies of adjacent siliciclastic sedimentary cycles.
... The Kaoko and Damara belts were formed by a succession of intracontinental rifting, seafloor spreading, intracontinental subduction, and continental collision including lateorogenic collapse and exhumation that lasted from ~750 Ma to ~530 Ma in the Kaoko belt and from ~750 to ~470 Ma in the Damara Belt (Goscombe et al., 2003;Goscombe et al., 2017;Miller, 2008). Two suites of approximately 750 Ma old, rift-related alkali rhyolites and syenites (Hawkesworth et al., 1983;Hoffman et al., 1996;Jung et al., 2007;Miller and Burger, 1983) from the northern part of the Damara Belt record the age of rifting in the Damara Orogen. ...
Article
The 560.8 ± 2.4 Ma old Okamutambo Pluton is the easternmost intrusion of the syn-collisional Otjimbingwe Alkaline Complex (OAC; Damara Belt, Namibia) and consists of mafic to intermediate alkaline rocks that belong to the monzodiorite-syenite series. The studied samples are potassic (K2O/Na2O > 1) and have moderately high magnesium (MgO: 5.7–2.6 wt%), nickel (Ni: 66–26 ppm), and chromium (Cr: 223–62 ppm) concentrations. LILE (Ba: 2425–1243 ppm; Sr: 1359–941 ppm) and HFSE (Zr: 447–202 ppm, Nb: 32.8–16.4 ppm, Hf: 4.7–9.4 ppm, Ta: 1.2–2.0 ppm) contents are also high. Strontium and Nd isotope data reveal the existence of two magmatic suites indicating a multi-source origin. Group 1 monzonites-quartz monzonites have moderately evolved Sr and Nd isotopic compositions (initial ⁸⁷Sr/⁸⁶Sr: 0.7066 to 0.7073; initial εNd: −3.5 to −5.0) and radiogenic Pb isotope ratios (²⁰⁶Pb/²⁰⁴Pb: 17.65–18.02; ²⁰⁷Pb/²⁰⁴Pb: 15.62–15.67; ²⁰⁸Pb/²⁰⁴Pb: 38.19–38.32). In contrast, group 2 monzodiorites-syenites display more evolved Sr and Nd isotopic compositions (initial ⁸⁷Sr/⁸⁶Sr: 0.7088 to 0.7090; initial εNd: −6.7 to −7.1) but similar Pb isotope ratios (²⁰⁶Pb/²⁰⁴Pb: 17.63–17.82; ²⁰⁷Pb/²⁰⁴Pb: 15.64–15.66; ²⁰⁸Pb/²⁰⁴Pb: 38.21–38.37). Differentiation involved AFC processes in group 1 monzonites-quartz monzonites whereas group 2 monzodiorites-syenites were modified by fractional crystallization. Although second-order processes were operative, high total alkali contents, incompatible trace element concentrations in excess of bulk crustal values, and evolved isotopic compositions of the most primitive samples are source-controlled and provide insight into the origin of the parental magmas. With reference to experimental data from the literature, it is inferred that the Okamutambo alkaline rocks represent evolved melts that were generated through melting of enriched lithospheric mantle (phlogopite-lherzolite). The observed negative Nb-Ta and Ti anomalies and positive Pb anomalies in primitive mantle-normalized trace element patterns are in line with a mantle source that contains a recycled crustal component. Isotopic compositions indicate that mantle enrichment is an ancient feature that might be linked to Proterozoic subduction. At 561 Ma, the geodynamic regime in the Damara Orogen was mainly characterized by compression during convergence and continental collision between the Congo and Kalahari cratons which is difficult to reconcile with the generation of the OAC as alkaline magmatism is commonly associated with extensional tectonic regimes. The OAC is, however, associated with a major suture zone that may have been involved in localized transtensional tectonics during oblique flat subduction and thus enabled the generation and ascent of mantle-derived alkaline melts.
... The Gariep Orogen forms the southern extension of the coastal arm (Kroner, 1977;Martin and Porada, 1977;Ollila, 1987). The Damara Orogen formed during the Pan-African tectono-thermal event; dating of volcanic units within the Nosib Group indicates a span of activity between 750 and 440 Ma (De Kock et al., 2000;Hoffman et al., 1996). ...
Article
The study reveals the relationships between crystal-chemical and genetic properties of beryl from several rare-element granitic pegmatites of the LCT family from the Damara Belt, Namibia. Based on their chemical compositions, the studied beryl samples can be divided into two groups. Beryl from the first group (the Engelbrecht's and Etusis pegmatites) is poor in Fe, Mg and Mn; however, slightly enriched in Zn and alkali elements (Na > Li, Rb, Cs). The samples from the second group (Kuduberge and Tsaobismund) are slightly enriched in Fe and Mg, in which they are mostly charge-balanced by Na, while other octahedral and channel constituents are negligible. The affinity of the first group of beryl to the dominant tetrahedral substitution is manifested in cell dimensions and the c/a ratio. Based on Raman and infrared spectroscopy, the Kuduberge and Tsaobismund samples have a very similar position and intensity of water FTIR bands, indicating the presence of both water types in the channels. The Englebrecht's sample is similar, however, the Etusis sample exhibits a shift of H2O II ν1 band, which could be attributed to a smaller Na content and strong dominance of doubly coordinating water. Our results indicate preferential occupancy of Na in the octahedral Na + (Fe,Mg)²⁺ = + Al substitution, which is typical for beryl from less fractionated pegmatites (Kuduberge and Tsaobismund), however, Na can also be a part of the complex tetrahedral substitution found in Englebrecht's beryl. Higher concentrations of Mn, Sc and Ti indicate a less fractionated genetic environment due to their affinity to Mg and Fe²⁺, whereas Cs, Rb and Zn prefer tetrahedral substitution, which is typical in highly evolved pegmatites.
... In northern Namibia the Rasthof Formation overlies diamictite and conglomerate of the Chuos Formation of presumed Sturtian age. The age of the Rasthof Formation is constrained below by 746 ± 2 Ma (U-Pb TIMS zircon age) volcanic rocks of the Naauwpoort Formation (Hoffman et al., 1996) and above by a 635.6 ± 0.5 Ma (U-Pb TIMS zircon age) rhyolite in the Ghaub Formation (Hoffmann et al., 2004). ...
