Umkondo Large Igneous Province Research Articles

Placing constraints on the age and origin of basaltic dykes on the Ghaap Plateau, Griqualand West, South Africa

Abstract Well-preserved strata of the Ghaap Plateau along the western margin of the proto-Kalahari Craton have been intruded by a large number of mafic dykes. Owing to the scarcity of outcrops, these dykes are generally not well documented and intrusion ages remain poorly understood. In order to address this lack of knowledge, a total of seventeen Ghaap Plateau dykes that included seven north-northeast to northeast-trending dykes, eight northwest-trending dykes and two east to east-northeast-trending dykes were sampled for this study. Here, we report on the petrography and whole rock geochemistry of these dykes. We also report three new U-Pb baddeleyite ages along with some palaeomagnetic data. The mineralogy of the dykes is dominated by clinopyroxene and plagioclase showing varying degrees of alteration. Whole rock geochemical data identify the dykes as basalts with MORB, E-MORB or within plate-like signatures, suggesting an asthenospheric magma source. Our study indicates that the Ghaap Plateau dykes represent several magmatic events of markedly different age. Based on their whole rock geochemical composition, all but one of the dyke samples are tentatively arranged into four distinct groups (Groups A to D). Group A dykes (four dykes) are compositionally similar to the Umkondo Large Igneous Province (LIP). Two north-northeast-trending dykes of this Group yielded overlapping U-Pb baddeleyite ages of 1 103 ± 84 Ma and 1 112 ± 55 Ma, thereby adding to the known footprint of the Umkondo LIP along the western margin of the proto-Kalahari craton. In Group B (four dykes), one northeast-trending dyke yielded a U-Pb baddeleyite age of 1 929 ± 17 Ma. Its palaeomagnetic signature (Lat. 24.5°, Long. 317.6°, A95= 14.20°) is supported by a positive baked contact test and agrees well with the ~1.93 Ga Hartley LIP palaeopole. Group B dykes are thus interpreted to be related to the Hartley LIP, with our new 1 929 ± 17 Ma age supporting a wider extent for the Hartley magmatism than previously known. A new palaeopole for the Hartley LIP (Lat. 22.3°, Long. 327.4°, A 95 = 9.25°) was calculated by combining the data of our 1 929 Ma dyke with data reported previously from other sites. Whole rock geochemical data for the remaining dykes are consistent with the Karoo magmatism (two east to east-northeast-trending dykes of Group D) or the ~2.4 Ga Ongeluk LIP (north-trending dyke). The final four dykes (Group C) are difficult to match with other known mafic units. Precise geochronology is needed to confirm the exact age of these unassigned dykes.

Major, trace element and Sr-Nd isotope evidence for a sublithospheric mantle source for the Umkondo large igneous province

The Mesoproterozoic (1.11 Ga) Umkondo large igneous province (LIP) in southern Africa and Antarctica was emplaced in < 5 Myr and is dominated by low-Ti tholeiitic doleritic-gabbroic sills. It is of particular interest because it is the least studied LIP in southern Africa with both sublithospheric and lithospheric mantle sources proposed and it coincides with the early assembly of Rodinia, so it has importance in understanding the nature of magmatism and tectonics in and around the Kalahari craton during the Mesoproterozoic. In this study, we compiled a large database of existing (∼750) and new (∼100) major and trace element data for the Umkondo province, as well as 42 new Sr-Nd isotopic measurements, to provide constraints on its magma sources and geochemical evolution. Major element compositional variations in the low-Ti tholeiites are explained by low-pressure (1 kbar) three-phase fractional crystallisation (olivine, clinopyroxene and plagioclase) of a parent magma with ∼ 10 wt.% MgO in oxidising conditions (QFM + 1). Inverse models show that the low-Ti tholeiitic magmas were derived as residual melts after the crystallization of 12%–33% olivine from primary komatiitic-basaltic magmas (up to ∼ 20 wt.% MgO) in equilibrium with mantle olivine (Fo90). Low Sm/Yb and TiO2/Yb-Nb/Yb indicate that the primary magmas were derived by 2%–20% shallow (40–50 km) partial melting of spinel lherzolite. High Sm/Yb is restricted to dyke swarms and may imply limited magma production from deeper (up to ∼ 70 km) garnet lherzolite-like sources. The low-Ti tholeiites of the Umkondo province are enriched in large ion lithophile elements (Rb-Sr-Cs-K) and depleted in high-field strength elements (Zr-Hf-Nb-Ta), indicating the involvement of crustal material and/or the subcontinental lithospheric mantle. This is supported by covariations in Th/Nb, Nb/Yb, Nb/La and Ce/Sm with generally negative ΔNb. Sr-Nd isotopes lend support to the notion that the Umkondo magmas were derived from depleted and/or enriched sublithospheric mantle sources and subsequently contaminated by enriched lithospheric material during emplacement (initial (at 1.11 Ga) 87Sr/86Sr between 0.704820 and 0.737464 and εNd between −8.9 and +5.3). The Vredefort sills are significant as they display the most depleted Sr-Nd isotopic signatures (average initial 87Sr/86Sr of 0.705342 and average εNd of 0.4) and are the least contaminated magma suite in the Umkondo province. Because of (i) the large volume of low-Ti magmas, (ii) evidence of a primary hot and MgO-rich (komatiitic) magma, and (iii) the short duration of magmatism, we suggest that the Umkondo province was formed by plume-induced melting of the sublithospheric mantle beneath the Kalahari craton in an extensional setting. This contrasts with previous suggestions that the heat source developed in response to the “thermal insulation” of the mantle beneath a thickened Kalahari craton in the absence of a mantle plume. There is further evidence from the elevated Zn/Fe that the sublithospheric mantle was lithologically heterogeneous and consisted of mixed peridotite and pyroxenite domains. There is a general lack of ultramafic cumulates in the low-Ti magma suite that may imply there was deeper ponding and storage of the primary magmas that fractionated large quantities of ultramafic rocks. There is also a paucity of high-Ti rocks in the Umkondo province that may reflect limited direct melting of the lithospheric mantle or that they are simply not as well-preserved in this province compared to the Karoo province. The similar trace element and Sr-Nd isotopic compositions of the Umkondo sills in southern Africa with the Borgmassivet sills in Antarctica support the concept that the Kalahari craton and Grunehogna terrane were adjoined at 1.11 Ga. The timing of the Umkondo province indicates there was localised lithospheric extension and upwelling asthenospheric mantle during a time of dominantly compressional tectonics on Earth at the end of the ‘boring billion’.

