Granitoid diversity in the early Archean felsic crust revealed by geochemical data from detrital and igneous Paleoarchean apatite

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The chemistry of detrital apatite has increasingly been employed for provenance studies and to constrain the composition of the parent magma, yet this tool has to date rarely been applied to the topic of Archean crustal evolution. In this study, we present new halogen and trace element compositions analyzed via EPMA and LA-ICP-MS in detrital apatites collected from Paleoarchean metasediments in the Barberton Greenstone Belt (BGB), South Africa, and in magmatic apatites of the surrounding tonalite-trondhjemite-granodiorite (TTG) granitoid-gneisses. Detrital apatites from both the Hooggenoeg (deposited at ∼ 3.43 Ga) and Schapenburg (deposited at ∼ 3.24 Ga) (meta-)sedimentary rocks retained trace element chemistry consistent with a primary igneous origin. They show strong fractionation of their REE spectra (La N /Lu N = 29.4 and 5.43 respectively) and low Eu/Eu* (∼0.5 and 0.8 respectively). On the other hand, the Hooggenoeg apatites have high Sr (600–1200 ppm) and moderate Mn content (400–800 ppm), are F-rich (∼2.2 wt%) and have Cl content ∼ 0.4 wt%; whereas the Schapenburg apatites have uniformly low Sr (∼200 ppm), higher Mn (800–1800 ppm) together with higher F (∼3.2 wt%) and lower Cl (∼0.01 wt%) contents. Strikingly, the characteristics of neither of these populations match with those of apatites from the Barberton TTGs but rather overlap well with apatites crystallized in Neoarchean granitoids, notably sanukitoids, either low-SiO 2 ones in the case of the Hooggenoeg apatites, or high-SiO 2 for the Schapenburg ones. Therefore, rocks akin to the sanukitoid suite were already present and exposed to surface alteration in the Paleoarchean felsic crust at the time the sediments were deposited. This implies that the felsic crust around the BGB at 3.43–3.24 Ga was more compositionally diverse than the exposed rock record (dominated by TTGs) suggests. In turn, we propose that the onset of observed granitoid diversity in the Neoarchean is an artifact related to a preservation bias, i.e. the higher (upper crustal) emplacement level of K-rich granitoids compared with mid-crustal TTGs made them more prone to erosion before ca. 3 Ga, while the onset of collisional processes at 3.0–2.5 Ga allowed to “shield” these high-level intrusions in the newly amalgamated continental masses and preserve them to our time.