Article
The Capiru Group (Neoproterozoic) is a 4 km-thick succession deposited on the northern margin of the Curitiba microplate, Southern Ribeira Belt, Brazil. The Capiru Group successions are deformed within the Brasiliano - Pan African Ribeira Belt, which culminated in the amalgamation of Western Gondwana. Even though the rocks are metamorphosed and deformed, preserved sedimentary structures and outcrop exposures allowed the interpretation of depositional environments and stratigraphic analysis. The Capiru Group metasedimentary rocks were studied by field mapping, measured sections and bulk-rock chemical compositions. Six formations were identified, termed from oldest to youngest as: Santana (proximal to distal marine shelf), Juruqui (proximal to distal deltaic), Rio Branco (carbonate platform), Morro Azul (lagoonal to shallow marine), Morro Grande (proximal to distal deltaic/marine) and Bocaina formations (estuarine to shallow-marine). Two main basin stages were identified: a passive continental margin stage, with increasing tectonic quiescence followed by a marine regression, and a syn-orogenic stage at the Curitiba microplate margin. Chemical and mineralogical Fe- and Al-rich compositions suggest continental denudation during the passive continental margin stage, with low sediment input rates from a relatively low-gradient relief, highly weathered continental source, and stability since Mesoproterozoic time. Sedimentological and stratigraphic similarities were found among Capiru Group passive margin successions and units deposited on the Río de Plata and Southern Paranapanema margins. The paleoenvironmental interpretations, such as greenhouse conditions and high weathering rates alternated with depositional gaps, indicate climatic changes during the Capiru Group deposition, with far-field or Phantom Glacial influence related to the regional discordances. These features are important for the paleogeographic reconstructions that must consider a tropical or equatorial position ∼650 - 635 Ma for the Curitiba microplate. During deposition of the Bocaina Formation, the arkosic composition of sandstones and lithic conglomerate lags suggest source rejuvenation during the syn-orogenic stage, prior to the collisional thrusting that led to the Western Gondwana assembly.
... The Chuos and Ghaub diamictites were deposited on the Congo craton and are part of the Otavi platform (Le Heron et al., 2013). The age of the Chuos is bracketed by underlying 747 ± 2 Ma volcanic rocks (Hoffman et al., 1996) and the overlying Ghaub Formation, the latter of which has an age of 635.5 ± 1.2 Ma derived from zircon in an interbedded ash layer (Hoffmann et al., 2004). These data suggest that the Chuos Formation formed during the Sturtian glaciation and the Ghaub formed during the Marinoan glaciation. ...
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Diamictites produced by continental glaciations were deposited in the Mesoarchean (∼2.9 Ga), Paleoproterozoic (∼2.4–2.2 Ga), Neoproterozoic (∼0.75–0.58 Ga), and Paleozoic (∼0.3 Ga), and, with the exception of the Mesoarchean, occur on multiple modern continents. We examine the provenance of a selection of these diamictites from all four time periods and three continents via detrital zircon U-Pb geochronology and Hf isotope and whole-rock Nd isotope analyses. Mesoarchean samples from South Africa contain mostly Mesoarchean zircon, with a much smaller number of Paleoarchean grains. Zircon Hf and whole-rock Nd point towards their derivation from reworked Paleoarchean crust. Paleoproterozoic samples from North America share a similar unimodal zircon age distribution, with a peak at 2.7 Ga and a narrow range in Hf isotopes that is slightly superchondritic. Coupled with whole-rock Nd results, these data point to their derivation from reworked juvenile Mesoarchean crust. By contrast, the Nd isotope data and existing published detrital zircon U-Pb data for Paleoproterozoic diamictites from South Africa indicate a mix of Neoarchean and Mesoarchean sources. Neoproterozoic samples from Namibia share a Paleoproterozoic detrital zircon population with variable but subchondritic Hf isotope compositions, indicating derivation from reworked Neoarchean crust. Most samples also have Mesoproterozoic and Neoproterozoic zircon with both positive and negative εHf(i), and all have Nd isotope compositions yielding Paleoproterozoic model ages. A Neoproterozoic sample from Canada contains mostly Neoproterozoic detrital zircon with variable Hf isotope compositions, indicating a mix of juvenile and reworked Mesoproterozoic crust. Samples from different parts of the Paleozoic Dwyka Group in South Africa show significantly different provenance, with a western locality containing only zircon older than 2.0 Ga and a Neoarchean Nd model age. By contrast, eastern samples have Paleoproterozoic model ages and published detrital zircon ages are largely Mesoproterozoic and younger. Paleozoic samples from Bolivia have a Paleoproterozoic Nd model age. Overall, the difference between zircon initial Hf isotope compositions and the depleted mantle is smallest for Archean zircons, indicating major episodes of crustal reworking followed shortly after crust formation (perhaps reflecting the last stage of cratonization). Hf and Nd data together record major episodes of juvenile crust formation at 3.2, 2.8–2.7, 2.0–1.7, and 1.3–0.9 Ga, consistent with previously documented global crust formation events. The diamictites provide no evidence for pre-3.6 Ga felsic crust in these regions.
Chapter
The Damara Orogenic System is a well-exposed orogenic junction that preserves a rich record of West Gondwana assembly and crustal processes in classic examples of transpression (Kaoko Belt) and bivergent collisional orogenesis (Damara Belt). Both belts show typical orogenic cycles in common with orogenic belts universally: from rifted passive margin sequences, subduction at continental margin arcs with back-arc basins, collision, crustal overthickening, outwedging of orogenic margins, detachment of subducted lithosphere, upper-plate lithospheric thinning and eventual collapse. There is no controversy here, and, like all orogens, kinematic and metamorphic response is dynamic and strongly zonal, the patterns of which are strong evidence for crustal architectures and tectonic history. Large relational data sets are required to characterize these patterns of orogenic response, to test robustness, accuracy and internal consistency, and to build crustal and tectonic models. For this summary of the Damara Orogenic System, two large-scale internally consistent relational data sets have been integrated: (1) age-calibrated deformation histories have been correlated across the whole system on the basis of stress fields and absolute age constraints in common; and (2) deformation structures have been correlated with mineral growth and patterns of metamorphic response characterized by P–T evolutions, metamorphic maps and field gradients, quantified using a large data set of PT determinations. Collision of the Rio De La Plata Craton at ~590 Ma resulted in west over east obduction of the Coastal Terrane arc over the Congo Craton passive margin. The Kaoko Belt subsequently evolved through ~45° clockwise rotation of the stress field, showing progressive transpressional orogenesis, steepening and strain partitioning, resulting in a strike-slip shear system between ~550 and 530 Ma. Collision in the Damara Belt at ~555–550 Ma involved subduction of the Kalahari Craton margin, with a highly attenuated Congo Craton passive margin in an upper-plate setting. The Kaoko and Damara orogenic fronts were both operating between 550 and 530 Ma, and with the same northwest-southeast shortening direction. At ~530–525 Ma a stress switch to north-northwest-south-southeast shortening resulted in transtensional reactivation of the Kaoko Belt shear zones, rapid exhumation and cooling, terminating orogenesis in this belt. At this time, main phase orogenesis, burial and metamorphism peaked in the Damara Belt, and subsequent contraction in this belt dominated the stress field, which evolved through a roughly 70° clockwise rotation to northeast-southwest shortening by ~512–508 Ma. Barrovian metamorphism in the southern orogenic margin was diachronous, 530–522 Ma in the north to 517–514 Ma in the south, and accompanied ongoing contraction. Deep burial to 9.5–11.5 kb followed by rapid isostatic readjustment gave successive foreland propagating exhumation events at <522 and <517 Ma, by southward transport of crustal wedges along basal thrusts. Outwedging was accommodated by top down to the north transport of hanging walls at higher structural levels, indicated by major metamorphic discontinuities and extensional structures, resulting in extensional telescoping of the southern margin. In contrast, medium-P/high-T granulite facies metamorphic conditions persisted in the orogen core from 540 to 505 Ma, following low ΔP/ΔT clockwise P–T paths indicating moderate burial and stable high-heat flow conditions, best explained by the detachment of subducted lithospheric during collision. A rapid stress switch to east–west directed shortening along the orogen at ~508 Ma generated crossfolding in the orogen core and northern margin. This stress field is inconsistent with any plausible trajectory between the Rio De La Plata, Congo and Kalahari Cratons and is interpreted as a far-field effect from the orogenic margin of Gondwana at that time: arc collisions in the Ross Orogen. This established a north–south extension direction exploited by 508–504 Ma decompression melts in many parts of the system, and at ~505–500 Ma triggered gravitational collapse and extension of the thermally weakened orogen core, resulting in a broad bivergent core complex, rapid exhumation and cooling from 700 to 400 ℃ between 500 and 470 Ma.