Latest Mesoproterozoic (ca. 1.2–1.1 Ga) amphibolite-facies metamorphism from the Dete-Kamativi Inlier, NW Zimbabwe: Implications for a Rodinia-related intracratonic orogen in Southern Africa

The Magondi Belt has been regarded as a Paleoproterozoic orogen formed by the collision of the Zimbabwe Craton and an unknown continental block. The Dete-Kamativi Inlier (DKI) located approximately 200 km west-southwest of the main Magondi Belt has been regarded as an extension of the belt. Here, we report new geochronological data for pelitic schists and a felsic orthogneiss from the DKI using monazites (CHIME method) and zircons (LA-ICP-MS analysis), and discuss the tectonic evolution of the region. Zircons from a felsic orthogneiss in the Kamativi area yielded magmatic and metamorphic ages of 2279 ± 25 Ma and 2020 ± 28 Ma, respectively. Similar Paleoproterozoic ages of ca. 2.1–1.8 Ga were also obtained from subhedral and rounded monazite grains in the pelitic schists. In contrast, irregular-shaped monazite intergrown with biotite in a different pelitic schist gave three latest Mesoproterozoic isochron ages of 1196 ± 37 Ma, 1143 ± 32 Ma, and 1070 ± 25 Ma, suggesting a long-lived (>120 million years) thermal event with several monazite-growing stages. Consistent isochron ages of 1062 ± 41 Ma and 1061 ± 26 Ma were obtained from monazites in the felsic orthogneiss and metapelite samples from an adjacent region. The application of phase equilibrium modeling for the peak mineral assemblage in the garnet-andalusite-biotite-cordierite-bearing pelitic schist with 1196–1070 Ma metamorphic ages indicated a peak P–T condition of 520–600 °C and 1.5–2.5 kbar, suggesting low-pressure amphibolite-facies metamorphism. The condition is lower than that obtained from the hornblende-plagioclase geothermometry of amphibolites (>700 °C) from the southwest part of the DKI, which probably corresponds to an earlier (ca. 2.0 Ga) high-grade metamorphic condition. The youngest thermal event, 994–982 Ma, from monazite rim in a mylonitic orthogneiss might correspond to the timing of later deformation. The latest Mesoproterozoic (1.2–1.1 Ga) amphibolite-facies metamorphism was likely associated with an intracratonic orogeny related to the activity of broadly coeval orogenic events, such as the Namaqua-Natal orogenic belt related to the amalgamation of the Rodinia supercontinent. Regional magmatic activity of the Umkondo large igneous province at 1112–1108 Ma could have also been an additional heat source.

Geochemical characterization of a reconstructed 1110 Ma Large Igneous Province

1110 Ma Large Igneous Province (LIP) fragments in the Kalahari craton, southern Africa (Umkondo LIP); Dronning Maud Land, Antarctica; Bundelkhand portion of Indian craton (Mahoba dolerite dykes); Congo craton (Huila-Epembe dolerite dykes); and Amazonia (Rincon del Tigre-Huanchaca LIP) have been reconstructed as a single LIP with plume centre beneath the NW part of the Kalahari Craton. This paper offers the best estimates for the paleoposition of the Indian and Amazonian cratons along with conjoined Kalahari-SF/Congo reconstruction. This 1110 Ma mafic magmatism is dominantly tholeiitic, ranging from basalt to andesitic basalt in composition, generated over a range of mantle melting depths [(Gd/Yb)N = 1.2–2.3], exhibit low to high contamination with crustal components (negative Nb anomalies, eNd (0 to −12), and elevated Th/Yb). The data fall into two Groups based on TiO2 content, with Group 1 (low Ti) of andesitic basalt composition, and Group 2 (high Ti) exhibiting a basaltic affinity. Group 1 magmas were generated in the spinel lherzolite field followed by significant contamination likely during passage through metasomatised lithospheric mantle in crustal magma chambers. A greater melting depth reaching into the garnet lherzolite field is proposed for the Group 2 magmas. The majority of Group 2 units are located in the Kalahari and Congo cratons, and this reflects onset of deeper melting closer to the interpreted plume axis in our reconstruction.

