3.30 Ga high-silica intraplate volcanic–plutonic system of the Gavião Block, São Francisco Craton, Brazil : evidence of an intracontinental rift following the creation of insulating continental crust.

Abstract

High-silica rhyolites having U–Pb zircon ages of 3303±11Ma occur along the eastern border of the Gavião Block (Brazil) associated with the Contendas-Mirante and Mundo Novo supracrustal belts. Unlike many Archean greenstone sequences, they are not interlayered with mafic to intermediate units. Instead, they belong to an inter-related plutonic–volcanic system, together with granitic massifs having similar zircon crystallization ages of ca. 3293 ± 3 Ma and 3328 ± 3 Ma and plotting along the same geochemical trends as the rhyolites. The rhyolites show well-preserved primary volcanic features such as magma flow textures and euhedral phenocrysts. High emplacement temperatures are indicated by petrographic evidence (β-quartz phenocrysts), zircon saturation temperatures (915–820 °C) and geochemical data, especially high SiO2 (74–79 wt.%) togetherwith elevated Fe2O3(T) (~3 wt.%), MgO (0.5–1.5 wt.%) and low Al2O3 (b11 wt.%). The rhyolites show homogeneous trace element ratios (La/YbN 4.8 ± 1.8; EuN/Eu* ~0.55; Sr/Y ~0.7) and negative ԐHf(3.3 Ga) from 0 to −7, indicating derivation from a single crustal source for both occurrences. Specifically, the rhyolites would have derived from extraction and eruption of highly silicic residual liquid formed by crystallization of granitic magma in a relatively shallow (b10 km) reservoir, now represented by the granite massifs. The granite magma was formed by melting or differentiation of material similar to the diorite gneiss that occurs regionally. The 3.30 Ga volcanic– plutonic systems formed after a period of crustal growth and stabilization of a thick continental lithosphere, represented by massive 3.40–3.33 Ga TTG and medium to high-K calk-alkaline magmatism in the Gavião Block. The 3.30 Ga-old rhyolites and graniteswould therefore have formed in an intracontinental tectonic setting after the formation and stabilization of newcontinental crust, and accordinglywould represent the first stages of rifting and continental break-up. Intraplate magmatism and intracrustal differentiation processes took place on Earth at 3.3 Ga and produced magmas that were distinct from Archean TTGs, questioning the reliability (or at least the uniqueness) of “intraplate models” to explain the origin of the latter.