Chapter
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Closure of the southern Adamastor Ocean led to the development of the Neoproterozoic-Early Cambrian orogens on both margins of the present South Atlantic Ocean, which preserve a rich record of West Gondwana assembly and large-scale crustal evolution. Here we describe the distinct stages and tectonic regimes related to the evolution of the southern Adamastor Ocean as reconstructed from the geological record in southeastern Brazil, Uruguay and southwestern Africa. The welding of the Rio de la Plata/Paranapanema and African cratons was the result of a long history that generated the Dom Feliciano Belt in southeastern Brasil and Uruguay and its African counterparts, the Kaoko, Gariep and Saldania belts. Recent ideas and previous hypotheses are discussed and integrated into a tectonic model for the evolution of the southern Adamastor Ocean, largely based on comparison of the Neoproterozoic Kaoko and Dom Feliciano belts. The history of the southern Adamastor Ocean spans between 900 and 590 Ma, from the earliest records of magmatism related to Rodinia break-up (980–780 Ma), through the climax of the extensional phase that led to the opening of a vast ocean (780–640 Ma). Tectonic inversion led to subduction towards the east (in today’s coordinates), which generated an extensive magmatic arc on the western margin of the Congo and Kalahari cratons (640–600 Ma) and eventually the collision between the Congo and Rio de La Plata Cratons (600–590 Ma) juxtaposing the magmatic arc and the western South American schist belts. At around 530 Ma, still under the influence of plate convergence, a positive flower structure was generated causing the extrusion of the Granite Belt, the eroded remnant of the magmatic arc, and reactivation of the doubly verging thrusts that affected the supracrustal units of the Kaoko and Dom Feliciano belts. Only at that time did deformation reach the Itajaí and Nama foreland basins.
Article
This paper presents a systematic study of major and trace elements and Sm-Nd isotopes in leucogranites closely related to uranium mineralization in the Gaudeanmus area, Namibia. The results illustrate that the uraniferous leucogranites possess high SiO2 (68.8wt%–76.0wt%, average 73.1wt%) and K (4.05wt%–7.78wt%, average 5.94wt%) contents, and are sub-alkaline and metaluminous to weakly peraluminous, as reflected by A/CNK values of 0.96–1.07 with an average of 1.01. The leucogranites are rich in light rare earth elements (LREE/HREE = 2.53–7.71; (La/Yb)N = 2.14–10.40), have moderate Eu depletion and high Rb/Sr ratios (2.03–5.50 with an average of 4.36); meanwhile, they are enriched in Rb, K, Th, U and Pb, and depleted in Ba, Nb, Ta, and Sr. The ∊Nd(t) values of uraninites range from –14.8 to –16.5, and the two-stage Nd model ages are 2.43–2.56 Ga. Detailed elemental and Sm-Nd isotopic geochemical characteristics suggest that the leucogranites were formed in a postorogenic extensional environment. The U-rich pre-Damara basement was the main source of uranium during the primary mineralization event, which is disseminated in leucogranites, whereas the uranium mineralization in veins possibly resulted from remobilization of the primary uranium minerals.
Thesis
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Despite a wealth of research on the Kalahari Copperbelt over the past 30 years, two crucial aspects of the mineralizing systems have remained elusive. First, the age of the rift sequence hosting the deposits and, second, the nature of the fluid pathways for the mineralizing fluids. Laser ablation-inductively coupled plasma mass spectrometry (LA-ICPMS) U-Pb isotopic analysis on one igneous sample of the Makgabana Hills rhyolite (Kgwebe Formation) within the central Kalahari Copperbelt in Botswana constrains the depositional age of the unconformably overlying Ghanzi Group to after 1085.5 ± 4.5 Ma. The statistically youngest detrital zircon age populations obtained from the uppermost unit of the Ngwako Pan Formation (1066 ± 9.4 Ma, MSWD = 0.88, n = 3), the overlying D’Kar Formation (1063 ± 11, MSWD = 0.056, n = 3), and the lower Mamuno Formation (1056.0 ± 9.9 Ma, MSWD = 0.68, n = 4) indicate that the middle and upper Ghanzi Groups were deposited after ~1060 to ~1050 Ma. Lu-Hf analysis of detrital zircon suggests that the Mesoproterozoic and Paleoproterozoic rocks of the Namaqua Sector and the Rehoboth Basement Inlier were the primary sediment sources for the siliciclastic rocks of the Ghanzi Group and lesser material was derived from the basin-bounding footwall margin of the northwest Botswana rift, the Paleoproterozoic Magondi Belt and the Okwa Block, and possibly parts of the Limpopo Belt on the northern margin of the Kalahari Craton in southern Africa. A molybdenite Re-Os age of 981 ± 3 Ma provides a minimum depositional age constraint on D’Kar Formation sedimentation. Authigenic xenotime U-Th-Pb ages of ~925 and 950 Ma further the evidence for an earliest Neoproterozoic (Tonian) age for the D’Kar Formation. Re-Os ages of 549 ± 11.2 Ma (low-level highly radiogenic chalcocite-idaite) and 515.9 ± 2.4 Ma (molybdenite), and a U-Th-Pb age of 538.4 ± 8.3 Ma (xenotime inclusion in chalcopyrite) from several Cu-Ag deposits in the central Kalahari Copperbelt suggest prolonged mineralizing events during basin inversion related to the Pan-African (~600 to 480 Ma) Damara orogen. High-resolution aeromagnetic maps were utilized to define the stratigraphy and structure of the Ghanzi Group of the central Kalahari Copperbelt. Maps of the second vertical derivative transformation were compared with detailed stratigraphic data from drill holes. These data reveal previously unrecognized thinning of the Ngwako Pan Formation onto rocks of the underlying Kgwebe Formation and suggest the presence of syn-sedimentary horst and graben and/or half-graben structures. Truncation of the aeromagnetics fabric of the uppermost Ngwako Pan Formation rocks suggests that the rocks were gently folded and eroded above paleotopographic highs prior to the ensuing basin-wide marine transgression and sedimentation of the unconformably overlying mixed marine siliciclastic and carbonate rocks of the D’Kar Formation. Detailed facies architecture derived from both drilling and magnetic data at the Zone 5 Cu-Ag deposit, located east of the Kgwebe and Makgabana Hills, suggests that its physical (stratigraphic) and chemical (organic material and in-situ bacteriogenic sulfide) nature were influenced by the underlying basin architecture, which was critical in development of trap sites and in funneling epigenetic hydrothermal fluids into those traps. The presented new data indicate that the basin architecture underlying the Kalahari Copperbelt strongly influenced the localization of deformation and hydrothermal fluid flow during epigenetic events. The results of this study can be used to help vector exploration for Cu-Ag deposits through the recognition of buried paleotopographic highs and associated favorable trap sites.
