Zircon ages defining deposition of the Palaeoproterozoic Soutpansberg Group and further evidence for Eoarchaean crust in South Africa

doi:10.1016/j.precamres.2014.05.020

Precambrian Research

Volume 249, August 2014, Pages 247-262

https://doi.org/10.1016/j.precamres.2014.05.020 Get rights and content

Highlights

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Lower succession of the Soutpansberg Group commenced deposition at around 1830 Ma.
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Limpopo Belt Central Zone is the source for zircons in Soutpansberg rocks.
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The identified 3937 ± 4 Ma zircon age is one of the oldest zircon ages in Africa.

Abstract

The precise age of the volcano-sedimentary Soutpansberg Group, which was deposited upon the Palala shear belt separating the Kaapvaal Craton from the Central Zone of the Limpopo Belt, has long been debated. The Soutpansberg Group is subdivided into a lower and upper succession, which are separated from each other by a prominent regional unconformity. Zircon grains from silicic pyroclastic rocks of both successions were investigated in order to constrain the timing of deposition of the Soutpansberg Group rocks.

The zircon grains of the investigated samples from both successions yield a wide range of ages, spanning from 1831 to 3937 Ma. Most of the zircon grains have rounded shapes, however it is not clear whether they are mainly xenocrystic or detrital, or have been rounded by resorption in a silicic magma chamber. The youngest zircon grain ages obtained come from the lower succession and are 1832 ± 9 and 1831 ± 15 Ma. We interpret these youngest zircon grains as magmatic grains that have been rounded by resorption. This view is corroborated by the fact that no magmatic rocks of this particular age have been observed in the Kaapvaal Craton or the Central Zone of the Limpopo Belt, and that no apparent sedimentary admixtures are present in the well exposed pyroclastic rocks. We therefore conclude that deposition of the Soutpansberg volcano-sedimentary succession commenced around 1830 Ma.

The Soutpansberg rocks were deposited apparently over a lengthy period of time (ca. 230 Ma), as provided by the published age of 1604 Ma for pyroclastic rocks of the upper succession in Botswana. Zircon grain age spectra of our Soutpansberg samples show prominent peaks at 2.0, 2.6 and 3.2 Ga, indicating the Central Zone of the Limpopo Belt as the source area, but excludes the adjacent northern part of the Kaapvaal Craton. The oldest zircon grain identified in the Soutpansberg samples has an age of 3937 ± 4 Ma, one of the oldest zircon grain ages yet reported from the African continent.

Introduction

The northern part of the Kaapvaal Craton, Southern Africa, is largely made up of Archaean granitoid-greenstone rocks, whereas the adjacent Central Zone of the Limpopo Belt is dominated by Archaean to Palaeoproterozoic granulite-facies ortho- and paragneisses (Robb et al., 2006, Kramers et al., 2006; Fig. 1). The granitoid-greenstone suites are predominantly Meso- to Neoarchaean in age, and the granulite-facies assemblages in the Central Zone of the Limpopo Belt are interpreted to have resulted from collision between the Kaapvaal and Zimbabwe Cratons at about 2.65 Ga and from dextral transpression in the Central Zone at about 2.0 Ga, aided probably by magmatic underplating (Holzer et al., 1998, Schaller et al., 1999, Kramers et al., 2011, Kramers and Mouri, 2011, Zeh et al., 2011, Zeh and Gerdes, 2012, Laurent et al., 2013). The Central Zone of the Limpopo Belt contains metasedimentary rocks that are at least partly derived from Eo- to Palaeoarchaean crust (Jaeckel et al., 1997, Kröner et al., 1999, Buick et al., 2003, Zeh et al., 2007, Zeh et al., 2008). The above rock types are locally overlain by erosional remnants of red bed successions that include the Blouberg Formation and the Waterberg, Palapye and Soutpansberg Groups (Fig. 2). These mainly clastic sedimentary sequences are interpreted as molasse-type deposits, sourced by erosion of an extensive mountain chain as a result of the Limpopo orogeny (Callaghan et al., 1991, Barker et al., 2006, Dorland et al., 2006, Corcoran et al., 2013), but their depositional ages are not well known. This study is aimed at better defining the depositional age of the Soutpansberg Group, one of the most widespread post-orogenic successions, and to identify the source region of the sediments.

Section snippets

Geological framework

The geological relationships of the post-Limpopo orogeny clastic deposits are briefly described below. Rocks of the steeply dipping Blouberg Formation, which are entirely clastic, occur as a string of minor scattered occurrences with a maximum preserved thickness of 1200 m (Bumby et al., 2001, Barker et al., 2006; Fig. 2). The sediments overlie the ca.10 km wide Palala Shear Belt, a crustal-scale strike-slip fault zone, separating the Kaapvaal Craton from the Limpopo Belt farther north. The

