Field observations and CA-LA-ICP-MS U–Pb zircon ages and Hf isotope compositions obtained from migmatitic orthogneisses and granitoids from the Belo Horizonte Complex, southern São Francisco Craton, indicate a major period of partial melting and production of felsic rocks in the Neoarchean. Our observations show that the complex is an important site for studying partial melting processes of Archean crystalline crust. Much of the complex exposes fine-grained stromatic migmatites that are intruded by multiple leucogranitic veins and sheeted dikes. Both migmatites and leucogranite sheets are crosscut by several phases of granitoid batholiths and small granitic bodies; both of which are closely associated with the host banded gneisses. Chemical abrasion followed by detailed cathodoluminescence imaging revealed a wide variety of zircon textures that are consistent with a long-lived period of partial melting and crustal remobilization. Results of U-Pb and Hf isotopes disclose the complex as part of a much wider crustal segment, encompassing the entire southern part of the São Francisco Craton. Compilation of available U-Pb ages suggests that this crustal segment was consolidated sometime between 3000 Ma and 2900 Ma and that it experienced three main episodes of partial melting before stabilization at 2600 Ma. The partial melting episodes took place between 2750 Ma and 2600 Ma as a result of tectonic accretion and peeling off the lithospheric mantle and lower crust. This process is likely responsible for the emplacement of voluminous potassic granitoids across the entire São Francisco Craton. We believe that the partial melting of Meso-Archean crystalline crust and production of potassic granitoids are linked to a fundamental shift in the tectonics of the craton, which was also responsible for the widespread intrusion of large syenitic bodies in the northern part of the craton, and the construction of layered mafic–ultramafic intrusions to the south of the BHC.

Compilation of U-Pb data for magmatic zircons in the southern São Francisco Craton. The major peaks of rock crystallization are divided into major periods of magmatic activity known as Santa Barbara (SB), Rio das Velhas I (RVI), Rio das Velhas II (RVII), and Mamona (M) events (modified from Moreira et al., 2016). (B) 2920–2850 Ma Rio das Velhas I event with continental crust formed by multiple additions of TTG magmas and tectonic accretion of mafic and ultramafic rocks (basal units of the Rio das Velhas Supergroup and by greenstone belt remnants around the margin of the Southern São Francisco Craton). (C) Rio das Velhas II event with subduction of an oceanic crust under a continental block and the formation of medium-K granitoids by mixing of different components of the meso-Archean crust. Most of the continental crust formed during the Rio das Velhas II event was produced by reworking older orthogneisses and by partial melting of the mafic oceanic crust. (D) The 2760–2680 Ma Mamona event taking place with tectonic accretion of large and stable continental blocks in the Neoarchean.
 Compilation of crystallization ages vs. metamorphic/migmatization ages for the crystalline rocks intruded in the basement rocks of the Belo Horizonte Complex.
176Hf/177Hf(t) versus apparent 207Pb/206Pb age diagrams for individual analyses with >95% concordance. Reference lines are: depleted mantle according to Griffin et al. (2000) (176Hf/177Hf = 0.283250 and 176Lu/177Hf = 0.0384) and TDM calculations using the 176Lu/177Hf and 176Hf/177Hf ratios of 0.03933 and 0.283294 (Blichert-Toft and Puchtel, 2010), CHUR constants (176Hf/177Hf = 0.282785 and 176Lu/177Hf = 0.0336) of Bouvier et al. (2008). Red symbols are from migmatites and black symbols from granites or leucosomes. Vertical bars indicate Rio das Velhas I (RV I), Rio das Velhas II (RV II) and Mamona magmatic events.