Article
Damara Belt is well-exposed mid-crustal section through a collisional orogen of Cambrian age that closed the Khomas Ocean basin between passive margins on the Congo and Kalahari Cratons. Collision resulted in a bi-vergent orogen with distinct paired metamorphic pattern of foreland-vergent high-P/low-T orogenic margins and a broad high-grade, low-P/high-T orogen core. Spatial and temporal patterns of the metamorphic response to collision have been characterized for all parts of the belt using; a large dataset (n ~ 240) of internally consistent quantitative PT determinations, evolution of mineral parageneses and semi-quantitative P-T paths, metamorphic mapping and quantitative metamorphic field gradients. Integration with deformation history, structural profiles, metamorphic chronology, magmatic history and stratigraphy, constrains a dynamic model of crustal architecture during peak metamorphic events. The pattern of zonal metamorphic response is demarcated by three major metamorphic discontinuities (MD) with steep pressure gradients, inferring crustal-scale structures that accommodated lateral exhumation of crustal wedges. Discontinuities are confirmed by deformation features in the field, and metamorphic mineral growth indicates that vertical flattening at the peak of metamorphism progressing through ductile to brittle extensional structures. Crustal wedges along the orogenic margins experienced steep clockwise P-T paths with peak-PT conditions terminated by isothermal decompression during rapid exhumation in isostatic response to deep burial. Metamorphic chronology and over-printing metamorphic fabrics constrain a sequence of foreland propagating out-wedging of crustal thrust wedges that resulted in telescoping of the orogenic margins. Peak metamorphism at a geothermal gradient of 20–25 °C/km and 8–9 kb in the Southern Zone (Wedge I) was attained between ~ 517–530 Ma, followed by south-directed out-wedging at the Uis-Pass Suture, accommodated by relative extension (MD1) at high structural levels near the boundary with the high-grade orogen core. Out-wedging of the Southern Zone, further buried the Southern Margin Zone (Wedge II) in the footwall below the Uis-Pass Suture. Peak metamorphism at 17 °C/km and 9.5–11.5 kb in the Southern Margin Zone was attained at 517 ± 4 Ma and followed by out-wedging on basal thrusts, accommodated by vertical flattening and extensional reactivation of the Uis-Pass Suture (MD2). Peak metamorphism at 17 °C/km and 10.5 kb in a high-P/low-T crustal wedge in the northern margin (Wedge III) occurred at 510 ± 4 Ma. Peak metamorphism in this wedge was terminated by isothermal decompression during north-directed thrusting, accommodated by extension at higher structural levels near the high-grade orogen core (MD3). Granulite facies metamorphism in the high-grade orogen core, which formed the upper plate to the deeper thrust wedges occurred at significantly lower pressures (4.3–6.0 kb), higher T/depth ratios (≥ 38–47 °C/km) and low ∆ P/∆ T clockwise P-T paths indicating only moderate burial and protracted high-grade conditions. Metamorphic chronology confirms high-grade conditions (540–505 Ma) persisted beyond isostatic adjustment of the high-P/low-T orogenic margins. High-heat flow conditions and long thermal lag are best explained by lithospheric breakoff during collision. In contrast to the high-P/low-T margins that experienced lateral exhumation in a convergent system, the high-grade orogen core was exhumed vertically as a broad core complex during extensional collapse of the orogen. Granulite grade conditions in the central part of the orogen core record higher pressures than marginal lower-grade zones to both the north and south. The contrasting post-peak vertical exhumation after ~ 505 Ma of the central core was accommodated by brittle extensional detachments at high stratigraphic levels with km-scale omission of crustal section.
Preprint
The end-Cryogenian glaciation (Marinoan) is portrayed commonly as the archetype of snowball Earth, yet its duration and character remain uncertain. Here we report U-Pb zircon ages for two ash beds from widely separated localities of the Marinoan-equivalent Ghaub Formation in Namibia: 639.29 ± 0.26 Ma and 635.21 ± 0.59 Ma. These findings verify, for the first time, the key prediction of the snowball Earth hypothesis for the Marinoan glaciation, i.e., longevity, with a duration of ≥4 m.y. They also show that the nonglacial interlude of Cryogenian time spanned 20 m.y. or less and that glacigenic erosion and sedimentation, and at least intermittent open-water conditions, occurred 4 m.y. prior to termination of the Marinoan glaciation.
Article
The eastern portion of the Dom Feliciano Belt in southernmost Brazil, known as Pelotas Terrane, is composed of Neoproterozoic rocks presenting metamorphic xenoliths in granitoid rocks, described and interpreted as “basement inliers”. Based on novel petrographic, mineral/whole rock/Sr–Nd geochemistry and U–Pb geochronological data, this paper presents the first record of a Paleoproterozoic metagabbro xenolith in the Pelotas Terrane. The studied xenolith exhibits a sharp and angular contact with the host rock and a predominant granoblastic texture, with sub-vertical mm-to cm-thick metamorphic banding, marked by the felsic and mafic bands, composed respectively of bytownite (An85) ± quartz, and diopside ± tremolite ± epidote. Zircon, apatite, and iron/titanium oxides occur as accessory minerals. The peak metamorphism for this rock is estimated at amphibolite facies conditions. The whole-rock chemistry of the metagabbro suggests a basic derivation (e.g., SiO2 52 wt%; Al2O3 12 wt%; Ni 33 ppm). The spidergrams demonstrate high LILE and HFSE concentrations. Chondrite-normalized REE patterns show enrichment in LREE. The Sr–Nd isotopic data show ⁸⁷Sr/⁸⁶Sr(i) = 0.701024, εNd(i) = 5.44, and Nd-TDM = 2.1 Ga. The metagabbro yielded a U–Pb Concordia age with analytical points arranged in a Discordia line. The upper intercept was interpreted as a magmatic crystallization age of 2.1 Ga (zircon cores), and the lower intercept indicated a resetting age of 616 Ma (zircon rims). The obtained Archean age (2.8 Ga) is interpreted as inherited. Gabbro's metamorphism likely reached the amphibolite facies, elemental and isotopic geochemistry. The remote zircon inheritance of xenoliths that have had polycyclic survival is used to link the occurrence of a gabbro xenolith, in the Pelotas Terrane (Dom Feliciano Belt; Southern Brazil), whose magmatic crystallization age is Paleoproterozoic, with the basement terranes before the Namaquan orogeny, in the Rehoboth region (Gariep Belt and Bushmanland/Namaqualand; Namibia). These units could be related to the evolution of an ancient Paleoproterozoic magmatic arc formed by the convergence of two Archean continents during the evolution of the Atlantica Supercontinent.
