Crustal thickening, exhumation and metamorphic cooling of Neoproterozoic eclogites in NE Brazil: Timescale for the assembly of West Gondwana

Abstract

The high-pressure followed by high-temperature metamorphism related to the evolution of collisional orogens is the 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 area presents an exhumed Archean crust with heterogeneous retrograde eclogites overprinted by amphibolite-facies in NE Brazil. Retrogressed eclogites in the area core are formed by Mn-rich garnet + clinopyroxene porphyroblasts layer with a minor amphibole + quartz + titanite assemblage, whereas the margins display dismembered lenses of Mg-rich garnet porphyroblasts with plagioclase + amphibole coronae and clinopyroxene + plagioclase + ortopyroxene sympleticte mainly within shear zones. Phase equilibria modelling reveals that these distinct retro-eclogites record similar eclogite conditions (up to 18 kbar and 660 ± 20 °C). Mn-rich eclogites display widespread magmatic zoned zircon grains from Mesoarchean to Paleoproterozoic and recrystallized metamorphic grains of 607.7 ± 5.4 Ma. Zircon from Mg-rich eclogites crystallized at 2.67 Ga and overgrowth rims formed at 2.0 Ga and 590.1 ± 1.8 Ma. REE in zircon lack Eu anomaly, displaying flat HREE patterns in 590.1 ± 1.8 Ma Neoproterozoic grains indicate recrystallization under eclogite-facies conditions. In situ U-Pb titanite dating yielded two younger age sets at 574.7 ± 0.5 and 562.8 ± 2.1 Ma. Lastly, in situ U-Pb dating of rutile inclusions in garnet cores present ages of 606.1 ± 4.0 Ma, whereas rutile included in retrograde coronae and sympleticte yielded concordant ages from 559.0 ± 2.3 to 523.2 ± 1.9 Ma. Thus, combined phase equilibria modelling and U-Pb ages reveal a progressive change in metamorphic conditions from eclogite conditions (at least 607.7 ± 5.4 to 590 ± 1.8 Ma; zircon dating and rutile ages included in eclogite garnet) to nearly isothermal decompression (574.7 ± 0.5 Ma; titanite age) of ancient Archean deep crust, which was followed by suggested final exhumation and metamorphic cooling at shallower crustal levels between 559.0 ± 2.3 and 523.2 ± 1.9 Ma (rutile age in retrogressed assemblage). Therefore, the high-pressure conditions followed by decompression and metamorphic cooling during sin- to post-collisional settings were associated with the final assembly of West Gondwana.