September 2021 LIP of the Month

The Paleoproterozoic Black Hills Dyke Swarm: key to resolve Kalahari within Columbia

Cedric Djeutchou¹, Michiel O. de Kock¹, Herve´ Wabo¹, Camilo E. Gaitán², Ulf Söderlund², Ashley P. Gumsley³

¹Department of Geology, University of Johannesburg, PO Box 524, Auckland Park 2006, South Africa

²Department of Geology, Lund University, Lund 223 62, Sweden.

³Institute of Earth Sciences, University of Silesia in Katowice, Sosnowiec 41-200, Poland.

Extracted and modified after:

The Black Hills Dike Swarm

The Black Hills Dike Swam (BHDS; Fig. 1) is a >300 km wide swarm of mainly NE to NNE trending mafic dikes. The swarm intrudes Archean basement in north-eastern South Africa as well as the eastern 2.68-2.06 Ga Transvaal Supergroup, the eastern 2.06-2.05 Ga Bushveld Complex, and the smaller 2.06 Ga Phalaborwa Complex (Olsson et al., 2016; Djeutchou et al., 2021). It is named after the prominent Black Hills dyke, which has been dated at 1860.9 ± 2.7 Ma (Olsson et al., 2016). Baddeleyite U-Pb crystallization ages of 12 dated dikes belonging to this swarm range from ca. 1.87 to ca. 1.84 Ga (Olsson et al., 2016; Wabo et al., 2019). However, until recently, only two dated dikes were also studied paleomagnetically (Lubnina et al., 2010; Wabo et al., 2019). Paleoproterozoic magnetizations were reported for several undated dykes that may belong to the BHDS (Letts et al., 2005, 2011; Lubnina et al., 2010). These pass a reversal and baked-contact tests, and the Paleoproterozoic age assignment is further supported by studies that combined U-Pb geochronology, geochemistry, and paleomagnetic data sets (Klausen et al., 2010; Olsson et al., 2016; Wabo et al., 2019). Dike ages do not discriminate between trend and location but subdivide the swarm into an older, 1.88-1.86 Ga, more-primitive group and a younger, 1.85-1.84 Ga, more chemically enriched group (Olsson et al., 2016).

The BHDS is spatially, geochemically, and temporally associated with several magmatic provinces in the region. These magmatic provinces are the 1.89-1.87 Ga Mashonaland sill province (MSP; Hanson et al., 2011; Söderlund et al., 2010) and the 1.88-1.87 Ga post-Waterberg sill province (PWSP; Hanson et al., 2004). The BHDS is further associated with  <1.83 Ga magmatism present in the Soutpansberg Basin (Geng et al., 2014; Olsson et al., 2016). The less well-dated, but likely ca. 1.8 Ga Mazowe dike swarm of Zimbabwe (Hanson et al., 2011) has also been identified as a possible member of wide-spread late Paleoproterozoic magmatism from Kalahari, see Figure 1. Paleomagnetic data from the Mashonaland and post-Waterberg sill provinces differ significantly and require large tectonic displacement between the Kaapvaal and Zimbabwe cratons (Hanson et al., 2011). Such offset before 1.8 Ga is currently unsupported by  geological evidence, and the possibility of a rapid true polar wander (TPW) was proposed as an alternative explanation for the mismatch between paleomagnetic results (Mitchell et al., 2010; Antonio et al., 2017). Unfortunately, only few Mashonaland intrusions are both precisely dated and paleomagnetically constrained. Furthermore, the BHDS was emplaced in the immediate aftermath of the proposed TPW. This limits careful evaluation of the discrepancy.

Paleomagnetism

Sixty-one dikes (NE, SE, and N-oriented) were sampled by Djeuchou et al. (2021) in north-eastern South Africa (Fig.1). The study specifically targeted precisely dated dikes. Each paleomagnetic site sampled a distinct dike and corresponds to a unique cooling unit. After stepwise alternating field and thermal demagnetization, high-temperature magnetizations were identified in 36 dikes and interpreted as being Paleoproterozoic. These high-temperature remanence components are northwest (downward) or southeast (upward), with moderate to steep inclinations. The remanence groups share common precision and are ∼180° apart, illustrating a class-C reversal test (Fig. 2). Sampling thus spans one or more reversals of the geomagnetic field. Samples from two dated ca. 1.85 Ga dikes (Olsson et al., 2016), one north-trending and one north-east trending were used to illustrate positive baked-contact tests, thus further supporting the primary Paleoproterozoic nature of the magnetization (Fig. 3). Where directly dated, the crystallization age is assumed to be the timing of remanence acquisition. The data from 34 dikes were combined by Djeuchou et al. (2021) with published BHDS results from 29 previously studied dikes to define a mean paleopole for the BHDS at 15.3°N, 14.9°E and A₉₅ (radii of 95% confidence) = 5.6° (quality, Q = 7; after Van der Voo, 1990). This pole represents ∼30 m.y. and is indistinguishable from poles for the 1.88-1.87 Ga Post-Waterberg sill province and <1.83 Ga Soutpansberg Basin.