Article
Significance Carbonate sediments of Neoproterozoic age exhibit large secular excursions of carbon isotope composition outside the range of modern seawater dissolved inorganic carbon (DIC), but their origins are controversial. We show that in a Neoproterozoic carbonate platform in Namibia, such excursions disappear on the flanks of the platform, where compositions are more compatible with modern seawater. We attribute the observed spatial variation to early fluid-buffered alteration on the flanks of the platform, where seawater invaded the sediment in response to geothermal porewater convection. Accordingly, the isotope excursions in the platform interior are decoupled from open-ocean DIC, which remained close to the modern range. Our interpretation is testable and, if confirmed, has important ramifications for the origins of ancient carbon isotope excursions.
Article
Early Cretaceous (135-130 Ma) continental rupture of Western Gondwana to form the South American and African plates closely paralleled the elongate trends of Precambrian and Paleozoic orogenic belts. These orogenic belts were produced as a result of the Neoproterozoic convergent and strike-slip assembly of Gondwana that re-deformed during later, Paleozoic orogenic events. Continued continental rifting led to the formation of conjugate, South Atlantic volcanic passive margins (VPM’s) whose widths vary from 55-180 km. Along-strike variations in crustal stretching, as measured from deep-penetration seismic reflection profiles, correlate with parallel and oblique orientations of rifts relative to the trend of the orogenic, basement fabric. Where orogenic fabric trends parallel to the north-south South Atlantic rift direction such as in the Dom Feliciano Uruguay/Brazil and Kaoko orogenic Belt of Namibia, we observe narrow (55-90 km) rift zones with modest continental beta factors of 2.5-3.5 because smaller amounts of rifting were needed to stretch the weaker and parallel, orogenic, basement fabric. Where basement fabric trends near-orthogonally to the north-south South Atlantic rift direction such as in the Salado suture of Southern Uruguay and the Damara Belt of Namibia, we observe wider (185-220 km) rift zones with higher beta factors of 4.3-5 because greater amounts of stretching were needed to rupture the orthogonal, orogenic, basement fabric. The rift-oblique Gariep Belt intersects the South Atlantic continental rupture at an intermediate angle (30°) and exhibits a predicted intermediate beta factor of 4. A compilation of published beta factors from thirty-six other rifted margins worldwide supports the same basement-trend-degree of stretching relationship we propose - with rift-parallel margins having lower beta factors in a range of 1.3-3.5 and rift-orthogonal or oblique-margins have higher beta factors in a range of 4-8.
Article
We interpret 27,550 km of deep-penetrating, 2D-seismic reflection profiles across the South Atlantic conjugate margins of Uruguay/Southern Brazil and Namibia. These reflection profiles show in unprecedented detail the lateral and cross-sectional, asymmetrical distribution of voluminous, post-rift volcanic material erupted during the Barremian-Aptian (129-125 Ma) period of early seafloor spreading in the southernmost South Atlantic. Using this seismic grid, we mapped the 10-200-km-wide, continental margin-parallel limits of seaward-dipping-reflector complexes (SDRs) - that are coincident with interpretations from previous workers using seismic refraction data from both the South American and West African conjugate margins. Subaerially-emplaced and tabular SDRs have rotated downward 20° in the direction of the Mid-Atlantic spreading ridge and are up to 22 km thick near the limit of continental crust (LoCC). The SDR package is wedge-shaped and thins abruptly basinward towards the limit of oceanic crust (LoOC) where it transitions to normal, 6-8-km-thick oceanic crust. We propose a model for the conjugate rifted margins that combines diverging tectonic plates and north-westerly plate motion with respect to a fixed mantle position of the mantle plume. Our proposed model explains an approximately 30% greater volume of SDRs/igneous crust on the trailing Namibian margin than on the leading Brazilian margin during the syn- and post-rift phases. Our model for volcanic margin asymmetry in the South Atlantic does not require a simple shear mechanism to produce the asymmetrical volcanic material distribution observed from our data and from previously published seismic refraction studies. Determining the basinward extent of extended continental basement is crucial to understanding basin evolution and for hydrocarbon exploration. Although the conjugate margins are asymmetric in evolution, their proximity during the early post-rift stage suggests a near-equivalent, early basin evolution and hydrocarbon potential. Similarly, understanding the processes and conditions that produce these asymmetries is critical for understanding volcanic passive margin (VPM) evolution.
Article
The Zambezi Belt located in southern Zambia and northern Zimbabwe forms part of the E–W-trending Kuunga orogen associated with the amalgamation of Gondwana supercontinent. The belt consists of a crystalline basement unconformably overlain by a supracrustal sequence of meta-pelites/psammites, calc-silicates, meta‑carbonates, and magmatic rocks. Monazite-bearing meta-pelites are distinguished by petrography, geothermobarometry, monazite chemistry, and detrital provenance. The studied samples include six garnet-bearing schists with biotite, plagioclase, quartz, muscovite ± staurolite ± kyanite from the Nega Formation, and three kyanite-mica schists with chlorite, quartz, and accessory rutile, ilmenite, tourmaline from the Chipongwe Formation. Phase equilibria modelling and isopleth geothermobarometry identified Barrovian-type medium-T/ medium-P facies peak conditions of 570–665 °C/ 6.5–9 kbar on garnet-bearing schists, and 600–663 °C/ ~8.8–12 kbar on kyanite-mica schists. Evidence from detrital geochronology suggests that the two formations are stratigraphically equivalent, but electron microprobe dating on monazite suggests they underwent different tectonothermal histories. Monazite ages indicate metamorphism and crustal growth from ~650–579 Ma in 3 of the garnet-bearing schists of the Nega Formation, and ~ 550–500 Ma in one of the kyanite-mica schists of the Chipongwe Formation. Dating of detrital zircon by LA-ICP-MS indicates local and distal sediment sources including Neoarchean to Paleoproterozoic (2633–1800 Ma) and Meso- to Neoproterozoic (1700–852 Ma). A tectonic cycle of burial metamorphism to exhumation is indicated by clockwise P-T-t paths along different geothermal gradients. The rock units were brought together during collision between the Congo and Kalahari Cratons and final assembly of Gondwana.
