Rhyacian TTG rocks in the Mineiro Belt, Brazil: constraints on sources and magmatic processes

Tonalite-trondhjemite-granodiorite (TTG) rocks form the backbone of the Earth’s continents. Typically formed during the Archean era (>2.5 billion years ago), they have been preserved within cratons in response to younger lithospheric additions, reworking, and erosion. Their formation is associated with the melting of hydrous basaltic rocks, which leaves amphibolite or eclogite residues. Fractional crystallization and/or mineral accumulation processes could explain magma differentiation, implying mineral and chemical variabilities. In the Paleoproterozoic Mineiro Belt, southern São Francisco Craton in southeastern Brazil, TTG rocks are exposed as large plutons, suites, and complexes that preserve magmatic textures and locally show features of partial melting. Representative rocks range from tonalite to trondhjemite and are composed of plagioclase, quartz, biotite, minor to absent K-feldspar and amphibole, and accessory minerals, such as zircon, titanite, epidote, and apatite. Plagioclase crystals locally form centimetric aggregates that suggest cumulus textures. Their chemistry shows moderate to highly fractionated rare earth element (REE) patterns [(La/Yb)_N >20] with positive to absent Eu anomalies and elevated Sr/Y ratios (>50). Large ion lithophile element (LILE) content is high (e.g., Sr + Ba >800 ppm), while high field strength element (HFSE) and mantle-compatible element (e.g., V, Ni, Cr) content is low. These chemical and mineralogical characteristics suggest melting of basaltic rocks at high temperature (>850 °C) and medium pressure (7 kbar) conditions. Amphibole fractionation and plagioclase accumulation could explain the medium-REE depletion and positive Eu anomalies, as well as the highest Sr/Y ratios. Zircon crystals reveal successive TTG magma crystallization at 2.15, 2.13, and 2.11 Ga and a single partial melting event at 2.04 Ga. Their radiogenic Hf compositions suggest major Paleoproterozoic and minor Archean sources. Crustal assimilation and magma contamination are ruled out due to the absence of field relationships, chemical heterogeneities, and minor inherited zircon grains. The silicon stable isotope signatures of zircon crystals are lighter compared to Archean TTGs, suggesting a less significant silicification process of the source before melting. This correlates with the secular evolution of dissolved silicon in the oceans. Source fertilization and LILE enrichment were likely caused by oceanic crust alteration and the formation of secondary hydrous minerals.