Titanite petrochronology of the southern Brasília Orogen basement: Effects of retrograde net-transfer reactions on titanite trace element compositions

Abstract

Abstract Combining U-Pb geochronology and trace element characteristics of accessory minerals more reactive during metamorphism and deformation than zircon, such as titanite (CaTiSiO5), is a powerful tool for elucidating pressure-temperature-time (P-T-t) evolution of orogenic basement complexes. This study reports in-situ titanite U-Pb and trace element data from upper-amphibolite facies metatexitic orthogneisses of the Pouso Alegre Complex in the basement of the Neoproterozoic southern Brasilia Orogen, SE Brazil. The titanite data is combined with thermodynamic modeling, major phase compositions and U-Pb zircon dating and used to constrain the P-T-t path of the complex during the Neoproterozoic orogenesis. The investigated garnet amphibole gneisses record peak metamorphic conditions of 750–775\u202f°C and 10.5–12.5\u202fkbar, associated with partial melting and dated at ca. 630–620\u202fMa based on U-Pb analyses of HREE+Y depleted titanite cores, inferred to have crystallized in the presence of garnet near the peak P-T conditions. Extensive melt crystallization at ca. 616\u202fMa is constrained by U-Pb dating of leucosome zircons. The exhumation of the complex and re-equilibration to ca. 550–625\u202f°C at 6–9\u202fkbar led to widespread generation of biotite\u202f+\u202fepidote/clinozoisite, partially replacing amphibole + garnet along the main foliation. Trace element signatures of these retrograde net-transfer reactions are recorded in the titanite rims which are HREE+Y enriched, strontium depleted and were dated at 605.7\u202f±\u202f6.2\u202fMa. The average zirconium contents of the titanite rims are higher than cores suggesting disequilibrium partitioning of zirconium into titanite during the retrograde path resulting in overestimated Zr-in-titanite temperatures for the rims. The exhumation of the complex was concomitant with the intrusion of folded and recrystallized leucogranite dykes dated at 607.0\u202f±\u202f5.7\u202fMa, attesting the persistence of deformation after their intrusion. Assuming that the Pouso Alegre Complex is part of the Sao Francisco paleocontinent reworked during the Brasilia orogeny, the data constrain the minimum age for the collision between the Sao Francisco paleocontinent and the Paranapanema block at ca. 620–630\u202fMa and agree with the idea of long-lived metamorphism and deformation in the southern Brasilia Orogen.