Article
This study provides new constraints on the paleoenvironmental, eustatic and carbon isotopic models of the cap carbonate-like rocks of the Dolomie Tigrée Formation in the Katanga region, Democratic Republic of the Congo. The study integrates detailed lithofacies analysis combined with sequence stratigraphy to reveal the high-resolution architecture framework of deposition. Eight lithofacies types are defined: shale, siltstone and sandstone (LF1), dolosiltite (LF2), dolarenite (LF3), massive dolomudstone (LF4), laminated dolomudstone (LF5), intraclastic dolopackstone (LF6), laminar dolomudstone (LF7), and fenestral dolopackstone (LF8). These lithofacies types show that the Dolomie Tigrée carbonate succession accumulated on a distally steepened slope to proximal carbonate ramp setting. The sequence stratigraphy analysis revealed 17 elementary parasequence sets (5th order) that are grouped into 3 distinct successive marine transgressive cycles (PSQ1 to PSQ3) of 4th order magnitude. Each cycle records, from the base to the top, a dominant transgressive system tract, vertically-upward evolving to short time highstand and lowstand system tracts bounded by a sequence boundary. PSQ1 records an early marine transgression marked by the negative shift in δ¹³C values near −5‰ at the base, then returned to positive values in the upper part of PSQ1 and PSQ2. This carbon isotopic excursion is here interpreted as the hydrological and tectonic reorganizations of outer-ramp-derived allopadic materials from deeper to shallow carbonate-oversaturated reservoirs, coupled with a glacial-isostatic rebound, which enhanced the development of biogenic carbonates in a proximal carbonate ramp setting. Our results show that the changes of sea level triggered the carbonate deposition occurring during the marine transgression. This trangression was related to extensional tectonics of the Nguba rift basin, which were possibly enhanced by the isostatic deglacial rebound in the aftermath of the Sturtian glaciation event.
Article
Lithospheric cusps occur where arcs are joined end to end. Where a subducting plate moves directly into a cusp, the slab experiences lateral constriction due to the cusp geometry. Buckled slabs of Cenozoic age occur at cusps (also known as ‘syntaxes’) in the Arabian, Indian, Pacific, Juan de Fuca and other plates. Here I report an Ediacaran example from the cusp of the Congo Craton where Pan-African collision zones meet at a right angle in NW Namibia. The craton was blanketed by syn- and post-rift Neoproterozoic marine carbonate, disconformably overlain by collision-related foredeep clastics. The disconformity has little stratigraphic relief in a 900 km-long fold belt rimming the craton, except within 60 km of the cusp apex where foredeep deposits bury a megakarst landscape floored by exhumed crystalline basement. Forebulge uplift, estimated from palaeokarst relief, was ≥1.85 km. This far exceeds characteristic forebulge heights of c. 0.5 km and matches the deepest part of the Grand Canyon of Arizona (USA). Coeval with megakarst development, map-scale mass slides moved coherently westwards and southwards towards the advancing accretionary prisms. Rapid burial by foredeep clastics preserved the megakarst palaeosurface and associated mass slides; folding them brought protection from complete destructive resurfacing for eons.
Article
Full-text available
One of the major issues in Neoproterozoic geology is the extent to which glaciations in the Cryogenian and Ediacaran periods were global in extent and synchronous or regional in extent and diachronous. A similarly outstanding concern is determining whether deposits are truly glacial, as opposed to gravitationally initiated mass flow deposits in the context of a rifting Rodinia supercontinent. In this paper, we present 115 publically available, quality-filtered chronostratigraphic constraints on the age and duration of Neoproterozoic glacial successions, and compare their palaeocontinental distribution. Depositional ages from North America (Laurentia) clearly support the idea of a substantial glacial epoch between about 720-660 Ma on this palaeocontinent but paradoxically, the majority of Australian glacial strata plot outside the previously proposed global time band for the eponymous Sturtian glaciation, with new dates from China also plotting in a time window previously thought to be an interglacial. For the early Cryogenian, the data permit either a short, sharp 2.4 Ma long global glaciation, or diachronous shifting of ice centres across the Rodinia palaeocontinent, implying regional rather than global ice covers and asynchronous glacial cycles. Thus, based on careful consideration of age constraints, we suggest that strata deposited in the ca. 720-660 Ma window in North America are better described as belonging to a Laurentian Neoproterozoic Glacial Interval (LNGI), given that use of the term Sturtian for a major Neoproterozoic glacial epoch can clearly no longer be justified. This finding is of fundamental importance for reconstructing the Neoproterozoic climate system because chronological constraints do not support the concept of a synchronous panglacial Snowball Earth. Diachroneity of the glacial record reflects underlying palaeotectonic and palaeogeographic controls on the timing of glaciation resulting from the progressive breakup of the Rodinian supercontinent.
Article
Central Gondwana was assembled by three continental collisions in relatively quick succession: late Cryogenian East Africa Orogen, early Ediacaran West Antarctica Orogen and late Ediacaran Kuunga Orogen. The Kuunga Orogen involved diachronous closure of the South Adamastor–Khomas–Mozambique Oceans and accretion of Kalahari Craton and cratonic elements in Antarctica, with a previously assembled North Gondwana. The two older orogens were still hot and deforming at the time of final assembly by the Kuunga Orogen, and were therefore reworked and re-metamorphosed. The Central Kuunga Orogen is comprised of the Lufilian Arc, Zambezi Belt, Malawi–Unango Complex and the Lurio Belt. This region was the site of earliest collision in the Kuunga Orogen at ~575 Ma, and involved collision of two buoyant, previously metamorphosed rigid basement promontories. Pivoting on the Zambezi Belt indenters led to clockwise rotation of the Kalahari Craton and oblique collision within the Damara Belt ~20–30 m.y. later. The Central Kuunga Orogen is a relatively cold collisional belt dominated by eclogite, whiteschist and Barrovian series metamorphic parageneses, and contrasts with the paired metamorphic response in the Damara Belt to the west, and low-P/high-T metamorphism in the East Kuunga Orogen. Metamorphic parageneses are preserved from each stage of the full Wilson Cycle: from initiation of continental lithosphere thinning at ~940 Ma, widespread rifting between 725 and 805 Ma, and passive margin sedimentation until ~580 Ma. Eclogite-facies subduction parageneses indicate consumption of ocean lithosphere was underway by ~630–660 Ma. Collision at ~575 Ma involved deep burial of continental crust and formation of very high-P, low T/depth metamorphic parageneses, followed by Barrovian series thermal peaks at ~545 and ~525 Ma. Isostatic compensation and stress switches associated with plate reconfigurations once Gondwana was assembled, resulted in exhumation and local extension in an intra-continental setting from ~518 Ma.
