Lateritic formations with iron crusts are very frequent in the Amazon. They, in addition to their great importance in containing a large part of the most voluminous ores (iron, manganese, bauxite and kaolin), may be valuable proxies for paleoclimatic reconstitution. In the Carajás region, the oldest lateritic formations are well represented on the high surface (plateaus) and the youngest on the low landscape. The latter have practically not yet been investigated, unlike those of the plateaus, and, considering their paleoenvironmental importance, we carried out a chemical-mineral characterization and geochemical discussion of these crusts (massive, nodular and with lithorelics) sampled in the geological domains of Canaã and Rio Maria. They were analyzed by optical microscopy, XRD, SEM/EDS and whole chemistry. The main minerals are hematite, goethite, kaolinite, quartz, in addition to chromite and possible anatase. SiO2, Fe2O3 and Al2O3 are the mains chemical constituents, consequently. These data, along with the interpretations of trace elements and rare earths, allowed us to conclude that the crusts were derived from both granitoids and mafic-ultramafic rocks (high Cr-Ni). The crusts can be well correlated to the immature lateritic profiles of the Amazon, formed during the Miocene-Pliocene and partially weathered in the Pleistocene already under hot and humid climate. Its low land terrains, suggests that after the partial denudation of the now elevated terrains of the Carajás Plateaus, exposing the rocky and saprolite under dry, sub-arid climate, and then a hot and humid climate was established in that area, promoting lateritic formation, in which the crusts represent its upper portion. Resumo Formações lateríticas com crostas ferruginosas são muito frequentes na Amazônia. Elas, além de sua importância por conterem grande parte dos minérios mais volumosos (ferro, manganês, bauxita e caulim) da região e País, podem ser valiosos registros para reconstituição paleoclimática. Na região de Carajás, as formações lateríticas mais antigas estão bem representadas nos platôs e as mais jovens nos terrenos mais baixos. Estas últimas praticamente ainda não foram investigadas, ao contrário daquelas dos platôs, e, considerando sua importância paleoambiental, esta pesquisa foi desenvolvida visando sua caracterização químico-mineral e discussão geoquímica. Amostras de crostas ferruginosas (maciças, nodulares e com litorelictos) dessa região foram coletadas nos domínios geológicos de Canaã e Rio Maria e analisadas por microscopia ótica, DRX, MEV / EDS e química total. Os principais minerais são hematita, goethita, caulinita, quartzo, além de traços de cromita e anatásio. Consequentemente, SiO2, Fe2O3 e Al2O3 são os principais constituintes químicos. Esses dados, juntamente com as interpretações de elementos traço e terras raras, nos permitiram concluir que as crostas derivaram tanto de granitóides quanto de rochas máfico-ultramáficas (alto Cr-Ni). Todas as características dessas crostas confirmaram sua relação com as formações lateríticas imaturas da Amazônia, formadas durante o Mioceno-Plioceno, parcialmente intemperizadas no Pleistoceno já sob clima quente e úmido. Sua paisagem, sugere que após a denudação parcial dos já elevados terrenos de Carajás, rochas sãs e saprólitos foram expostos sob clima seco e semiárido, sendo em seguida afetados por clima quente e úmido que promoveu a formação laterítica imatura, em que as crostas investigadas representam sua porção superior.
The upland lakes (ULs) in Carajás, southeastern Amazonia, have been extensively studied with respect to their high-resolution structural geology, geomorphology, stratigraphy, multielement and isotope geochemistry, palynology and limnology. These studies have generated large multiproxy datasets, which were integrated in this review to explain the formation and evolution of the ULs. These ULs evolved during the Pliocene–Pleistocene periods through several episodes of a subsidence of the lateritic crust (canga) promoted by fault reactivation. The resulting ULs were filled under wet/dry and warm/cool paleoclimatic conditions during the Pleistocene period. The multielement geochemical signature indicates that the detrital sediments of these ULs were predominantly derived from weathered canga and ferruginous soils, while the sedimentary organic matter came from autochthonous (siliceous sponge spicules, algae, macrophytes) and allochthonous (C3/C4 canga and forest plants and freshwater dissolved organic carbon) sources. Modern pollen rain suggests that even small ULs can record both the influence of canga vegetation and forest signals; thus, they can serve as reliable sites to provide a record of vegetation history. The integrated data from the sedimentary cores indicate that the active ULs have never dried up during the last 50 ka cal BP. However, subaerial exposure occurred in filled ULs, such as the Tarzan mountain range during the Last Glacial Maximum (LGM) and the Bocaína and S11 mountain ranges in the mid-Holocene period, due to the drier conditions. Considering the organic proxies, the expansion of C4 plants has been observed in the S11 and Tarzan ULs during dry events. Extensive precipitation of siderite in UL deposits during the LGM indicated drier paleoenvironmental conditions, interrupting the predominantly wet conditions. However, there is no evidence of widespread forest replacement by savanna in the Carajás plateau of southeastern Amazonia during the late Pleistocene and Holocene.
The Vila Jussara Suite (VJS) is formed by Neoarchean (~ 2.75 Ga) granites that are located in the Sapucaia Domain of the Carajás Province (CP), Amazonian Craton. Four petrographic varieties were identified in the VJS: biotite-hornblende monzogranite (BHMzG); biotite-hornblende tonalite (BHTnl); biotite monzogranite (BMzG); and hornblende-biotite granodiorite (HBGd). In terms of magnetic signature, BHMzG has two subgroups: the first subgroup has low magnetic susceptibility (MS) values (0.16 × 10 -3 to 0.81 × 10 -3 ) and more commonly contains ilmenite with titanite rims; the second subgroup shows moderate to high MS (1.91 × 10 -3 to 6.02 × 10 -3 ) and magnetite dominant over ilmenite. BHTnl has moderate MS (0.85 × 10 -3 to 1.36 × 10 -3 ) and dominance of pyrite followed by magnetite. BMzG and HBGd have comparatively high MS (3.35 × 10-3 to 19.3 × 10 -3 and 2.14 × 10 -3 to 25.0 × 10 -3 , respectively), with magnetite dominant over pyrite. The granite varieties of the VJS were formed under different oxygen fugacity (fO2) conditions, varying from reducing (< fayalite-magnetite-quartz (FMQ)) to oxidizing conditions (nickel-nickel oxide (NNO) to NNO+1). In addition, biotite-hornblende syenogranite occurs subordinately and shows high MS values and extremely high FeO t /(FeO t + MgO) ratios, both in whole-rock and amphibole and biotite. The granites of the VJS are similar to other Neoarchean granites of the Carajás province. The reduced VJS granites are akin to the ferroan granites of Planalto suite, Estrela Complex and Vila União area and the magnesian granites of VJS approach the magnesian granites of Vila União area.
We present this manuscript as a methodological approach and general guidelines for geochemical mapping and background/baseline projects for environmental assessment in tropical areas. A case study was carried out in the Itacaiúnas River watershed (IRW), Eastern Amazon, to fill in a gap in knowledge on the distribution of chemical elements, particularly those potentially toxic, in the near-surface environment of the area. The high-impact results of this research project revealed the need for similar scientific investigation across the globe with the implementation of a systematic methodology. The study shows, for example, the importance of well-planned field activities, multi-medium sampling, analytical methods, laboratory procedures, database construction, and general aspects of data processing and statistical treatment. The importance of this contribution is that it can be used as a reference in support of geospatial analysis in research within the scope of geochemical mapping and background-baseline projects. The database is accessible through a web-based geographic information system front-end; a Geochemical Atlas of the IRW will be available as soon as possible.
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