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Título

Pressure-temperature-time and tectonic evolution of the Southern Espinhaço Front in Serra do Cipó – Minas Gerais

Texto do resumo

The Southern Espinhaço Front in Serra do Cipó marks the boundary between the São Francisco Craton (west) and the Ediacaran-Cambrian Araçuaí-West Congo Orogen (east), defined by a major thrust heaving the Mesoproterozoic Espinhaço Supergroup over the Ediacaran-Cambrian Bambuí Group. The west-verging fold-and-thrust belt (FTB) is essentially made up of the Espinhaço Supergroup, strongly deformed. The same unit is present in the foreland region (FLD) but as a stable basement to the Bambuí Group, representative of the associated foreland basin. To describe the tectonic evolution of the area, we carried out a multiproxy approach involving field-based structural observations, quantitative strain, quartz crystallographic textures, fluid inclusions in quartz veins and illite crystallinity (IC) analyses, as well as in situ muscovite Rb-Sr dating and K-Ar dating of fine fractions (< 2 μm and < 0.2 μm) of white mica. At all scales, local structures (reverse faults, shear zones and asymmetric folds), tectonic foliations, stretching lineations and kinematic indicators are coherent with the regional fold-and-thrust system of the Southern Espinhaço Range. In the FLD, the Espinhaço Supergroup sits practically undeformed below a detachment surface separating it from Bambuí Group, which, in turn, accommodated most of the orogenic deformation in the FLD. X-strain axes and flattening planes dip gently towards east. The strain ellipsoids are oblate, with eccentricities (Rf) between 1.4 and 1.8, characterizing flattening strain and suggesting a loss of volume by the rocks. Quartz crystallographic textures indicate internal strain accommodation by dislocation glide with activation of the rhombohedral system (with minor contribution of the basal plane in the FLD, suggesting lower temperatures) in grains above 100 μm in diameter, while finer grains experienced dissolution-reprecipitation. Fluid inclusions are exclusively aqueous and biphasic, lacking gases typical of metamorphic contribution. In addition, inclusions from the FTB and the FLD form statistically separate groups in terms of salinity signature. These data suggest intrabasinal fluid circulation from local sources (formation waters). The IC values indicate epizonal conditions, and metamorphic biotite is present in the FTB and absent in the FLD, serving as a zonal indicator and indicating higher temperature in the former. We estimated PT fields by combining the mineral parageneses, microthermometric data from fluid inclusions, deformation mechanisms associated with microstructures, quartz crystallographic textures and IC values. The FLD experienced 300-411 ºC and 1.7-6.0 kbar, and the FTB experienced 385-450 ºC and 2.8-8.0 kbar. In situ Rb-Sr dating of muscovite yielded an age of 548.8 ± 17.1 Ma in the FTB, attributed to the metamorphic peak. K-Ar ages of white mica < 2 μm were 510.3 ± 5.3 Ma and 495.8 ± 9.3 Ma in the FLD and 476.2 ± 7.5 Ma and 456.1 ± 8.6 Ma in the FTB, suggesting faster cooling of the FLD, and white mica < 0.2 μm were dated at 432.6 ± 7.5 Ma. We interpret the K-Ar ages to represent later deformation stages during cooling that were active until the Mid Silurian. We pose that the deformation can be explained by the critical taper model, with involved progressive thickening of the crustal wedge up to a critical state, when the following deformation increment would cause the basal detachment to propagate into the foreland.

Palavras Chave

Fold-and-thrust belt; strain analysis; Southern Espinhaço Range; tectonics; EBSD.