Archaean and Proterozoic diamond growth from contrasting styles of large-scale magmatism

Precise dating of diamond growth is required to understand the interior workings of the early Earth and the deep carbon cycle. Here we report Sm-Nd isotope data from 26 individual garnet inclusions from 26 harzburgitic diamonds from Venetia, South Africa. Garnet inclusions and host diamonds comprise two compositional suites formed under markedly different conditions and define two isochrons, one Archaean (2.95 Ga) and one Proterozoic (1.15 Ga). The Archaean diamond suite formed from relatively cool fluid-dominated metasomatism during rifting of the southern shelf of the Zimbabwe Craton. The 1.8 billion years younger Proterozoic diamond suite formed by melt-dominated metasomatism related to the 1.1 Ga Umkondo Large Igneous Province. The results demonstrate that resolving the time of diamond growth events requires dating of individual inclusions, and that there was a major change in the magmatic processes responsible for harzburgitic diamond formation beneath Venetia from the Archaean to the Proterozoic.

The geology and geochemistry of the Straumsnutane Formation, Straumsnutane, western Dronning Maud Land, Antarctica and its tectonic setting on the western margin of the Kalahari Craton: additional evidence linking it to the Umkondo Large Igneous Province

Abstract The study focuses on whole-rock major and trace element chemistry, as well as radiogenic isotope data from the Straumsnutane Formation lavas in western Dronning Maud Land, Antarctica. The data are compared with those from the Espungabera Formation lavas of central Mozambique, published data from the Borgmassivet intrusions in Dronning Maud Land, Antarctica and other intrusions in southern Africa which are correlated with the approximately 1100 Ma Umkondo Igneous Province. Petrographical studies indicate that the Straumsnutane lavas are dominated by plagioclase, clinopyroxene, amphibole and Fe–Ti oxides. Secondary mineral assemblages include chlorite, pumpellyite, white mica and epidote, indicating that the Straumsnutane lavas have been metamorphosed under low-grade greenschist-facies conditions followed by retrograde prehnite–pumpellyite-facies conditions. The chemical data for the Straumsnutane Formation lavas are dominantly tholeiitic and basaltic andesitic in composition, and indicate that they are of continental origin. Trace element ratio values for the Straumsnutane lavas suggest that fractional crystallization and/or crustal contamination have been significant processes in the magma evolution. Low to high 87 Sr/ 86 Sr isotopic ratios (0.682–0.720) are evident from the Straumsnutane lavas suggesting varying degrees of hydrothermal alteration/low-grade metamorphism. The calculated 87 Sr/ 86 Sr values and the negative ε Nd values at 1100 Ma suggest contamination by older continental crust during the genesis of the Straumsnutane Formation lavas. Isotopic modelling shows that the Straumsnutane lavas may have been formed from the mixing of a mid-ocean ridge basalt (MORB)-like source with approximately 4% of older crust similar to the Messica Granite Gneiss of central Mozambique. Comparison of the geochemical data and petrography of the Straumsnutane lavas with those of the Espungabera Formation lavas of central Mozambique shows that they are similar. Additional comparisons show that the Straumsnutane lavas are geochemically similar to rock units of the Umkondo Igneous Province in southern Africa. It is therefore concluded that the Straumsnutane Formation lavas also formed part of the Umkondo Igneous Province.

Using palaeomagnetism to determine late Mesoproterozoic palaeogeographic history and tectonic relations of the Sinclair terrane, Namaqua orogen, Namibia

Abstract The Sinclair terrane is an important part of the Namaqua orogenic province in southern Namibia containing well-preserved Mesoproterozoic volcano-sedimentary successions suitable for palaeomagnetic and geochronological studies. The Guperas Formation in the upper part of the Sinclair stratigraphic assemblage contains both volcanic and sedimentary rocks cut by a bimodal dyke swarm with felsic members dated herein by U–Pb on zircon at c. 1105 Ma. Guperas igneous rocks yield a pre-fold direction and palaeomagnetic pole similar to that previously reported. Guperas sedimentary rocks yield positive conglomerate and fold tests, with a maximum concentration of characteristic remanence directions at 100% untilting. The combined Guperas data generate a palaeomagnetic pole of 69.8° N, 004.1° E ( A 95 =7.4°, N= 9). The 1105 Ma post-Guperas dykes yield stable remanence directions with positive baked-contact tests and a palaeomagnetic pole at 62.3° N, 031.9° E ( A 95 =6.9°, N= 26), which is coincident with that of the Kalahari-wide Umkondo large igneous province, demonstrating tectonic coherence of the Sinclair terrane with the Kalahari craton at the time of dyke emplacement. These results show that palaeomagnetic and geochronological studies of the Sinclair terrane can provide kinematic constraints on the tectonic evolution of the Namaqua–Natal–Maud orogenic belt and its role in the formation of the Rodinia supercontinent.