Magmatic Episodes and Magnetostratigraphy

Djeuchou et al. (2021) noted that some units from regional magmatic provinces are coeval, and calculated weighted mean crystallization ages from U-Pb ID-TIMS baddeleyite dates (Fig. 4) at 1873 ± 1 Ma (MSWD = 0.62), 1860 ± 2 Ma (MSWD = 0.7), and 1848 ± 2 Ma (MSWD = 0.7). These episodes refine the old (1860 Ma) and young (1848 Ma) BHDS groupings previously defined by Olsson et al. (2016). Units of the ca. 1873 Ma (oldest) episode all have positive inclinations (Fig. 4), while the dikes of the younger episodes have exclusively negative inclination (Fig. 4). This defines a late Paleoproterozoic magnetostratigraphic record for Kalahari (Djeuchou et al., 2021). Besides polarity, the chemical composition provides another discriminator between episodes (Olsson et al., 2016).  Djeuchou et al. (2021)  used all undated BHDS dikes that recorded positive inclinations to calculate a ca. 1873 Ma episode pole (11.8°N, 10.6°E and A₉₅ = 11.8°; Q = 7). Poles at ca. 1860 Ma and ca. 1848 Ma have larger uncertainties but are statistically indistinguishable from the ca. 1873 Ma pole (Djeuchou et al., 2021).

Implications of results

A comparison of absolute polarity can constrain the relative positions of Superior, Fennoscandia and Kalahari. Hoffman and Grotzinger (1993) used trade-wind orographic patterns across the Slave craton to assign absolute polarity to Laurentian paleomagnetic data in the Paleoproterozoic. Driscoll and Evans (2016) follow this rational and assign positive inclinations from Superior as normal polarity. In this same way, Fennoscandia show normal polarity before ~1.88 Ga (Fig. 4). A single dated MSP sill define normal polarity at this time from Kalahari. Between ca. 1.88 Ga and ca. 1.86 Ga dual magnetic polarities are described from the ca. 1870 ± 9 Ma Svecofennian Keuruu dikes. The reversal(s) recorded by these dikes can perhaps be correlated to either the 1873 Ma or pre-1873 Ma reversals of Kalahari (Djeuchou et al., 2021). On Superior, the ca. 1.88 Ga Molson dikes record dual polarities as does the ca. 1.87 Ga Haig/Flaherty/Sutton mean (Fig. 4). Reversals represented by these data may correlate to the post-1873 Ma Kalahari reversal (Djeuchou et al., 2021). The shared normal polarity before ~1.88 Ga, and shared record of a reversal at ~1.87 Ga suggests that Fennoscandia, Superior and Kalahari were in the same hemisphere (Djeuchou et al., 2021).

At 1.88 Ga, Kalahari restores to 45-59° paleolatitude, similar to Superior, that was located at 32-58° (Fig. 5), but at a much higher paleolatitude than Fennoscandia, which was located between 11-28°. Djeuchou et al. (2021) placed the Superior-Nain block in a position northeast of Fennoscandia preceding the Northern Europe-North America (NENA) configuration (following the example of Klein et al., 2016) by overlapping the 1.88 Ga Molson dikes pole with 1.89-1.84 Ga Fennoscandian poles (Fig. 5). The 1.89-1.83 Ga Kalahari poles are further overlapped with the 1.88 Ga Molson dikes pole (Djeuchou et al., 2021), and Kalahari was placed to the west of reconstructed Superior (Fig. 5). The modern southwestern margin of Superior was kept open, as it is occupied by the 1.77-1.60 Ga Central Plains, Yavapai and Mazatzal orogens after 1.8 Ga (Whitmeyer and Karlstrom, 2007). Djeuchou et al. (2021) note that although it is permissible to place Kalahari to the east of Superior in terms of paleolatitude, that such a placement is unlikely, given that this area is occupied by the Manikewan Ocean (Fig. 5). Restoration of Kalahari to the west of Superior aligns the BHDS and the MDS into a radiating pattern around the ca. 1.88 Ga Circum-Superior LIP magmatic centre (Minifie et al., 2013). This event according to Djeuchou et al. (2021) is thus likely related to the intra-plate magmatism of Kalahari and Fennoscandia (Fig. 5). In this position, overlap is further also achieved between ~2.0 Ga Kalahari and Superior poles (Fig. 5). This suggests a longer-lived link, and it is interesting to note that the Kaapvaal craton was similarly reconstructed at ca. 2.43 Ga by Gumsley et al. (2017).

Figure 5. Reconstruction of Kalahari at 1.88 Ga relative to West Superior revealing radiating pattern of 1.88-1.83 Ga magmatism around the Circum-Superior LIP centre (star). TBL = Thabazimbi-Murchison lineament. Other abbreviations as per Fig. 1. From Djeutchou et al. (2021).

It should be noted that there is possible disagreement with the interpretation of the absolute polarity from Kalahari relative to Superior in terms of 1850-1830 Ma normal polarities recorded by the ~1850 Ma, but poorly dated Sudbury irruptive and the ca. 1838 Ma Boot-Phantom pluton (Hood, 1961; Symons and McKay, 1999). Both these results were, however, excluded from quality-filtered Laurentian data (Swanson-Hysell, 2021). The interpretation of polarity data and the reconstruction of Djeuchou et al. (2021) thus needs to be tested as new data becomes available.

Unfortunately, a 1.83-1.40 Ga gap in reliable paleomagnetic data from the Kalahari craton (De Kock et al., 2021) prevents evaluation of Kalahari’s position throughout the existence of Columbia. The 1.88 Ga reconstruction, however, does suggest a peripheral position for the Kalahari craton early on during Columbia assembly.

References