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Dropstones of ice-rafted origin are typically cited as key cold-climate evidence in Cryogenian strata and, according to conventional wisdom, should not occur in postglacial, warm-water carbonates. In Namibia, the Chuos Formation (early Cryogenian) contains abundant dropstone-bearing intervals and striated clasts. It is capped by the Rasthof Formation, composed of laminites in its lower portion and microbial carbonates above. These laminites are locally found to contain pebble- and granule-sized lonestones in abundance. At the Omutirapo outcrop, meter-thick floatstone beds occur at the flanks of a Chuos paleovalley and are readily interpreted as mass-flow deposits. At Rasthof Farm, however, the clasts warp, deflect, and penetrate hundreds of carbonate laminations at both the outcrop and thin-section scale. We propose that these are dropstones, and we infer an ice-rafting mechanism. Evidence for vestigial glaciation concomitant with cap carbonate deposition thus merits a reappraisal of the depositional conditions of cap carbonates and their paleoclimatic significance.
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The Tonian and Cryogenian periods together span from 1000 to c. 635.5 Ma and are currently chronometrically divided at 720 Ma. The early Tonian followed the amalgamation of the Rodinia supercontinent and is a time for which the stratigraphic, chemostratigraphic, and fossil record is relatively sparse and poorly dated. The initiation of intracratonic basins on many cratons c. 850 Ma, while Rodinia was still intact, is responsible for a much richer late Tonian record. This record preserves evidence for eukaryotic diversification and the first documented pronounced negative carbon isotope anomaly in the Neoproterozoic—the Bitter Springs Anomaly. Much of the second half of the Tonian Period is characterized by high carbon isotope values (δ¹³C of carbonate >5‰), but recent studies indicate that at least one and probably two deep negative δ¹³C excursions occurred after c. 740 Ma, the latter immediately preceding the onset of Cryogenian glaciation. This glaciation appears to have initiated globally at c. 717 Ma, based on consistent, high-precision U–Pb zircon ages from multiple sedimentary successions. These ages will support formal definition of the Global Stratotype Section and Point for the base of the Cryogenian System. This first Cryogenian glaciation, commonly referred to as the Sturtian glaciation, was long-lived, ending c. 660 Ma. Because the second and briefer late Cryogenian (i.e., Marinoan) glaciation is known to have initiated prior to 639 Ma and ended c. 635.5 Ma, the Cryogenian nonglacial interval must have been relatively short-lived (c. 20 Myr). Nevertheless, this interval is well represented on many cratons, due in part to the formation of widespread rift basins and passive margins as Rodinia began to break up. Although molecular clock and biomarker data suggest that the earliest animals had appeared by this time, no unambiguous metazoan fossils have been recovered from Cryogenian strata, which show low overall fossil diversity.
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Plate reorganization events involve fundamental changes in lithospheric plate-motions and can influence the lithosphere-mantle system as well as both ocean and atmospheric circulation through bathymetric and topographic changes. Here, we compile published data to interpret the geological record of the Neoproterozoic Arabian-Nubian Shield and integrate this with a full-plate tectonic reconstruction. Our model reveals a plate reorganization event in the late Tonian period about 720 million years ago that changed plate-movement directions in the Mozambique Ocean. After the reorganization, Neoproterozoic India moved towards both the African cratons and Australia-Mawson and instigated the future amalgamation of central Gondwana about 200 million years later. This plate kinematic change is coeval with the breakup of the core of Rodinia between Australia-Mawson and Laurentia and Kalahari and Congo. We suggest the plate reorganization event caused the long-term shift of continents to the southern hemisphere and created a pan-northern hemisphere ocean in the Ediacaran. A Proterozoic full-plate reconstruction reveals a late Tonian plate reorganization event that resulted in consumption of the Mozambique Ocean and the concentration of continents in the Southern Hemisphere.
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Otavi Group is a 1.5−3.5-km-thick epicontinental marine carbonate succession of Neoproterozoic age, exposed in an 800-km-long Ediacaran−Cambrian fold belt that rims the SW cape of Congo craton in northern Namibia. Along its southern margin, a contiguous distally tapered foreslope carbonate wedge of the same age is called Swakop Group. Swakop Group also occurs on the western cratonic margin, where a crustal-scale thrust cuts out the facies transition to the platformal Otavi Group. Subsidence accommodating Otavi Group resulted from S−N crustal stretching (770−655 Ma), followed by post-rift thermal subsidence (655−600 Ma). Rifting under southern Swakop Group continued until 650−635 Ma, culminating with breakup and a S-facing continental margin. No hint of a western margin is evident in Otavi Group, suggesting a transform margin to the west, kinematically consistent with S−N plate divergence. Rift related peralkaline igneous activity in southern Swakop Group occurred around 760 and 746 Ma, with several rift-related igneous centres undated. By comparison, western Swakop Group is impoverished in rift-related igneous rocks. Despite low paleoelevation and paleolatitude, Otavi and Swakop groups are everywhere imprinted by early and late Cryogenian glaciations, enabling unequivocal stratigraphic division into five epochs (period divisions): (1) non-glacial late Tonian, 770−717 Ma; (2) glacial early Cryogenian/Sturtian, 717−661 Ma; (3) non-glacial middle Cryogenian, 661−646±5 Ma; (4) glacial late Cryogenian/Marinoan, 646±5−635 Ma; and (5) non-glacial early Ediacaran, 635−600±5 Ma. Odd numbered epochs lack evident glacioeustatic fluctuation; even numbered ones were the Sturtian and Marinoan snowball Earths. This study aimed to deconstruct the carbonate succession for insights on the nature of Cryogenian glaciations. It focuses on the well-exposed southwestern apex of the arcuate fold belt, incorporating 585 measured sections (totaling >190 km of strata) and >8,764 pairs of δ13C/δ18Ocarb analyses (tabulated in Supplementary On-line Information). Each glaciation began and ended abruptly, and each was followed by anomalously thick ‘catch-up’ depositional sequences that filled accommodation space created by synglacial tectonic subsidence accompanied by very low average rates of sediment accumulation. Net subsidence was 38% larger on average for the younger glaciation, despite its 3.5−9.3-times shorter duration. Average accumulation rates were subequal, 4.0 vs 3.3−8.8 m Myr−1, despite syn-rift tectonics and topography during Sturtian glaciation, versus passive-margin subsidence during Marinoan. Sturtian deposits everywhere overlie an erosional disconformity or unconformity, with depocenters ≤1.6 km thick localized in subglacial rift basins, glacially carved bedrock troughs and moraine-like buildups. Sturtian deposits are dominated by massive diamictite, and the associated fine-grained laminated sediments appear to be local subglacial meltwater deposits, including a deep subglacial rift basin. No marine ice-grounding line is required in the 110 Sturtian measured sections