The Neoarchean transition between medium- and high-K granitoids : Clues from the Southern São Francisco Craton (Brazil)

Field observations, geochemical data and LA-ICP-MS U-Pb ages for orthogneisses and granitoids from the Southern São Francisco Craton (SE, Brazil) indicate a major change in the composition of the continental crust during the Neoarchean. The crystalline basement comprises fine-grained banded gneisses that are intruded by leucogranitic sheets and dikes, by large granitoid batholiths and by small granitic bodies closely associated with the host banded gneisses. Granites and gneisses have high silica content (70-76wt.%) and, based on their major and trace element composition are subdivided in medium- and high-K granitoids. The distinctive geochemistry between these rock-types reflects melting of different sources. Medium-K rocks have chemical affinity similar to rocks of the tonalite-trondhjemite-granodiorite series (TTGs): high Na2O, Al2O3 and Sr content, low heavy-rare earth content. However, their overall chemical features suggest mixing between two end-members: a magma generated by partial melting of metamafic rocks and a component derived by recycling TTG rocks of older continental crust. The composition of high-K granites is similar to that of late-Archean biotite and two-mica granites. These rocks cannot be generated by melting of TTGs, but rather require melting of Archean metasediments capable of producing large volumes of K2O-rich magmas. Medium-K rocks were volumetrically dominant during two earlier periods of magmatism and associated greenstone belt deposition (2920-2850 and 2800-2760Ma), while high-K rocks mark the final cratonization event and consolidation of the granitic crust of the craton between 2750 and 2720Ma. The relatively sharp transition between the two rock types reflects the onset of basin deposition during the Neoarchean followed by melting of the neo-formed basins at depth.