. Regional setting

The Soutpansberg Group, a volcano-sedimentary suite, is developed and preserved above the Palala Shear Belt, which separates the Kaapvaal Craton in the south (Southern Marginal Zone) from the Central Zone of the Limpopo Belt in the north (Fig. 1b). This prominent structure is a crustal-scale, deep-seated terrane boundary with the mylonitic rocks attaining a width of over 10 km (Schaller et al., 1999). The Southern Marginal Zone is mainly composed of ortho- and paragneisses with zircon U–Pb ages

Previous geochronology

In general, the upper age limit for initiation of Soutpansberg deposition is constrained by the waning phases of strike-slip movement along the Palala Shear Belt and the intrusion of the Entabeni Granite (Fig. 2). Ductile movement along the Palala Shear Belt ceased at about 1.97 Ga (Schaller et al., 1999), and the Entabeni Granite with SHRIMP zircon ages of 2021 ± 5 Ma (Dorland et al., 2006) and 2023 ± 6 Ma (Zeh et al., 2009) is overlain by the Soutpansberg strata.

There are as yet no precise

Analytical methods

X-ray fluorescence (XRF) analyses were carried out at the Council for Geoscience Geochemistry Laboratory in Pretoria. A PANalytical Axios X-ray fluorescence spectrometer, equipped with a 4 kW Rh tube, was used to determine major element concentrations on fused glass disks and trace element concentrations on pressed powder pellets. Quality control was ascertained by repeat analyses of an internal standard.

The samples selected for zircon dating weighed about 5–7 kg, and were reduced by rock

Petrography and geochemistry

A simplified stratigraphic column of the Soutpansberg Group, also showing the stratigraphic position of the samples, is given in Fig. 4. GPS co-ordinates of the sample localities are presented in Table 1. The samples studied are all pyroclastic rocks. Their geochemical compositions are shown in Table 2.

Sample GB 07-20 is from the lower succession (Sibasa Formation; Fig. 4) and is an ash flow tuff, informally known as Natal House tuff, which is underlain by shale and quartzite. The rock is

Zircon U–Pb dating

Zircon grains from Sample GB 07-20 are transparent to semi-transparent and light yellow in colour. They occur essentially as rounded crystals with some preserving stubby, prismatic morphologies (100–150 μm long, length to width ratio of 1.1–2.0, Fig. 5). Many are zoned zircon grains with core-rim relationships, whereas some grains show oscillatory zoning (Fig. 5). Thirty eight zircon grains were analysed, and the results are presented in Table 3 and Fig. 6. Twenty one spots yielded concordant

Depositional age of the Soutpansberg Group

Commonly zircons with a rounded shape are interpreted as inherited (xenocrystic) or detrital grains, with the latter thought to have been mechanically rounded during transportation in a sedimentary environment (e.g. Heubeck et al., 2013).

A detrital origin for all the zircon grains in this study, however, seems at odds with our thin section studies and field observations, which do not indicate any reworking of the ash flows or an admixture of sedimentary material. We therefore suggest that at

Conclusions

The surprising, yet disappointing aspect of our study is that we were not able to unambiguously identify igneous zircon grains that were undoubtedly associated with magmatic activity and would therefore reflect emplacement of the Soutpansberg pyroclastic rocks. However, our data set and fieldwork strongly suggest that at least the youngest zircon grains are not detrital, but rather magmatic, with their rounded shapes caused by resorption in a magma chamber prior to eruption. We therefore

Acknowledgements

This contribution is from the Chemical Geodynamics Joint Laboratory of HKU and Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, and was supported by research grants from the University of Hong Kong, the Germany/Hong Kong Joint Research Scheme sponsored by the Research Grant Council of Hong Kong and the German Academic Exchange Service (DAAD) (grant_HK033/12), and was also supported by a research grant of the State Key Laboratory of Isotope Geochemistry, Guangzhou Institute of

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Zircon ages defining deposition of the Palaeoproterozoic Soutpansberg Group and further evidence for Eoarchaean crust in South Africa

Highlights

Abstract

Introduction

Section snippets

Geological framework

. Regional setting

Previous geochronology

Analytical methods

Petrography and geochemistry

Zircon U–Pb dating

Depositional age of the Soutpansberg Group

Conclusions

Acknowledgements

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Intracrustal radioactivity as an important heat source for Neoarchean metamorphism in the Central Zone of the Limpopo Complex

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A proposed geotectonic model for the Soutpansberg Group within the Limpopo Mobile Belt, South Africa

Spec. Publ. Geol. Soc. S. Afr.

The Soutpansberg and Waterberg Groups and the Blouberg Formation

The lower unconformity-bounded sequence of the Soutpansberg Group and its correlatives – remnants of a Proterozoic large igneous province

S. Afr. J. Geol.

High precision, U–Pb analyses of single grains of zircon from quartzite in the Beit Bridge Group yield a discordia

S. Afr. J. Geol.

Preliminary results of single zircon studies from various Archaean rocks of the Northeastern Transvaal

The Kaapvaal Craton

The geology of the Alldays area

Petrogenesis of metamorphic rocks

Carbon and U–Pb evidence for a Palaeoproterozoic crustal component in the Central Zone of the Limpopo Belt, South Africa

J. Geol. Soc. Lond.