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PhD Tittle: Southern Brasília Orogen: From subduction to collision (unravelling the evolution from the arc generation to the suture) PhD Sandwich Plan: Migmatization in the Southern Brasília Orogen: unravelling the pressure-temperature-time evolution and its relation to the building of the Brasília Orogen

Author Mahyra TEDESCHI
Director of thesis Professor Daniela Rubatto - University of Bern
Co-director of thesis Professor Joerg Hermann -University of Bern
Summary of thesis

The thesis aims to investigate the origin and metamorphism of the rocks from Socorro-Guaxupé nappe in the southern Minas Gerais, part of the Meridional Brasília Orogen. The Sandwich Project has the purpose of explore the process leading to, and the conditions and timing of high-grade metamorphism and partial melting of this crustal section, and the consequences that this metamorphic event had on the building of the orogen. The dynamics of crustal melting (timing, duration and productivity) in collisional settings has major implications for the evolution of the overriding orogeny being able, for example, to change the rheology of the thickened crust and trigger exhumation. The combination of detailed geochronological studies and the P-T paths is the most effective way to understand the dynamic of collision and the process leading to migmatization, being the aim of the Doctorate Sandwich project. Professor Daniela Rubatto which has long pioneered the use of trace element geochemistry and accessory mineral thermometry in high grade terranes to achieve correct age interpretation and detailed P-T-time paths, as well as has been using oxygen isotope analysis to track fluids and melts in metamorphic systems, will be the supervisor of this project. The Neoproterozoic NNW-SSE trending southern Brasília Orogen is located in southeastern Brazil. Previous studies have described the southern portion of the orogen as the result of the collision between the passive margin of the Sao Francisco paleocontinent from the east, with the active margin of the Paranapanema block, on the western side of the suture (subduction from east to west). The collision produced a set of flat lying nappes along the suture verging to the east. The Socorro-Guaxupé Nappe corresponds to a segment of the lower and middle continental crust being mainly composed of orthogneisses of calc-alkaline signature with crystallization ages in the range 670 and 625 Ma and negative εNd (t) values (20 to 13), interpreted as an active continental margin setting. Two ages of migmatization have been proposed: 625-635 Ma in the granulitic rocks and 617-613 Ma in the amphibolitic rocks. The reasons for different intervals have been assigned to the accretionary stage and to the collisional stage, consecutively. Focusing on the Socorro-Guaxupé unit, a detailed map at a scale of 1:100.000 was realized in the first year of the doctorate together with a cross-section in the retro-eclogite, allowing the differentiation and evaluation of the gneisses and migmatitic structures that will be examined in this project. During the internship, research will focus on the partial melting by investigating the migmatites. Questions to be answered are: When did migmatization occur? Is the distribution of ages homogenous across the area or are there patterns of age distribution? How long did melting last and what was the chronological evolution (i.e., timing and duration) of melting across the belt? Which stage of metamorphism do the zircon and monazite U-Pb ages really date? Are there differences between the migmatization processes in the granulitic and the amphibolitic rocks? Has the migmatization of the lower and upper plate occurred at the same time? Is the migmatization seen in the field related to compressional tectonic during collision, or to subduction or exhumation in an extensional setting? In the latter, did the migmatization indeed facilitated exhumation of the subducted continental plate by weakening its rheology? The approach considered consists of performing a detailed geochronological study in zircon and monazite crystals together with trace element distribution, oxygen isotopes and the P-T evolution. Knowledge of the age and its paragenesis allows the determination of P-T-time-fluid paths, and linking the migmatization event with the orogen related events (e.g., collision, exhumation). Both leucosome (representing the melt) and mesosome (restite or non-melted portion) samples will be investigated. The P-T evolution of the samples and distribution of accessory minerals will be investigated using a mapping approach with XMapTools, a program for electron microprobe X-ray image processing used to estimate the pressure–temperature conditions of crystallization of minerals in metamorphic rocks. This approach will be the base for thermobarometry and for the construction of the pseudosections of the leucosome versus mesosome samples. U, Th and Pb analyses of the selected domains will be obtained with the sensitive high-resolution ion microprobes (SHRIMP II) at USP and with the LA-ICP-MS at the University of Bern and Lausanne. After geochronological analyses the domains will be investigated for trace element with a Laser Ablation-ICPMS in the laboratories of the University of Bern, in Switzerland. Trace element patters of dated domains will be used to relate ages to major mineral assemblages. The studied samples do not contain significant garnet, usually used in these studies, but the main mafic phases are amphibole and pyroxene. In this sense, this project will use a similar approach as pioneered for garnet, and asses if the trace element composition of monazite or zircon reflects the abundance of amphibole or pyroxene in the rock. Being able to relate zircon chemistry to the presence of such major minerals will provide constraints for age interpretation. Such a geochemical indicator has the potential to become a versatile petrological tool for application in a large number of high grade terranes of tonalitic-granodioritic, as those broadly found in Brazil. Since oxygen isotopes within a rock system can be modified by the influx of external fluids or melts, they will be used, in zircon and monazite, for tracking the influx of fluids, particularly in the earlier stages of melting. The hypothesis that partial melting can significantly shift the oxygen isotopic composition of coexisting zircon in leucosome versus melanosome will be tested.

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