A new grand mean palaeomagnetic pole for the 1.11 Ga Umkondo large igneous province with implications for palaeogeography and the geomagnetic field

© The Authors 2015. We present a new grand mean palaeomagnetic pole (Plong: 222.1°, Plat: -64.0°, A95: 2.6°, N = 49) for the ca. 1110 Ma Umkondo large igneous province (LIP) of the Kalahari Craton. Newpalaeomagnetic data from 24 sills in Botswana and compiled reprocessed existing data are used to develop a palaeomagnetic pole as the Fisher mean of cooling unit virtual geomagnetic poles (VGPs). The mean and its associated uncertainty provide the best-constrained pole yet developed for the province. Comparing data from individual cooling units allows for evaluation of palaeosecular variation at this time in the Mesoproterozoic. The elongation of the population of VGPs is consistent with that predicted by the TK03.GAD model lending support to the dipolar nature of the field in the late Mesoproterozoic. In our new compilation, 4 of 59 (~7 per cent) of the igneous units have northerly declinations while the rest are southdirected indicating that a geomagnetic reversal occurred duringmagmatic activity. Interpreting which of these polarities corresponds with a normal or reversed geomagnetic field relative to other continents can constrain the relative orientations between cratons with time-equivalent data. This interpretation is particularly important in comparison to Laurentia as it bears on Kalahari's involvement and position in the supercontinent Rodinia. The dominance of southdirected declinations within the Umkondo Province was previously used to suggest that these directions are the same polarity as reversed directions from the early magmatic stage of the Keweenawan Midcontinent Rift of Laurentia. Two Umkondo sills with northerly declinations have U-Pb baddeleyite ages of ca. 1109 Ma that are temporally close to dated Midcontinent Rift units having reversed directions. Based on this comparison, and palaeomagnetic data from younger units in the Kalahari Craton, we favour the option in which the sites with northerly declinations from the Umkondo Province correspond to the reversed polarity directions from the early magmatic stage in the Midcontinent Rift. This interpretation allows for the Namaqua-Natal metamorphic belt of Kalahari to be a conjugate to the Grenville margin of North America and for Kalahari to have become conjoined with Laurentia within the supercontinent Rodinia subsequent to Umkondo LIP magmatic activity.

Palaeomagnetic and geochronological data from Late Mesoproterozoic redbed sedimentary rocks on the western margin of Kalahari craton

Abstract Redbeds of the Aubures Formation constitute the uppermost stratigraphic unit in the Mesoproterozoic Sinclair succession of southern Namibia. Aubures palaeomagnetic remanence vectors, held almost exclusively by hematite, document at least one geomagnetic polarity reversal in the stratigraphy, a positive intraformational conglomerate test indicating primary magnetization and greatest concentration of characteristic directions at 50–60% untilting, indicating that deformation was coincident with sedimentation. The new Aubures palaeomagnetic pole, at 56.4°N and 018.0°E with A 95 =11.3°, is located on the apparent polar wander path of the Kalahari craton, between poles of the 1110 Ma Umkondo igneous event and the c. 1090 Ma Kalkpunt redbeds of the Koras Group near Upington, South Africa. This distinctive concordance suggests that Aubures sediments have an age of approximately 1100 Ma, that the Sinclair region was probably part of Kalahari at that time and that the Aubures and Kalkpunt redbeds are broadly correlative. New laser-ablation inductively coupled plasma mass spectrometry detrital zircon results from the Aubures Formation, including a youngest age component (1108±9 Ma) that is coincident with the Kalahari-wide Umkondo large igneous province, conform to this interpretation. Palaeomagnetism and geochronology of the Sinclair succession can provide kinematic constraints on the tectonic evolution of Kalahari as it approached other cratons in the growing Rodinia supercontinent.

Full vector low‐temperature magnetic measurements of geologic materials

AbstractThe magnetic properties of geologic materials offer insights into an enormous range of important geophysical phenomena ranging from inner core dynamics to paleoclimate. Often it is the low‐temperature behavior (&lt;300 K) of magnetic minerals that provides the most useful and highest sensitivity information for a given problem. Conventional measurements of low‐temperature remanence are typically conducted on instruments that are limited to measuring one single‐axis component of the magnetization vector and are optimized for measurements in strong fields. These instrumental limitations have prevented fully optimized applications and have motivated the development of a low‐temperature probe that can be used for low‐temperature remanence measurements between 17 and 300 K along three orthogonal axes using a standard 2G Enterprises SQuID rock magnetometer. In this contribution, we describe the design and implementation of this instrument and present data from five case studies that demonstrate the probe's considerable potential for future research: a polycrystalline hematite sample, a polycrystalline hematite and magnetite mixture, a single crystal of magnetite, a single crystal of pyrrhotite, and samples of Umkondo Large Igneous Province diabase sills.

