New U–Pb geochronologic and palaeomagnetic constraints on the late Palaeoproterozoic Hartley magmatic event: evidence for a potential large igneous province in the Kaapvaal Craton during Kalahari assembly, South Africa

Abstract The Palaeoproterozoic Hekpoort Formation of the Pretoria Group is a lava-dominated unit that has a basin-wide extent throughout the Transvaal sub-basin of South Africa. Additional correlative units may be present in the Kanye sub-basin of Botswana. The key characteristic of the formation is its general geochemical uniformity. Volcaniclastic and other sedimentary rocks are relatively rare throughout the succession but may be dominant in some locations. Hekpoort Formation outcrops are sporadic throughout the basin and mostly occur in the form of gentle hills and valleys, mainly encircling Archaean domes and the Palaeoproterozoic Bushveld Complex (BC). The unit is exposed in the western Pretoria Group basin, sitting unconformably either on the Timeball Hill Formation or Boshoek Formation, which is lenticular there, and on top of the Boshoek Formation in the east of the basin. The unit is unconformably overlain by the Dwaalheuwel Formation. The type-locality for the Hekpoort Formation is the Hekpoort farm (504 IQ Hekpoort), ca. 60 km to the west-southwest of Pretoria. However, no stratotype has ever been proposed. A lectostratotype, i.e., the Mooikloof area in Pretoria East, that can be enhanced by two reference stratotypes are proposed herein. The Hekpoort Formation was deposited in a cratonic subaerial setting, forming a large igneous province (LIP) in which short-termed localised ponds and small braided river systems existed. It therefore forms one of the major Palaeoproterozoic magmatic events on the Kaapvaal Craton.

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Timing and tempo of the Great Oxidation Event

Proceedings of the National Academy of Sciences ◽

10.1073/pnas.1608824114 ◽

2017 ◽

Vol 114 (8) ◽

pp. 1811-1816 ◽

Cited By ~ 164

Author(s):

Ashley P. Gumsley ◽

Kevin R. Chamberlain ◽

Wouter Bleeker ◽

Ulf Söderlund ◽

Michiel O. de Kock ◽

...

Keyword(s):

Atmospheric Oxygen ◽

Kaapvaal Craton ◽

Large Igneous Province ◽

Snowball Earth ◽

Low Latitude ◽

Great Oxidation Event ◽

Carbon Isotope Excursion ◽

Igneous Province ◽

Age Constraints ◽

Exact Timing

The first significant buildup in atmospheric oxygen, the Great Oxidation Event (GOE), began in the early Paleoproterozoic in association with global glaciations and continued until the end of the Lomagundi carbon isotope excursion ca. 2,060 Ma. The exact timing of and relationships among these events are debated because of poor age constraints and contradictory stratigraphic correlations. Here, we show that the first Paleoproterozoic global glaciation and the onset of the GOE occurred between ca. 2,460 and 2,426 Ma, ∼100 My earlier than previously estimated, based on an age of 2,426 ± 3 Ma for Ongeluk Formation magmatism from the Kaapvaal Craton of southern Africa. This age helps define a key paleomagnetic pole that positions the Kaapvaal Craton at equatorial latitudes of 11° ± 6° at this time. Furthermore, the rise of atmospheric oxygen was not monotonic, but was instead characterized by oscillations, which together with climatic instabilities may have continued over the next ∼200 My until ≤2,250–2,240 Ma. Ongeluk Formation volcanism at ca. 2,426 Ma was part of a large igneous province (LIP) and represents a waning stage in the emplacement of several temporally discrete LIPs across a large low-latitude continental landmass. These LIPs played critical, albeit complex, roles in the rise of oxygen and in both initiating and terminating global glaciations. This series of events invites comparison with the Neoproterozoic oxygen increase and Sturtian Snowball Earth glaciation, which accompanied emplacement of LIPs across supercontinent Rodinia, also positioned at low latitude.

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Spatial variability in Karoo dolerites

10.5194/egusphere-egu21-5975 ◽

2021 ◽

Author(s):

Arnold Kotze ◽

R. James Roberts

Keyword(s):

South Africa ◽

Trace Element ◽

Basaltic Magma ◽

Principal Component ◽

Large Igneous Province ◽

Appreciable Amount ◽

Data Set ◽

Igneous Province ◽

Positive Loading ◽

Project Data

AD Kotze and RJ RobertsDepartment of Geology, University of Pretoria, Hatfield, Pretoria, South Africa; [email protected]The Karoo Large Igneous Province (KLIP) in South Africa consists of both a spatially limited extrusive basalt suite (Drakensberg Group) and a spatially extensive dolerite suite, both generally considered to be remarkable homogenous and of a &#8220;low-Ti&#8221; character (Luttinen, 2018). The homogeneity of the rocks requires that statistical analysis is necessary to look for spatial and geochemical trends in the data, which may yield clues to the mantle processes producing the 60 000 km2 expanse of basaltic magma. In this project, data derived from several locations are used as proxies to check for lateral variability in the Karoo dolerites. A principal component analysis (PCA) on trace element data using a covariance matrix was performed, and comparisons based on variables that are 1) common to the Karoo dolerites and Lesotho basalts and, 2) responsible for the most amount of variation to the data set are made. Trace element modelling is then used to test different mantle melting scenarios possibly responsible for the variation seen in the dolerites.Principal component analyses revealed several trace elements are responsible for most of the variability in the dolerites. Cr and Ni has the strongest positive loading on Component 1 whereas Cr and Ba has the strongest positive loading on Component 2. Ba has a strong negative loading on Component 1. Cu, Sr, V and Zr do impart an appreciable amount of variation to the data, but all four variables have weak negative loadings on both components. Interestingly, the activity of Cu and V seems to be the inverse of that of Cr and Ni.Due to the nature of a PCA, this work is afforded an opportunity to place the geochemistry of the Karoo dolerites within a larger geodynamic context without bias. From the observed variation, the activity of Ba and Cr is interpreted as an assimilation-oxidation process, whereas the Ni signature reflects the mantle origin of the magmas. Further modelling of these processes will allow the testing of suggested mechanisms for the formation of the KLIP, especially whether the magmatism is plume-related or related to the foundering of crustal blocks.Luttinen, A., 2018. Bilateral geochemical asymmetry in the Karoo large igneous province. Scientific Reports, 8(5223).

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