The Paleoarchean (3.8 to 3.2 Ga), a pivotal time in the Earth’s history, was marked by major geological events such as important crustal growth episodes, craton formation, and a shift in tectonic modes from stagnant lid to mobile lid. The Paleoarchean crustal record, therefore, provides valuable insights into fundamental processes that defined a crucial transitional period in the evolving Earth. In this research we focus on the Mairi complex of the São Francisco craton, Brazil, which is host to the oldest known rocks in South America and preserves an important Paleoarchean rock record [1,2]. Here, we report new zircon U-Pb age and Hf isotope data and whole-rock Hf and Nd isotopes from granitic gneisses and a leucosome from the Mairi complex. The U-Pb data have well-defined zircon populations with 207Pb/206Pb ages at ~3.6Ga. One gneiss yields a 207Pb/206Pb age of 3.4 Ga with evidence of 3.6 Ga inherited zircons. No zircon xenocrysts older than 3.6 Ga were determined in any samples. The Hf isotope compositions of all zircon are consistently subchondritic, with a narrow range of initial ƐHf(3600) from -1.6 to -3.2 and ƐHf(3400) of -3.6. The bulk-rock data, on the other hand, exhibit a large range in Hf and Nd isotope compositions with initial ƐHf(3.6-3.4 Ga) ranging generally from +2 to -5 with a couple of outliers with ƐHf(i) of -8 and -14 and ƐNd(3.6-3.4 Ga) with mostly positive values from +7 to 0, with outliers at -4 and -17. Complex samples with evidence of ancient Pb-loss and multi-component zircons show the higher discrepancies between bulk-rock and zircon isotope compositions, similar to what has been documented in Greenland Eoarchean gneisses [3]. The isotopic variability in whole-rock data reflects mixing of different components during analysis and /or open-system behaviour due to post-crystallization tectono-thermal events. In samples where the age can be tightly constrained, the Hf isotope composition in the zircon is extraordinarily homogenous. Accordingly, we rely our interpretations on the robust zircon Hf isotope Data. The subchondritic Hf isotope compositions of the 3.6-3.4 Ga gneisses indicate derivation from a homogenous reservoir and are consistent with melting and/or reworking/assimilation of ~3.8 Ga pre-Existing chondritic crust. Our interpretation on the genesis of these rocks is at odds with previous studies that suggest Hadean-early Eoarchean precursors for these rocks based on the assumption of derivation from a depleted mantle reservoir [1,2]. Instead, we suggest that the Hf isotope data reported in this study for the 3.6-3.4 Ga Mairi complex gneisses are consistent with derivation from melting of a ~3.8 Ga precursor of broadly chondritic composition, similar to the Hf isotope compositions of other 3.8-3.6 Ga gneisses worldwide [4-7].

Reference(s)

[1] Oliveira et al., 2019, TerraNova; 

[2] dos Santos et al., 2022, Precambrian Research; 

[3] Vervoort and J. Blichert-Toft, 1998, Geochimica et Cosmochimica Acta; 

[4] Amelin et al., 2011, Canadian Journal Earth Sciences; 

[5] Bauer et al., 2017, Earth and Planetary Science Letters; 

[6] Fisher and Vervoort, 2018, Earth and Planetary Science Letters; 

[7] Kemp et al., 2019, Geochimica et Cosmochimica Acta.