The geology and geochemistry of the Espungabera Formation of central Mozambique and its tectonic setting on the eastern margin of the Kalahari Craton

Whole rock major and trace element chemistry as well as radiogenic isotope data from the Espungabera Formation of central Mozambique are compared with published data from the Umkondo Formation lavas in SE Zimbabwe and Straumsnutane Formation lavas in western Dronning Maud Land, Antarctica. These formations form part of the ∼1100Ma Umkondo Igneous Province in southern Africa and are now preserved on the Grunehogna (in Antarctica) and Zimbabwe (in Zimbabwe) Cratons.The chemical data indicate that the Espungabera Formation lavas are dominantly tholeiitic and basaltic to basaltic andesitic in composition. The Espungabera lavas are dominated by plagioclase, clinopyroxene and Fe–Ti oxides. Metamorphic mineral assemblages indicate the lavas have been metamorphosed under mid-greenschist facies on a retrograde path to prehnite-pumpellyite facies conditions. The decrease in FeOt with increasing MgO content in the Espungabera lavas and the slight decrease in TiO2 with increasing MgO indicates fractionation of Fe–Ti oxides. The lavas are characterised by negative Nb anomalies; enriched LILE’s and high 87Sr/86Sr isotopic ratios. The 87Sr/86Sr data calculated at 1100Ma suggest contamination by continental crust during the petrogenesis of the lavas. The Espungabera volcanics have negative εNd values (−2.83 to −3.49) also suggesting that the magma was contaminated by older crust.Comparison of the chemical data from the Espungabera Formation with data from the Umkondo Group basalts from SE Zimbabwe and the Straumsnutane Formation lavas from Dronning Maud Land, Antarctica shows that they are similar. These similarities, along with similarities in the available geochronological data suggest that these rocks are comagmatic. Both units are also geochemically similar to some rock units that form part of the Umkondo Large Igneous Province (i.e. Zimbabwe basalts that were regarded as Umkondo basalts by Munyanyiwa (1999), Waterberg sills, Umkondo sills and Type III Mutare and Guruve dykes identified by Ward (2002)), and therefore we conclude that the Espungabera lavas in Mozambique also form part of the Umkondo Igneous Province.The craton-based tholeiitic Umkondo Igneous Province is broadly coeval with tonalitic calc-alkaline and granitic gneisses in the Nampula and Maud Terranes in Mozambique and Antarctica respectively, immediately east of the Kalahari Craton in a reconstructed Gondwana. These data can be interpreted to indicate that the Espungabera and Straumsnutane lavas form part of a back-arc complex, west of a volcanic arc/subduction zone along the eastern margin of the Kalahari Craton at ∼1100Ma.

Dykes of the 1.11 Ga Umkondo LIP, Southern Africa: Clues to a complex plumbing system

The Umkondo Large Igneous Province (LIP) is represented by widespread (∼2.0×106km2) mafic intrusions that were rapidly emplaced (1112–1108Ma) into the Kalahari craton of southern Africa and the formerly adjacent Grunehogna Province of Antarctica during Rodinia assembly. Very few Umkondo-aged dykes have been identified before, resulting in a poor understanding of this LIP's plumbing system and origin. Here we report six new ∼1110Ma U–Pb TIMS baddeleyite ages for various dolerite dykes, which, when coupled with geochemistry from some of the dykes, suggest association with the Umkondo LIP. The distribution of dykes defines distinct radiating swarms, which locate two separate magmatic centers on the northern margin of the Kalahari craton, and a third less robust center on the SE margin. The Umkondo intrusions’ geochemistry indicates significant partial melting of the sub-continental mantle lithosphere (SCLM) and requires a transient thermal anomaly in the mantle. A viable model sees a mantle plume ascend beneath the craton and split into different portions that moved and ascended to different lithospheric thin-spots along the margins of the craton. As an alternative, the rise in mantle temperature associated with continental aggregation at this time is considered sufficient to cause partial melting of the SCLM without any plume involvement. Specific features of the Umkondo LIP's plumbing system are supportive of either model, and an approach of multiple working hypotheses is recommended.

Reconnaissance paleomagnetic studies of Mesoproterozoic alkaline igneous complexes in the Kaapvaal craton, South Africa

We report paleomagnetic data from three different parts of a diffuse Mesoproterozoic alkaline igneous province in the eastern part of the Kaapvaal craton in South Africa. Except for the ∼1.3Ga Pilanesberg dikes, these rocks have not previously been studied paleomagnetically. Isotopic dates from igneous rocks in the province generally indicate emplacement between 1.4 and 1.35Ga, which falls in a time frame where the apparent polar wander path (APWP) for the Kaapvaal craton is poorly known. In order to help address this problem, we report paleomagnetic data for syenitic rocks in the large Pilanesberg Complex near the western limit of the alkaline province, two separate syenitic intrusions in the Pienaars River Complexes near Pretoria, and the Spitskop ijolite–nepheline syenite–carbonatite complex near the eastern limit of the province. These intrusive units yield poles that allow a tentative extension of the Mesoproterozoic Kaapvaal APWP back to 1.4Ga. The data suggest a pronounced bend or cusp in the APWP at 1.1Ga, which may record a major change in plate motion occurring at the same time as emplacement of the Umkondo large igneous province in southern Africa.

