Closing an Archean ocean over 350 Mill. years – the Age-Hf isotope record of granitoids from the Limpopo Belt and adjacent Pietersburg Block

The Limpopo Belt in southern Africa represents a high-grade gneiss terrane located between the Kaapvaal Craton to the south and the Zimbabwe Craton to the north, and is subdivided into 3 zones: (i) Northern Marginal Zone (reworked part of the Zimbabwe Craton), (ii) Southern Marginal Zone (reworked part of the Pietersburg Block, forming the northern edge of the Kaapvaal Craton), (iii) Central Zone (a polymetamorphic gneiss terrane in between). Results of combined U-Pb dating and Hf isotope analyses of zircon grains from ca. 30 granitoids (TTG’s, sanukitoids, biotite granite, hybride granites) from the Pietersburg Block, Southern Marginal Zone and Central Zone indicate that most granitoids were formed periodically during the Meso- to Neoarchean between 2.97 and 2.62 Ga (from south to north), and plot on a unique crustal array with 176Lu/177Hf = 0.002. This array and the geological-geographic background suggest emplacement of granitoids from different sources (slab melting, enriched mantle wedge and minor crust) over a period of ca. 350 Ma, most likely during successive closure of wide oceanic basin (>10.000 km[1]) located between the Zimbabwe and proto-Kaapvaal Craton. Opening of this basin started ca. 70 Ma earlier, as suggested by Hf model ages of ca. 3.05 Ga, and closure by continent collision occurred at 2.62 Ga. We note that the derived timing for Archean ocean formation and closure is similar to that of modern Wilson cycles (ca. 340 Ma[2]).

[1] Zeh, A. & Kirchenbaur, M. 2022. Precambrian Research 373, 106631. [2]Granot, R. 2016. Nature Geosciences 9, 701-705.