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Lithology plays a fundamental role in rock weathering and erosion, and in landscape evolution. When weathering‐ and erosion‐prone lithologies are protected from erosion by more resilient rock types (e.g., quartzites and banded iron formations) unusual weathering products result. At the Southern Espinhaço Range, Minas Gerais, Brazil, bauxitic weathering profiles are found in a unique geomorphological–lithological–climatic setting. Resistant quartzites acted as a barrier against erosion of interbedded hematite‐phyllite lenses, channelling solution flows and facilitating the formation of deep weathering profiles. The long‐term exposure of the hematite‐phyllites under alternating wet and dry tropical climates favoured widespread bauxitization. Here we investigate the geochemical, mineralogical, geochronological and micromorphological signatures of scaffolded bauxites in order to reconstruct their evolutionary history. Our data suggest that recurrent aluminium and iron mobilization within the profiles were mainly driven by mineral dissolution‐reprecipitation mediated by bioturbation and the influx of vegetation‐derived organic species. (U–Th)/He geochronology of Al‐goethite reveals that bauxitization started at least since the Lower Miocene, with important intensification of weathering in the Upper Miocene and Lower Pleistocene. The adjacent resilient quartzites acted as scaffolds for bauxitization and supported the preservation of more erodible weathering profiles developed over phyllites. Surface waters that could not infiltrate into the impermeable adjacent quartzites preferentially infiltrated into the more weathereable phyllites, enhancing their porosity and permeability, further enhancing weathering. The evolutionary history of Southern Espinhaço Range bauxites suggests a new model of bauxitization in ancient land surfaces evolution underlain by quartzites, where erosion‐prone lithologies are scaffolded by resilient quartzites and survive long‐term weathering with minimum erosion, producing bauxites. [ABSTRACT FROM AUTHOR]

Abstract: Urucum is one of the youngest banded iron-formations (BIFs) yet its exact age remains uncertain. 40Ar/39Ar geochronology on late-diagenetic to early metamorphic cryptomelane from the Urucum sequence reveals a minimum depositional age of 587±7Ma. Metamorphic braunite age spectra yield flat segments defining apparent ages of 547±3Ma to 513±4Ma, interpreted as recrystallization or cooling ages. Metamorphic muscovite grains from a meta-arkose interbedded with the BIF yield reproducible plateau ages of 513±3Ma. Cryptomelane ages are interpreted to record the minimum timing of the extensional tectonic events that generated the graben systems at the southeastern border of the Amazon craton, including the Chiquitos-Tucavaca aulacogen and the Jacadigo Basin, which hosts the Urucum banded iron-formation. Structurally controlled hydrothermal alteration resulted in crystallization of braunite and muscovite during the interval between 547 and 513Ma coeval with post-collisional decompression that resulted in granite emplacement (e.g., São Vicente Granite) in the Paraguay Belt. Subsequent uplift and erosion of the Precambrian sequence possibly succeeded metamorphism. A chemically distinct cryptomelane generation identified in surface samples yield ca. 60Ma results. These ages provide evidence for supergene recrystallization after exhumation in the Mesozoic/Cenozoic. 40Ar/39Ar systematics implies that the oldest cryptomelane samples experienced Ar-loss probably via thermally induced diffusion. This suggests that the Urucum BIF was buried and heated to significant temperatures in the late Proterozoic, consistent with present mineralogy and isotopic composition that do not reflect the original characteristics of the depositional environment. [Copyright &y& Elsevier]