Structural and metamorphic control on the exhumation of high-P granulites: The Carvalhos Klippe example, from the oriental Andrel?ndia Nappe System, southern portion of the Brasília Orogen, Brazil

摘要

High-pressure granulites are formed within the lowermost continental crust. If exhumation is driven only by erosion, these granulites will remain at their site of formation indefinitely. Their exposure, then, requires a complex process. In this study, the Andrelandia Nappe System is used to investigate the formation and exhumation of high-pressure granulites related to subduction-collision-extrusion processes. This nappe system is part of the southern portion of the Neoproterozoic Brasília Orogen in Brazil, which comprises a pile of syn-metamorphic rootless nappes in which high-pressure granulites are an important constituent. The Andrelandia Nappe System is thrusted by a magmatic arc allochthon, and it overrides the allochthonous passive margin that is represented by rocks of the Carrancas Nappe System. Metamorphism within the Andrelandia Nappe System presents an inverted pattern that varies from top to bottom and from granulite to amphibolite facies conditions under a high-pressure regime. The Carvalhos Klippe is part of the uppermost nappe and is located 60 km ahead of its original allochthon. This klippe is composed of rutile-kyanite-garnet-K-feldspar granulites, and it displays ternary feldspars and associated mafic granulites that in turn contain garnet-clinopyroxene-plagioclase-quartz as a peak mineral assemblage. Both granulite types crystallized at a higher pressure than the rocks within the orthopyroxene stability field. The kyanite-bearing granulites show a clockwise P-T path, which is inferred from reaction textures and thermobarometry. Peak conditions were attained at 825 °C and 12 kbar at 617.7 ± 1.3 Ma, which is determined by an ID-TIMS analysis of monazite. The convergent kinematic flow of the klippe was directed eastward, then northeastward, and finally changed northward to northeastward, which is contemporaneous with extension and thinning under high-temperature conditions. Melt-filled dilation shear zones were observed and the melt worked as a lubricant within the shear zones, facilitating granulite exhumation-extrusion.