Episodic weathering in Southwestern Amazonia based on (U[sbnd]Th)/He dating of Fe and Mn lateritic duricrust

Abstract

This study presents the results of mineralogical analysis and (UTh)/He dating of micrometric grains of hematite, goethite, cryptomelane and hollandite from six samples of bauxite, Fe, Mn, Mn-Al-Fe, MnAl and Mn-Al-P duricrusts located in southwestern Amazonia (Brazil). The samples were collected at different elevations to constrain temporal epochs of lateritization and to provide insights into the cratonic landscape evolution of Proterozoic and Paleozoic terrains in Brazil. Three main phases of duricrust formation were recognized, indicating a long period of lateritic weathering: The first phase, preserved as a fragment in Fe duricrust on the Paleozoic sedimentary rocks, began in the Permian (~290 Ma) and lasted until the Late Cretaceous (~65 Ma). The second (and best preserved) phase formed Mn duricrusts on the Paleoproterozoic Apuí lateritic surface (150–250 m a.s.l.) and on the Aripuanã lateritic surface (<170 m a.s.l.) between the Late Cretaceous (~90 Ma) and Oligocene (~32 Ma). The third phase of lateritic formation occurred from Late Oligocene to Early Miocene times (~28 to ~19 Ma). A degradation process, indicated by a goethite cortex on hematite fragments, took place in the Early to Late Miocene (~23 to ~13 Ma), affected all previous lateritic duricrusts, and can be correlated to an epoch of extensive bauxite duricrust formation in the region. These Fe, Mn and Al aggradation episodes indicate strong and widespread polyphasic weathering in southwestern Amazonia since ~290 Ma, as expected on stable cratonic terrains exposed to alternating hot, humid, and dry climate conditions over time.