Mesoproterozoic intraplate magmatic ‘barcode’ record of the Angola portion of the Congo Craton: Newly dated magmatic events at 1505 and 1110 Ma and implications for Nuna (Columbia) supercontinent reconstructions

In the Angola portion of the Congo Craton, the only Proterozoic large igneous province (LIP) dated prior to this study was the 1380–1370Ma (Kunene Intrusive Complex and related units). U–Pb TIMS ages on baddeleyite from dolerite sills and gabbro-noritic dykes, has revealed two additional Mesoproterozoic intraplate events: at ca. 1505 and ca. 1110Ma, that are each proposed to be part of the plumbing system for LIPs. The identification of these three Mesoproterozoic magmatic events (ca. 1505, 1380, and 1110Ma) represent an initial magmatic ‘barcode’ for this portion of Congo Craton (and formerly connected São Francisco Craton), which can be compared with the magmatic ‘barcode’ record of other blocks to identify former nearest neighbors in the Precambrian supercontinent Nuna (also known as Columbia).Specifically, a 1502±5Ma U–Pb TIMS baddeleyite age has been obtained for the prominent Humpata dolerite sill which is part of a wider sill province in SW Angola portion of the Congo Craton. The combined presence of both 1505Ma and 1380Ma magmatism in the Congo–São Francisco reconstructed craton is a match with similar ages published for two intraplate magmatic provinces in northern Siberia and suggests a nearest-neighbor relationship in the supercontinent Nuna in which northern Siberia is juxtaposed adjacent to the western São Francisco portion of the reconstructed São Francisco–Congo Craton.In addition, a precise U–Pb TIMS baddeleyite age of 1110±2.5Ma was obtained for a prominent NNW–NNE trending gabbro-noritic (GN) dyke swarm in southeastern Angola, but this age is currently unknown in Siberia suggesting that the breakup of Congo–São Francisco Craton from Siberia happened earlier, perhaps in association with the 1380Ma event. This 1110Ma age is however, a precise match with that of the Umkondo large igneous province (LIP) of the Kalahari Craton, and also with mafic intraplate magmatism on other blocks such as the Bundelkhand Craton (India) and the Amazonian Craton. We provisionally consider these three cratons to have been nearest neighbors to the Congo–São Francisco Craton at this time and to have shared this 1110Ma magmatic event as a LIP node. There is also an age match with the early part of the Keweenawan event (in the interior of the Laurentia); however, on previously discussed paleomagnetic grounds the Keweenawan event is likely to have been distant and unrelated (and on the other side of the Grenville orogen).

Geochemistry and petrogenesis of mafic sills in the 1.1 Ga Umkondo large igneous province, southern Africa

The detailed petrogenesis of mafic sills occurring throughout southern Africa provides strong support for the development of an Umkondo large igneous province on the eastern margin of the Kalahari craton at 1.1Ga. The sills are most extensively developed in the Waterberg and Middelburg basins in northern South Africa and south-eastern Botswana. They are typical fractionated continental tholeiites with subophitic to ophitic dolerites, gabbros and gabbro-norites, and largely basaltic andesite in composition. The vast majority of the sample set defines one major geochemical subgroup, here referred to as the Mesoproterozoic Post-Waterberg sills A (MPWA sills), which is characteristically LREE enriched with relatively unfractionated HREEs, and with normalised incompatible element profiles similar to modern island arc andesites. A small number from the sample set define a minor subgroup (MPWB sills), which has so far only been recognised in the Middelburg basin, South Africa and which is characterized by fractionated HREEs. Both the major and trace element geochemical signatures of the MPWA sills are indistinguishable from the type Umkondo sills and less common lavas documented from Eastern Zimbabwe and mafic sills on the Grunehogna craton in present day Eastern Antarctica. This provides strong supporting evidence for an Umkondo large igneous province developed on the Kalahari craton at 1.1Ga. Despite crustal-type Sr–Nd isotopic signatures in the MPWA sills, bulk contamination by the continental crust is ruled out in favour of derivation from a primitive mantle-like asthenospheric source with a contribution from the subcontinental lithospheric mantle modified by a previous subduction event. The smaller MPWB magma type could represent a smaller degree melt at greater depth from a modified MORB-like source, although the relationship between the two subgroups remains unclear.

Coats Land crustal block, East Antarctica: A tectonic tracer for Laurentia?

