630 Ma of Archean evolution in a crust segment at the southern São Francisco craton

The São Francisco craton (SFC) has proven to be a natural laboratory for the study of Archean-Paleoproterozoic rocks, due to the record of most Earth’s tectono-magmatic events. Still, many cratonic segments lack isotopic–petrological studies anchored on detailed mapping. This is the case for large portions of the Archean metamorphic complexes in the southern SFC. The late Archean is a period of great global changes, and its study is paramount to understanding past and present geodynamics. Here we present the cartography, petrography, geochemistry, and chronology of a small but geologically complex segment of Archean rocks in the Minas Gerais state, Brazil, focused on the orthogneisses and metagranitoids. The Archean units in the study area crop out in the West of the Jeceaba–Bom Sucesso lineament, which is a suture marked by the sedimentary units of the Bom Sucesso Supergroup (2.6–2.4 Ga), with BIFs. The study area revealed a ca. 630 Ma (3260–2630 Ma) long Paleo- to Neoarchean story, marked by five intervals (3.3–3.1 Ga; 2.84–2.82 Ga; 2.76–2.75 Ga; 2.75–2.69 Ga; 2.66–2.63 Ga). The first interval (3.3–3.1 Ga) is a relevant period for the whole southern SFC and includes the 3256 ± 18 Ma TTG xenoliths in the São Tiago orthogneiss and inherited zircon grains in intrusions of the study area. The second interval (2.84–2.82 Ga) is related to a crystallization age of 2842 ± 3 Ma obtained in the gray gneiss basement (granitic to tonalitic). The Aureliano Mourão batholith has a similar age of 2842 ± 7 Ma and comprises porphyritic granitic to granodioritic rocks with a transitional chemistry from TTG to high-K calc-alkaline. Other São Tiago xenoliths (potassic metatonalites) have an age of 2816 ± 30 Ma. The units formed at 2.84–2.82 Ga are subduction-related. The third interval (2.76–2.75 Ga) is recorded in many units: (i) Capela São Francisco metagranite, a highly potassic porphyritic rock crystallized at ca. 2760 Ma; (ii) biotite-rich equigranular metagranite (2762 ± 10 Ma); (iii) an undated (due to high common Pb) equigranular fine metagranodiorite, with TTG affinity and maximum estimated age of ca. 2760 Ma; and (iv) a quartz-feldspathic orthogneiss (2755 ± 10 Ma). These units were produced in convergent settings and intrude into the Aureliano Mourão batholith, except for the last one. The fourth interval (2.75–2.69 Ga) is associated with: (i) three different ages in the A-type Bom Sucesso metagranite at ca. 2750, 2720, and 2700 Ma; (ii) two meta-leucogranites (Estiva and Moraes) formed at ca. 2690 Ma; (iii) a deformed porphyritic Rio do Amparo granitic orthogneiss crystallized at 2705 ± 6 Ma; and (iv) Lavras granitic orthogneiss with a crystallization age of 2719 ± 8 Ma and cogenetic rocks (2724 ± 11 Ma, pegmatite; 2719 ± 18, fine metagranite). These units were generated in extensional settings. Both the Rio do Amparo and Lavras orthogneisses underwent ca. 2680–2670 Ma high-grade metamorphism. The fifth interval (2.66–2.63 Ga) is related to a major potassic magmatism of the São Tiago complex, which occurred at ca. 2660 and 2630 Ma, in late- and sin-collision settings, respectively. The Archean cratonic stability is attested by the intrusion of the Lavras I mafic dykes at the end of the Archean. The worldwide Archean sodic-to-potassic magmatism transition is blurred in the study area. The potassic magmatism in the study area lasted more than 200 Ma (ca. 2840–2630 Ma), which is uncommon in the Archean worldwide. The study of the area is difficult due to (i) the lack of continuous outcrops, hiding crosscutting relations, and (ii) localized deformation, which results in an irregular distribution of deformation–metamorphism aspects through time. Detailed geological mapping and geochemical-geochronological characterization have prevailed as the strongest tools for such a task.