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Late Paleoproterozoic Mafic Magmatism and the Kalahari Craton During Columbia Assembly Cedric Djeutchou1*, Michiel O

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Late Paleoproterozoic Mafic Magmatism and the Kalahari Craton During Columbia Assembly Cedric Djeutchou1*, Michiel O https://doi.org/10.1130/G48811.1 Manuscript received 11 January 2021 Revised manuscript received 17 May 2021 Manuscript accepted 24 May 2021 © 2021 The Authors. Gold Open Access: This paper is published under the terms of the CC-BY license. Late Paleoproterozoic mafic magmatism and the Kalahari craton during Columbia assembly Cedric Djeutchou1*, Michiel O. de Kock1, Hervé Wabo1, Camilo E. Gaitán2, Ulf Söderlund2 and Ashley P. Gumsley3 1 Department of Geology, University of Johannesburg, PO Box 524, Auckland Park 2006, South Africa 2 Department of Geology, Lund University, Lund 223 62, Sweden 3 Institute of Earth Sciences, University of Silesia in Katowice, Sosnowiec 41-200, Poland ABSTRACT be rapid true polar wander (TPW) (Mitchell The 1.87–1.84 Ga Black Hills dike swarm of the Kalahari craton (South Africa) is coeval et al., 2010; Antonio et al., 2017). Unfortunate- with several regional magmatic provinces used here to resolve the craton’s position during ly, few precisely dated Mashonaland intrusions Columbia assembly. We report a new 1850 ± 4 Ma (U-Pb isotope dilution–thermal ioniza- also have paleomagnetic constraints (Table S3 tion mass spectrometry [ID-TIMS] on baddeleyite) crystallization age for one dike and new in the Supplemental Material1). Furthermore, paleomagnetic data for 34 dikes of which 8 have precise U-Pb ages. Results are constrained the BHDS was emplaced in the immediate by positive baked-contact and reversal tests, which combined with existing data produce a aftermath of the proposed TPW. This limits 1.87–1.84 Ga mean pole from 63 individual dikes. By integrating paleomagnetic and geochro- careful evaluation of the discrepancy. Our data, nological data sets, we calculate poles for three magmatic episodes and produce a magneto- however, provide a high-resolution paleomag- stratigraphic record. At 1.88 Ga, the Kalahari craton is reconstructed next to the Superior netic record to test Kalahari’s 1.87–1.84 Ga craton so that their ca. 2.0 Ga poles align. As such, magmatism forms part of a radiating paleogeography. pattern with the coeval ca. 1.88 Ga Circum-Superior large igneous province. THE BLACK HILLS DIKE SWARM INTRODUCTION data from 1.89 to 1.83 Ga magmatic provinces, The BHDS is a >300-km-wide swarm of The Paleoproterozoic to Mesoproterozoic however, can provide constraints on Kalahari’s mainly northeast- to north-northeast–trend- supercontinent (referred to as Nuna, Hudson- role in Columbia’s assembly. ing mafic dikes intruding Archean basement, land, or Columbia) formed by assembly of Ar- The current 1.89–1.83 Ga paleomagnetic the 2.68–2.06 Ga Transvaal Supergroup, and chean cratons starting at 1.9 Ga and was fully record of Kalahari can be enhanced by study- the 2.06–2.05 Ga Bushveld and Phalaborwa amalgamated as late as 1.65–1.58 Ga (Meert, ing its magmatic provinces (Fig. 1). The 1.87– Complexes (Olsson et al., 2016; Fig. 1). Pa- 2012; Pisarevsky et al., 2014; Pourteau et al., 1.84 Ga northeast- to north-northeast–trending leoproterozoic magnetizations that pass rever- 2018). The position of the Kalahari craton (i.e., mafic Black Hills dike swarm (BHDS; South sal and baked-contact tests were reported for here the pre–1.2 Ga conjoined Kaapvaal and Africa) is one such province (Olsson et al., the BHDS (Letts et al., 2005, 2011; Lubnina Zimbabwe cratons, or the proto-Kalahari cra- 2016; Wabo et al., 2019). We present new pa- et al., 2010) and are confirmed by combined ton, sensu stricto; see de Kock et al., 2021) in leomagnetic data from 36 BHDS dikes togeth- U-Pb geochronology, geochemistry, and paleo- Columbia is obscured by limited paleomagnet- er with existing data from the 1.89–1.87 Ga magnetic data sets (Olsson et al., 2016; Wabo ic data. It does not appear in some reconstruc- Mashonaland sill province (Söderlund et al., et al., 2019). Baddeleyite U-Pb crystallization tions (e.g., Evans and Mitchell, 2011; Pehrs- 2010; Hanson et al., 2011) and the 1.88– ages of 12 dikes range between ca. 1.87 and ca. son et al., 2016), but in other reconstructions it 1.87 Ga post-Waterberg sill province (Hanson 1.84 Ga (Olsson et al., 2016; Wabo et al., 2019). is placed at Columbia’s periphery (e.g., Zhao et al., 2004). The BHDS is spatially, geochemi- However, only two dated dikes were also stud- et al., 2003) or is regarded as a “lone craton” cally, and temporally associated with <1.83 Ga ied paleomagnetically (Lubnina et al., 2010; (Pisarevsky et al., 2014). A craton’s position Soutpansberg Basin magmatism (Geng et al., Wabo et al., 2019). Dike ages do not discrimi- in Columbia is typically evaluated through 2014; Olsson et al., 2016) and the ca. 1.8 Ga nate between trend or location but subdivide 1.8–1.3 Ga apparent polar wander path com- Mazowe