Thickened crust, fast exhumation and slow metamorphic cooling of Neoproterozoic eclogites in NE Brazil: Phase modelling, timescale and tectonic implications for the assembly of West Gondwana

The high-pressure (HP) followed by high-temperature (HT) metamorphism related to the evolution of collisional orogens is the major expected Pressure-Temperature-time (P-T-t) path for modern-style plate tectonics. However, these P-T-t paths are rarely preserved even in Phanerozoic orogens. In this matter, the Campo Grande migmatite-gneiss dome presents an exhumed Archean crust with heterogeneous retrograde eclogites overprinted by amphibolite-facies in NE Brazil. Retrogressed eclogites in the dome core are formed by Mn-rich garnet + clinopyroxene porphyroblasts layer with a minor (<10%) amphibole + quartz + titanite assemblage, whereas the dome margins display dismembered lenses of Mg-rich garnet porphyroblasts with Pl+Amph coronae e Cpx+Pl+Opx sympleticte mainly within shear zones. Phase modelling reveals that these distinct retro-eclogites record similar eclogite conditions (up to 18 kbar and 650 ± 20°C). Mn-rich dome-core eclogites display widespread magmatic zoned zircon grains from 3.2 to 2.1 Ga old and recrystallized metamorphic grains of 603 ± 10.4 Ma. Mg-rich dome-margin eclogites were crystallized at 2.65 Ga, with 2.98 Ga inherited zircon grains, and two overgrowth rims at ~2.0 Ga and 590±1.8 Ma. REE in zircon displays slightly positive Eu anomaly and flat HREE patterns in 590 ± 1.8 Ma Neoproterozoic grains indicate recrystallization under eclogite-facies conditions. In situ U-Pb titanite dating yielded slightly younger ages from 580 ± 2.7 to 557 ± 3.9 Ma. Lastly, in situ U-Pb dating of rutile inclusions in garnet cores present ages of 604 ± 4 Ma, whereas rutile included in retrograde coronae and sympleticte yielded concordant ages from 559 ± 2.3 to 523 ± 1.6 Ma. Thus, combined phase modelling and U-Pb ages reveal a progressive change in metamorphic conditions from eclogite conditions (at least 603±10.4 to 590±1.8 Ma; zircon dating and rutile ages include in HP garnet) to nearly isothermal decompression (580 ± 2.7 Ma; titanite age) of ancient Archean deep crust, which was followed by suggested final exhumation and metamorphic cooling at shallower crustal levels between 559 ± 2.3 and 523 ± 1.6 Ma (rutile age in retrogressed assemblage). Therefore, the high-pressure conditions and subsequent exhumation of deep ancient protoliths were reached during sin- to post-collisional settings related to the assembly of West Gondwana.