in our survey. In contrast, the newly-opened southern foreslope was occupied by a Marinoan marine ice grounding zone, which became the dominant repository for glacial debris eroded from the upper foreslope and broad shallow troughs on the Otavi Group platform, which was glaciated but left nearly devoid of glacial deposits. On the distal foreslope, a distinct glacioeustatic falling-stand carbonate wedge is truncated upslope by a glacial disconformity that underlies the main lowstand grounding-zone wedge, which includes a proximal 0.60-km-high grounding-line moraine. Marinoan deposits are recessional overall, since all but the most distal overlie a glacial disconformity. The Marinoan glacial record is that of an early ice maximum and subsequent slow recession and aggradation, due to tectonic subsidence. Terminal deglaciation is recorded by a ferruginous drape of stratified diamictite, choked with ice-rafted debris, abruptly followed by a syndeglacial-postglacial cap-carbonate depositional sequence. Unlike its Sturtian counterpart, the post-Marinoan sequence has a well-developed basal transgressive (i.e., deepening-upward) cap dolomite (16.9 m regional average thickness, n=140) with idiosyncratic sedimentary features including sheet-crack marine cements, tubestone stromatolites and giant wave ripples. The overlying deeper-water calci-rhythmite includes crystal-fans of former aragonite benthic cement ≤90 m thick, localized in areas of steep sea-floor topography. Marinoan sequence stratigraphy is laid out over ≥0.6 km of paleobathymetric relief. Late Tonian shallow-neritic δ13Ccarb records were obtained from the 0.4-km-thick Devede Fm (~770−760 Ma) in Otavi Group and the 0.7-km-thick Ugab Subgroup (~737−717 Ma) in Swakop Group. Devede Fm is isotopically heavy, +4−8‰ VPDB, and could be correlative with Backlundtoppen Fm (NE Svalbard). Ugab Subgroup post-dates 746 Ma volcanics and shows two negative excursions bridged by heavy δ13C values. The negative excursions could be correlative with Russøya and Garvellach CIEs (carbon isotope excursions) in NE Laurentia. Middle Cryogenian neritic δ13C records from Otavi Group inner platform feature two heavy plateaus bracketed by three negative excursions, correlated with Twitya (NW Canada), Taishir (Mongolia) and Trezona (South Australia) CIEs. The same pattern is observed in carbonate turbidites in distal Swakop Group, with the sub-Marinoan falling stand wedge hosting the Trezona CIE recovery. Proximal Swakop Group strata equivalent to Taishir CIE and its subsequent heavy plateau are shifted bidirectionally to uniform values of +3.0−3.5‰. Early Ediacaran neritic δ13C records from Otavi Group inner platform display a deep negative excursion associated with the post-Marinoan depositional sequence and heavy values (≤+11‰) with extreme point-to-point variability (≤10‰) in the youngest Otavi Group formation. Distal Swakop Group mimics older parts of the early Ediacaran inner platform δ13C records, but after the post-Marinoan negative excursion, proximal Swakop Group values are shifted bidirectionally to +0.9±1.5‰. Destruction of positive and negative CIEs in proximal Swakop Group is tentatively attributed to early seawater-buffered diagenesis (dolomitization), driven by geothermal porewater convection that sucks seawater into the proximal foreslope of the platform. This hypothesis provocatively implies that CIEs originating in epi-platform waters and shed far downslope as turbidites are decoupled from open-ocean DIC (dissolved inorganic carbon), which is recorded by the altered proximal Swakop Group values closer to DIC of modern seawater. Carbonate sedimentation ended when the cratonic margins collided with and were overridden by the Atlantic coast-normal Northern Damara and coast-parallel Kaoko orogens at 0.60−0.58 Ga. A forebulge disconformity separates Otavi/Swakop Group from overlying foredeep clastics. In the cratonic cusp, where the orogens meet at a right angle, the forebulge disconformity has an astounding ≥1.85 km of megakarstic relief, and kmthick mass slides were displaced gravitationally toward both trenches, prior to orogenic shortening responsible for the craton-rimming fold belt.
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After tilt correction for Ediacaran thick-skinned folding, a pair of Cryogenian half grabens at the autochthonous southwest cape of Congo Craton (CC) in northwest Namibia restore to different orientations. Toekoms sub-basin trended east-northeast, parallel to Northern Zone (NZ) of Damara belt, and was bounded by a normal-sense growth fault (2 290 m throw) dipping 57° toward CC. Soutput sub-basin trended northwest, oblique to NZ and to north-northwest-trending Kaoko Belt. It was bounded by a growth fault (750 m down-dip throw) dipping steeply (~75°) toward CC. Soutput growth fault could be an oblique (splay) fault connecting a Cryogenian rift zone in NZ with a sinistral transform zone in Kaoko Belt. A transform origin for the Kaoko margin accords with its magma-poor abrupt shelf-to-basin change implying mechanical strength, unlike the magma-rich southern margin where a gradual shelf-to-basin change implies a mechanically weak extended margin. A rift−transform junction is kinematically compatible with observed north-northwest−south-southeast Cryogenian crustal stretching within CC. Post-rift subsidence of the CC carbonate platform varies strongly across the south-facing but not the west-facing shelf. A sheared western CC margin differs from existing Kaoko Belt models that posit orthogonal opening with hyper-extended continental crust. Carbonate-dominated sedimentation over southwest CC implies palaeolatitudes ≤35° between 770 and 600 Ma.
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The leucogranite-hosted uranium deposit in Namibia is a well-known intrusive uranium deposit in the world. In this paper, a systematic study of Sr-Nd isotopes has been carried out on uranium mineralized leucogranites in the Gaudeanmus area, Namibia. The results show that uranium bearing leucogranites were formed in the post-orogenic extensional environment, the ε Nd(t) values of the rock are from -13.5 to -17.4, with the initial ⁸⁷ Sr/ ⁸⁶ Sr ratios of 0.73035 ∼ 0.79345; and the ages of two-stage Nd model are between 2.32 ∼ 2.63 Ga. The Sr-Nd isotopic geochemical results of the pre-Damara basement, Damara sequence and uraniferous leucogranites indicate that the contribution of uranium rich pre-Damara basement to the ore-forming materials is dominant during the main metallogenic epoch of magmatic crystallization differentiation, while the extra uranium mineralization at the later hydrothermal stage may be from the primary uranium minerals such as uranite and coffinite.
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Detrital zircon provenance study of a metamorphosed sedimentary succession in the eastern part of the Kaoko Belt in Namibia has revealed two distinct sources for the Neoproterozoic sedimentation along the southwestern Congo Craton margin. The lower part of the succession shows detrital zircon ages consistent with erosion of Paleoproterozoic basement of the Congo Craton with an inferred Mesoproterozoic volcano-sedimentary cover. Within the middle part of the succession, which includes glaciogenic sediments correlated with the Sturtian (717–660 Ma) glaciation, the Mesoproterozoic zircon grains disappear and the signal is dominated by ages known from the Congo Craton basement. The sedimentation in these parts of the succession is interpreted as related to the early Neoproterozoic rifting.
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