Evidence for a 3.2–3.1 Ga accretionary orogeny along the south-eastern edge of the Kaapvaal Craton: a regional setting for late-stage gold mineralisation in Barberton

Taylor, Jeanne1,2

1Department of Earth Sciences, Stellenbosch University, South Africa. 2Institute of Geoscience, Goethe University Frankfurt am Main, Germany.

The Barberton Granite-Greenstone Belt (BGGB) of South Africa is one of only a few exceptionally preserved Paleo- to Mesoarchaean terranes in the world able to advance our understanding of tectonic processes operating on the early Earth. Together with the Pilbara Craton of Western Australia, it has fuelled the debate of uniformitarian (modern-style subduction-accretion) versus non-uniformitarian (vertical) geodynamic models for Archaean crustal evolution. Barberton is also known for its world-class lode-gold deposits, which formed late in its tectonic evolution during at least two episodes, at 3080 Ma and at 3040–3015 Ma. However, the thermal-tectonic processes responsible for gold mineralisation remain somewhat cryptic.

The Ancient Gneiss Complex (AGC) of Swaziland constitutes a fragment of pristine 3.7–2.7 Ga continental crust in direct contact with the south-eastern margin of the BGGB, and represents a unique opportunity to interrogate contrasting tectonic models. A large body of recent data from 3.23–3.22 Ma granitic rocks, and high-grade, aluminous clastic meta-sediments deposited at 3.53–3.43 Ga and ~3.2 Ga, is largely inconsistent with a non-uniformitarian geodynamic model. Granulite-facies meta-sedimentary rocks display extraordinarily complex metamorphic histories within single hand-samples, which have been deciphered by high-resolution, in situ dating of accessory phases. These rocks seemingly experienced an early thermal episode at 3.43 ̶ 3.40 Ga, followed by two granulite-facies events at 3.23 Ga and 3.15 ̶ 3.05 Ga. Peak metamorphic conditions of 830–875 °C and 6.5–7.6 kbar were reached by 3.11–3.07 Ga, accompanied by extensive in situ partial melting. A final retrograde thermal overprint took place at 2.73 Ga involving rehydration and further decompression. Syn- to post-peak metamorphic (i.e., 3.11–3.07 Ga) deformation fabrics in the granulites (NW-SE directed compression and synchronous NE-SW extension) are similar in character, and coaxial with large-scale deformation features in Barberton and the surrounding 3.23–3.07 Ga AGC granites.

Detrital zircon U-Pb age spectra from clastic meta-sediments deposited at ~3.2 Ga display patterns comparable to those found in modern convergent margin settings (after Cawood et al. 2012), in particular, trench and fore-arc basin environments. Significantly, detrital zircon εHf isotope signatures indicate that these (meta)-sediments were largely derived from an isotopically distinct (i.e., younger, more juvenile) source terrane compared to the BGGB or the AGC, such as a ≤ 3.32 Ga primitive island arc. Detrital zircon age data, combined with in situ dating of metamorphic monazite inclusions in the cores of high-grade garnets (from the same samples), further testify to the rapid, deep burial of these meta-sediments soon after their deposition to ± 25–30 km crustal depths by ~3.10 Ga. In combination, the AGC data provide strong evidence for a long-lived ~3.2–3.1 Ga accretionary margin (involving north-westward subduction) along the south-eastern edge of the proto-Kaapvaal Craton, and support the idea that gold mineralisation in Barberton was linked to late orogenic, transtensional tectonics following terrane accretion. Further connections can be made with long-lived southward subduction and tectonic accretion in the Pietersburg Block (the northernmost terrane of the Kaapvaal Craton), during a comparable time interval between 3.15 and 2.97 Ga.


Biography

My PhD and Postdoctoral research has focused on geodynamic processes operating on the early Earth and the evolution of Archaean cratons; high-grade metamorphism, partial melting of the crust and the production and extraction of granitic magma from their source; radiogenic isotope dating and its behaviour in accessory phases during poly-metamorphism.

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