August 2021 LIP of the Month

Identification of a new 485 MA post-orogenic mafic dyke swarm east of the pan-african saldania-gariep belt of south africa. 

C.G. Kingsbury^1,6, M. B. Klausen², U. Söderlund³, W. Altermann⁴, R. E. Ernst^5,6

¹Department of Geology, University of Pretoria, Private Bag x 20, Hatfield 0028, South Africa, ²Department of Earth Sciences, University of Stellenbosch, Private Bag X1, Matieland 7602, South Africa ³Department of Geology, Lund University, Sölvegatan 12, SE-223 62 Lund, Sweden ⁴Department of Geology, University of Johannesburg, Auckland Park 2006, South Africa ⁵Ottawa-Carleton Geoscience Centre, Department of Earth Sciences, Carleton University, 1125 Colonel By Drive, Ottawa, ON K1S 5B6, Canada ⁶Faculty of Geology and Geography, Tomsk State University, Tomsk 634050, Russia

Extracted and modified from: Kingsbury, C.G., Klausen, M.B., Söderlund, U., Altermann, W., Ernst, R.E., 2021. Identification of a new 485 Ma post-orogenic mafic dyke swarm east of the Pan-African Saldania-Gariep Belt of South Africa. Precambrian Res. 354. doi:10.1016/j.precamres.2020.106043

Introduction

Western South Africa, between Cape Town and the Namibian border, hosts many roughly coast-parallel dyke swarms that may be structurally related to a reconstructed plume-centre of the c. 130 Ma Parana-Etendeka Large Igneous Province (LIP) but are largely undated (Fig. 1). U-Pb ID-TIMS age determinations on baddeleyite from two ~100 km long dykes that outcrop along the Namaqualand shoreline and a ~55 km inland dyke (near the town of Garies), yield 487 ± 8 Ma and 482 ± 7 Ma ages, respectively, that lie within each others 2σ errors (Fig. 2). Thus, significantly older than the Parana-Etendeka LIP with which these dykes have previously been correlated via geochemical means (e.g. Trumbull et al, 2007) and, instead, revealing an independent and previously unrecognized magmatic event that coincides the end-Cambrian time boundary.

Field characteristics

Sample 19CK004 (at 30.7959 ?S, 18.1777?E) from the ~20 m wide Garies dyke was collected along a farmland track, 5 km east of the N7 road and 5 km north of the Northern Cape-Western Cape provincial boundary. The 19CK002A sample (at 30.5447?S, 17.4057?E) from the ~35 m wide Namaqualand dyke was collected from a micropegmatoidal bleb in an otherwise doleritic dyke. Together with the petrography and bulk rock geochemistry of additional samples from along both dykes, all of these data are more fully presented and discussed in the Kingsbury et al. (2021) paper, while this extended abstract focuses on some implications of this newly discovered LIP.

Potential links to the End Cambrian

The most recent end-Cambrian time boundary, based on a first appearance of the conodont species I. fluctivagus, is by Goldman et al. (2020) set at 486.9 ± 1.5 Ma and it lies within the errors of our 487 ± 8 and 482 ± 7 Ma ages for the Namaqualand-Garies dykes. In addition, Zhu and others (2006) have identified a coeval negative Carbon-13 excursion event that is commonly associated with environmental perturbations and that they refer to as the Top Of Cambrian Excursion (TOCE). Building on the notion that LIPs can be the driver of environmental and biotic change, including mass extinctions, (e.g. Courtillot and Renne, 2003; Ernst and Youbi, 2017; Kasbohm et al. 2021; Ernst et al., 2021), it is proposed that the volcanism associated with the Namaqualand-Garies dykes may have contributed to the TOCE and associated sudden environmental change. In possible conjunction with other major magmatic events, like the 470-490 Ma Kidryasovo Event in the Urals of Russia (Puchkov et al., 2021), these may collectively represent a squad of “smoking guns” that led to the End Cambrian environmental change. 

A supercontinental assembly LIP

While ~485 Ma U-Pb ID TIMS baddeleyite ages for the Namaqualand-Garies dyke pair rule out their emplacement during the opening of the South Atlantic and further link with the ~130 Ma Parana – Etendeka LIP, they constitute a new magmatic event that can be used for paleogeographic reconstructions. Dykes of this age are as yet unknown across southern Gondwana and constitute a supercontinental assembly-type LIP event that was missing from Klausen’s (2020) record for southern Africa. Since the dykes coincide both spatially and temporally with a local post-orogenic pull-apart rift system (e.g. Tankard, 2012; Fig. 1c), it is speculated whether this could have induced sufficient lithospheric thinning or additional delamination was needed to allow an ambient asthenospheric mantle to decompressionally melt. Thermal capping by Gondwana may also have increased the ambient mantle temperature, all the while that we cannot rule out the involvement of an even hotter and more enriched mantle plume. However, more research into how LIPs formed during supercontinental assemblies are needed, just as how these may lead to global environmental changes.

References