dike swarm of Zimbabwe (Fig. 1B) the swarm into an older, 1.88–1.86 Ga, more- parison (Evans and Mitchell, 2011). For the (Hanson et al., 2011). Paleomagnetic data primitive group and a younger, 1.85–1.84 Ga, Kalahari craton (simply Kalahari hereafter), from the Mashonaland and post-Waterberg sill more chemically enriched group (Olsson et al., post–1.8 Ga paleomagnetic data remain sparse provinces differ significantly and require large 2016). The combined BHDS, Mashonaland, and (de Kock et al., 2021) and its tenure in Co- tectonic displacement between the Kaapvaal Post-Waterberg sill provinces, and Soutpans- lumbia remains unconstrained. Paleomagnetic and Zimbabwe cratons (Hanson et al., 2011). berg Basin magmatism (Klausen et al., 2010; Currently there is no geological support for Lubnina et al., 2010; Olsson et al., 2016) form *E-mail: [email protected] such displacement. Another explanation may an ∼50 m.y. record. 1Supplemental Material. Geochronology, geochemistry, and paleomagnetic datasets. Please visit https://doi.org/10.1130/GEOL.S.14925186 to access the supplemental material, and contact [email protected] with any questions. CITATION: Djeutchou, C., et al., 2021, Late Paleoproterozoic mafic magmatism and the Kalahari craton during Columbia assembly: Geology, v. 49, p. XXX–XXX, https://doi.org/10.1130/G48811.1 Geological Society of America | GEOLOGY | Volume XX | Number XX | www.gsapubs.org 1 Downloaded from http://pubs.geoscienceworld.org/gsa/geology/article-pdf/doi/10.1130/G48811.1/5367144/g48811.pdf by guest on 29 September 2021 A B C Figure 1. Simplified geological maps showing outlines of the Kalahari craton (A), pre–1.8 Ga Kalahari craton (B), and northeastern Kaapvaal craton (C) (adapted from de Kock et al., 2019; Hanson et al., 2011; Olsson et al., 2016). U-Pb baddeleyite age constraints are from the Mashona- land sill province (MSP; Söderlund et al., 2010; Hanson et al., 2011), post-Waterberg sill province (PWSP; Hanson et al., 2004); and Black Hills dike swarm (BHDS; Olsson et al., 2016; Wabo et al., 2019; this study). 40Ar-39Ar age is from Layer et al. (1998). Soutpansberg Basin U-Pb zircon ages are from Geng et al. (2014). Paleomagnetic data are from the MSP (McElhinny and Opdyke, 1964; Bates and Jones, 1996), Mazowe dike swarm (MDS; Wilson et al., 1987), PWSP (Hanson et al., 2004), BHDS (Layer et al., 1998; Lubnina et al., 2010; Letts et al., 2011; Maré and Fourie, 2012; Wabo et al., 2019; this study); and Soutpansberg Group and post-Soutpansberg sill province (PSSP; Hanson et al., 2004; Gose et al., 2006). RESULTS Paleomagnetism mon precision and are ∼180° apart, illustrating Geochronology We sampled 61 dikes for paleomagnetic a class-C reversal test (Fig. S6). Sampling thus Baddeleyite grains extracted from an study using standard methods (see the Supple- spans one or more reversals of the geomagnet- 8–10-m-thick north-northeast–trending dike mental Material; Table S3). Directly dated dikes ic field. Two dated ca. 1.85 Ga dikes (Olsson (dike MAD01 in Fig. 1C) were divided into were explicitly targeted. Each paleomagnetic et al., 2016), one north trending and one north- three fractions and dated by isotope dilu- site sampled a distinct dike and corresponds to east trending (dikes CDL and BJL in Fig. 1C), tion–thermal ionization mass spectrometry a unique cooling unit. High-temperature magne- have positive baked-contact tests supporting the (ID-TIMS) at the Department of Geoscienc- tizations identified in 36 dikes are interpreted as primary Paleoproterozoic nature of the magneti- es, Swedish Museum of Natural History in Paleoproterozoic (Fig. 1; Fig. S2). The remain- zation (see the Supplemental Material). Where Stockholm. The full methodology and data, ing 25 dikes had heterogeneous demagnetization directly dated, the crystallization age is assumed including geochemistry (Table S2), are pro- behavior and are not discussed here (for more also to be the timing of remanence acquisition. vided in the Supplemental Material. Free re- detail, see the Supplemental Material). High- gression yields upper and lower intercepts temperature remanence components are grouped DISCUSSION at 1850 ± 4 Ma and 200 ± 250 Ma (mean as northwest (downward) or southeast (up- The 1.89–1.83 Ga Paleomagnetic Record squared weighted deviation [MSWD] = 1.13), ward), showing moderate to steep inclinations. Paleomagnetic Data by Magmatic Province respectively (Table S1; Fig. S1). The upper Site means with radii of 95% confidence α( 95) Our data (34 dikes) are combined with intercept is interpreted as the dike’s crystal- >16° were excluded (i.e., 2 of our 36 dikes). published BHDS results (29 dikes; Table lization age. The northwest and southeast groups share com- S3) to define two polarities Fig. 2A( ; see the 2 www.gsapubs.org | Volume XX | Number XX | GEOLOGY | Geological Society of America Downloaded from http://pubs.geoscienceworld.org/gsa/geology/article-pdf/doi/10.1130/G48811.1/5367144/g48811.pdf by guest on 29 September 2021 AB CD E Figure 2.
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