Undeformed rhyolite and granophyre in the Coats Land crustal block of East Antarctica, dated as 1112 ± 4 Ma, are identical in age to both the Umkondo large igneous province (LIP) of the Kalahari craton (southern Africa) and the early Keweenawan LIP of Laurentia (North America). Although marine and satellite data demonstrate that Coats Land was close to Kalahari within the Gondwana supercontinent, the Coats Land rocks yield Pb isotope compositions strikingly distinct from those of the Umkondo province, yet indistinguishable from rocks of the Keweenawan province. The anorogenic Red Bluff granitic suite, along the present-day southern Laurentian margin in the Franklin Mountains (Texas, USA), is of comparable age, general rock type, and Pb isotope composition to rocks of Coats Land and may provide a piercing point for a Coats Land–Laurentia link. Paleomagnetic poles permit the Coats Land block to be close to this part of Laurentia ca. 1100 Ma and allow juxtaposition of Kalahari and southern Laurentia ca. 1000 Ma. The Coats Land crustal block may therefore be a critical tectonic tracer for placing Laurentia within late Mesoproterozoic and Neoproterozoic paleogeographic reconstructions. If this hypothesis is correct, Laurentia collided with the Kalahari craton along Antarctica's Maud orogen, which would represent a continuation of the ca. 1000 Ma Grenville orogen of eastern and southern Laurentia.

Baddeleyite geochronology and geochemistry of mafic cobbles from the Dwyka diamictite: New insights into the sub-Kalahari basement, South Africa

U–Pb baddeleyite geochronology and geochemistry of mafic cobbles from the ~ 300 Ma Dwyka diamictite on the western boundary of the Kalahari Basin provide new insights into the sub-Kalahari basement. Palaeo-ice flow directions indicate that the origin of the Dwyka diamictite deposits on the border between South Africa and Namibia is from beneath the Kalahari sands to the east or north-east. This work shows that the mafic cobbles from three localities near Rietfontein come from intrusions related to the Umkondo Large Igneous Province (LIP), located in the Kheis or Rehoboth Provinces. The lack of Banded Iron Formation (BIF) cobbles in these glacial diamictites indicates that their source region lies west or north of the BIF-dominated Griqualand West Basin of the Kaapvaal Craton. The four dated mafic cobbles give U–Pb ages of 1111–1123 Ma, coeval with dolerites of the Umkondo LIP. The cobbles experienced greenschist facies metamorphism before the Dwyka glaciation, but are unfoliated. The geochemistry of the cobbles indicates that they are genetically similar and have a within-plate setting. They show many similarities to the Umkondo LIP dolerites and the coeval Rouxville Formation, an upper unit of the Koras Group. The most likely sources of the cobbles are the mafic intrusives and lavas along the Kalahari Line, southern equivalents of the ~ 1.11 Ga Tshane and Xade complexes or intrusive and extrusive mafic units related to the Umkondo LIP, now concealed beneath the Kalahari sands.

Evidence from Dwyka tillite cobbles of Archaean basement beneath the Kalahari sands of southern Africa

We have found a new source of information about what lies beneath the Kalahari sands. The Kheis and Rehoboth Provinces of southern Africa were thought to be underlain by either an ~1800Ma orogenic belt, or a northern branch of the ~1200Ma Namaqua–Natal Province. Glacial diamictites of the Permocarboniferous Dwyka Group exposed at Rietfontein west of the Kalahari sands carry cobbles plucked from the bedrock by the ice sheet which covered the Gondwana supercontinent about 300Ma ago. Despite altered mineralogy, the cobbles are not severely altered geochemically. Their normative mineral compositions give classifications as trondhjemites and granites, supported by rare earth element diagrams. Microbeam U–Pb zircon dating of the granitic cobbles shows that they contain no evidence of crustal growth or orogeny at either 1800 or 1200Ma. Rather they testify to the presence of 2500 to 2900Ma Archaean trondhjemitic and granitic crust beneath the Kalahari, with a lesser ~2050Ma granite component. The pebble assemblages from the diamictites we sampled lack the diagnostic banded iron formation (BIF), stromatolitic limestone and other supracrustal pebbles which characterise diamictites derived from the Kaapvaal Craton, thus we envisage shorter transport distances and derivation from the region now beneath the Kalahari sands. Three of the Archaean granite cobbles have unusual less-than-mantle zircon oxygen isotope values around +3 (δ18O VSMOW), which may reflect interaction of their source with high-temperature, originally meteoric water before melting to produce the granites. The mafic cobbles described in a companion paper are much younger and are related to intrusions of the 1.1Ga Umkondo Large Igneous Province, probably located on the Kalahari Line or Rehoboth Province.Five trondhjemitic granites from the westernmost outcrops of the Kaapvaal Craton were dated, the oldest being 3061±9Ma and four others between 2882±7Ma and 2854±7Ma, reflecting the cratonisation of the Kimberley Terrane. Four of the Archaean Dwyka cobbles we dated are younger than the 2.7Ga Kaapvaal cover sequence and are thus too young to be derived from the craton.All the Dwyka cobbles described here are most likely derived from either the Rehoboth Province or the Kalahari Line with origins from the Kheis Province, Kaapvaal Craton, or further afield considered unlikely. We envisage the Rehoboth Province to consist of an Archaean core supplemented by Palaeoproterozoic granitoids, which was joined to the Kaapvaal Craton at an early stage of crustal development and played an important role during later tectonic events. This has important implications not only for the tectonic framework and assembly of Southern Africa, but also for exploration for diamonds and other ore deposits.

