Bushveld superplume drove Proterozoic magmatism and metallogenesis in Australia

Marco L Fiorentini,Eunjoo Choi,Franco Pirajno,Craig O’Neill,Roland Maas,Andrea Giuliani,Stephen Foley

doi:10.1038/s41598-020-76800-0

Abstract

Large-scale mantle convective processes are commonly reflected in the emplacement of Large Igneous Provinces (LIPs). These are high-volume, short-duration magmatic events consisting mainly of extensive flood basalts and their associated plumbing systems. One of the most voluminous LIPs in the geological record is the ~ 2.06 billion-year-old Bushveld Igneous Complex of South Africa (BIC), one of the most mineralised magmatic complexes on Earth. Surprisingly, the known geographic envelope of magmatism related to the BIC is limited to a series of satellite intrusions in southern Africa and has not been traced further afield. This appears inconsistent with the inferred large size of the BIC event. Here, we present new radiometric ages for alkaline magmatism in the Archean Yilgarn Craton (Western Australia), which overlap the emplacement age of the BIC and indicate a much more extensive geographic footprint of the BIC magmatic event. To assess plume involvement at this distance, we present numerical simulations of mantle plume impingement at the base of the lithosphere, and constrain a relationship between the radial extent of volcanism versus time, excess temperature and plume size. These simulations suggest that the thermal influence of large plume events could extend for thousands of km within a few million years, and produce widespread alkaline magmatism, crustal extension potentially leading to continental break-up, and large ore deposits in distal sectors. Our results imply that superplumes may produce very extensive and diverse magmatic and metallogenic provinces, which may now be preserved in widely-dispersed continental blocks.

Highlights

The convective processes responsible for the dispersion of heat from the Earth’s core to its outer surface are reflected in the formation of mantle plumes[1,2], which can result in the emplacement of enormous amounts of magma into the crust over very short time spans, generally less than 5 Ma3,4
Re-Os dating of kimberlites is problematic—Re and Os are known to be hosted in both magmatic minerals and possibly unequilibrated x enocrysts22,23—the ~ 2.02 Ga apparent age for the Eastern Goldfields Superterrane (EGS) alkalic complexes provided encouragement to further radiometric dating, with the aim of searching for syn-Bushveld magmatic activity in the Yilgarn Craton
The geochronological results and 3D thermochemical modelling presented here support the hypothesis that a widely-dispersed ~ 2.06 Ga Proterozoic alkaline province in the eastern Yilgarn Craton of Western Australia represents a magmatic manifestation associated with the Bushveld superplume

Summary

Introduction

The convective processes responsible for the dispersion of heat from the Earth’s core to its outer surface are reflected in the formation of mantle plumes[1,2], which can result in the emplacement of enormous amounts of magma into the crust over very short time spans, generally less than 5 Ma3,4. The Bushveld Igneous Complex (BIC) of South Africa, which hosts the largest known reserves of platinum group elements (PGEs), vanadium and chromium, is thought to reflect the arrival of a large mantle plume under the Kaapvaal Craton at ~ 2.06 Ga9. This craton had already formed a deep, buoyant and refractory lithospheric root by ~ 3 Ga10. We have used geochronological constraints and excess temperature estimates from magma compositions to constrain a relationship between the radial extent of volcanism versus time, and plume size These results suggest that the Yilgarn alkaline province may be a distal expression of the Bushveld superplume, which was separated from its former location closer to the Kaapvaal Craton by subsequent terrane dispersal. Re-Os dating of kimberlites is problematic—Re and Os are known to be hosted in both magmatic minerals and possibly unequilibrated x enocrysts22,23—the ~ 2.02 Ga apparent age for the EGS alkalic complexes provided encouragement to further radiometric dating, with the aim of searching for syn-Bushveld magmatic activity in the Yilgarn Craton

Methods

Results

Discussion

Conclusion