A linear Hf isotope-age array despite different granitoid sources and complex Archean geodynamics: Example from the Pietersburg block (South Africa)

- Zircon Hf isotope-age arrays are interpreted as internal crust reworking.
- New zircon U-Pb and Lu-Hf data for 33 samples of the northern Kaapvaal Craton.
- They stem from various sources in different settings, yet define a Hf-age array.
- The dominant crustal lithology shapes Hf-age arrays, regardless how it is reworked.
- Information about zircon-hosting rocks is needed to interpret Hf-age arrays.

Combined U-Pb and Lu-Hf isotope data from zircon populations are widely used to constrain Hadean-Archean crustal evolution. Linear Hf isotope-age arrays are interpreted to reflect the protracted, internal reworking of crust derived from the (depleted) mantle during a short-lived magmatic event, and related 176Lu/177Hf ratios are used to constrain the composition of the reworked crustal reservoir. Results of this study, however, indicate that Hf isotope-age arrays can also result from complex geodynamic processes and crust-mantle interactions, as shown by U-Pb and Lu-Hf isotope analyses of zircons from well characterized granitoids of the Pietersburg Block (PB), northern Kaapvaal Craton (South Africa).Apart from scarce remnants of Paleoarchean crust, most granitoids of the PB with ages between 2.94 and 2.05 Ga (n=32) define a straight Hf isotope-age array with low 176Lu/177Hf of 0.0022, although they show a wide compositional range, were derived from various sources and emplaced successively in different geodynamic settings. The crustal evolution occurred in five stages: (I) predominately mafic crust formation in an intra-oceanic environment (3.4-3.0 Ga); (II) voluminous TTG crust formation in an early accretionary orogen (3.0-2.92 Ga); (III) internal TTG crust reworking and subduction of TTG-derived sediments in an Andean-type setting (2.89-2.75 Ga); (IV) (post-)collisional high-K magmatism from both mantle and crustal sources (2.71-2.67 Ga); and (V) alkaline magmatism in an intra-cratonic environment (2.05-2.03 Ga).The inferred array results from voluminous TTG crust formation during stage II, and involvement of this crust during all subsequent stages by two different processes: (i) internal crust reworking through both partial melting and assimilation at 2.89-2.75 Ga, leading to the formation of biotite granites coeval with minor TTGs, and (ii) subduction of TTG-derived sediments underneath the PB, causing enrichment of the mantle that subsequently became source for high-K granitoids and mafic rocks at 2.68 and 2.05 Ga. Some scatter along the array might have resulted either from significant assimilation of ancient crust, intracrustal Lu/Hf fractionation or melting of heterogeneous mantle sources. Those results show that without any information about the nature and composition of zircon-hosting granitoids, Hf isotope-age data are of limited use to constrain Hadean-Archean magmatogenesis and geodynamics.