Lithospheric mantle evolution monitored by overlapping large igneous provinces: Case study in southern Africa

Most of the studies on the large igneous provinces (LIPs) focus on Phanerozoic times, and in particular, those related to the disruption of Pangea (e.g. CAMP, Karoo, Parana–Etendeka) while Precambrian LIPs (e.g. Ventersdorpf, Fortescue) remain less studied. Although the investigation of Precambrian LIPs is difficult because they are relatively poorly preserved, assessment of their geochemical characteristics in parallel with younger overlapping LIP is fundamental for monitoring the evolution of the mantle composition through time. Recent 40Ar/ 39Ar dating of the Okavango giant dyke swarm (and related sills) in southern Africa showed that ~ 90% of the dykes were emplaced at 179 ± 1 Ma and belong to the Karoo large igneous province whereas ~ 10% of dykes yielded Proterozoic ages (~ 1–1.1 Ga). Here, we provide new major, trace and rare earth elements analyses of the low-Ti Proterozoic Okavango dyke swarm (PODS) that suggest, combined with age data, a cognate origin with the 1.1 Ga Umkondo large igneous province (UIP), southern Africa. The geochemical characteristics of the PODS and UIP basalts are comparable to those of overlapping low-Ti Karoo basalts, and suggest that both LIPs were derived from similar enriched mantle sources. A mantle plume origin for these LIPs is not easily reconciled with the geochemical dataset and the coincidence of two compositionally similar mantle plumes acting 900 Myr apart is unlikely. Instead, we propose that the Umkondo and Karoo large igneous provinces monitored the slight evolution of a shallow enriched lithospheric mantle from Proterozoic to Jurassic.

Paleomagnetic and rock magnetic study of the Vredefort impact structure and the Johannesburg Dome, Kaapvaal Craton, South Africa—Implications for the apparent polar wander path of the Kaapvaal Craton during the Mesoproterozoic

New paleomagnetic and rock magnetic results are presented for various pre-, syn- and post-impact lithologies from the region of the 2023 ± 4 Ma, originally ca. 250 km diameter, Vredefort impact structure, on the Kaapvaal Craton of South Africa. After removal of a viscous remanent magnetization (VRM) component, the characteristic remanent magnetization (ChRM) component from 16 Vredefort Granophyre and pseudotachylitic (PT) breccia samples ( D = 18.3°, I = 54.8°, α 95 = 8.1°) was isolated and a paleomagnetic pole at 25.1°N and 43.5°E ( A 95 = 10.6°) obtained. Since, such a ChRM component was isolated also from the Archean Basement rocks of the Vredefort Dome, we consider its nature as a primary component of the impactites proven. The pole falls onto the Paleoproterozoic part of the apparent polar wander path of the Kaapvaal Craton. Rock magnetic analysis and scanning electron microscopy (SEM) revealed that the pseudotachylitic breccia and Granophyre samples contain two distinct magnetite phases: one ultra-small (micrometer-size), and one altered and larger (>50 μm) phase. The ultra-small magnetite is interpreted as the carrier of the ChRM and the larger grains as the carriers of the VRM component. Additionally, rock magnetic and petrophysical data reveal unusually high Koenigsberger ratios ( Q values) in all pre-impact lithologies, in some Vredefort impactite samples, and in the much younger (1.1 Ga) Anna's Rust Sheet (ARS) gabbro samples. As the high Q values, which had also been reported by previous studies of Vredefort lithologies, are now also seen in samples from the Johannesburg Dome, a direct link to the Vredefort impact can be ruled out. This is also supported by the hysteresis data of this study. As the observed magnetization is rather hard and shows multiple components of remanent magnetization, we exclude lightning as a cause for all observed high Q values (except in case of ARS gabbros). It is instead suggested that the cause of the high Q values could be related to the high temperatures of the rocks that were uplifted by the impact event from a mid-crustal original setting, and to fluid circulation within the two domes that made the rocks vulnerable to acquire high thermochemical remanence. Moreover, paleomagnetic analysis of the rocks around the ARS gabbro intrusion in the northern part of the Vredefort structure revealed the presence of either a shallow north or a shallow south direction, which is tentatively related to emplacement of the Umkondo large igneous province. Analysis of all rocks, including the Vredefort impactites, yields occasionally distinct great circle paths towards these shallow directions. A likely explanation for this overprint direction is the heating caused by now eroded ARS-type gabbro in the area, or by regional, ca. 1.1–1.0 Ga orogenic effects. No evidence of Karoo-type (0.18 Ga) overprint is seen.