Transcontinental retroarc sediment routing controlled by subduction geometry and climate change (Central and Southern Andes, Argentina)

AbstractCentral Argentina from the Pampean flat‐slab segment to northern Patagonia (27°–41°S) represents a classic example of a broken retroarc basin with strong tectonic and climatic control on fluvial sediment transport. Combined with previous research focused on coastal sediments, this actualistic provenance study uses framework petrography and heavy‐mineral data to trace multistep dispersal of volcaniclastic detritus first eastwards across central Argentina for up to ca. 1,500 km and next northwards for another 760 km along the Atlantic coast. Although detritus generated in the Andes is largely derived from mesosilicic volcanic rocks of the cordillera, its compositional signatures reflect different tectono‐stratigraphic levels of the orogen uplifted along strike in response to varying subduction geometry as well as different character and crystallization condition of arc magmas through time and space. River sand, thus, changes from feldspatho‐litho‐quartzose or litho‐feldspatho‐quartzose in the north, where sedimentary detritus is more common, to mostly quartzo‐feldspatho‐lithic in the centre and to feldspatho‐lithic in the south, where volcanic detritus is dominant. The transparent‐heavy‐mineral suite changes markedly from amphibole ≫ clinopyroxene > orthopyroxene in the north, to amphibole ≈ clinopyroxene ≈ orthopyroxene in the centre and to orthopyroxene ≥ clinopyroxene ≫ amphibole in the south. In the presently dry climate, fluvial discharge is drastically reduced to the point that even the Desaguadero trunk river has become endorheic and orogenic detritus is dumped in the retroarc basin, reworked by winds and temporarily accumulated in dune fields. During the Quaternary, instead, much larger amounts of water were released by melting of the Cordilleran ice sheet or during pluvial events. The sediment‐laden waters of the Desaguadero and Colorado rivers then rushed from the tract of the Andes with greatest topographic and structural elevation, fostering alluvial fans inland and flowing in much larger valleys than today towards the Atlantic Ocean. Sand and gravel supply to the coast was high enough not only to promote rapid progradation of large deltaic lobes but also to feed a cell of littoral sediment transport extending as far north as the Río de la Plata estuary

Congo River sand and the equatorial quartz factory

A never solved problem in sedimentary petrology is the origin of sandstone consisting exclusively of quartz and most durable heavy minerals. The Congo River offers an excellent test case to investigate under which tectonic, geomorphological, climatic, and geochemical conditions pure quartzose sand is generated today. In both upper and lowermost parts of the catchment, tributaries contain significant amounts of feldspars, rock fragments, or moderately stable heavy minerals pointing at the central basin as the main location of the "quartz factory". In Congo sand, quartz is enriched relatively to all other minerals including zircon, as indicated by Si/Zr ratios much higher than in the upper continental crust. Selective elimination of old zircons that accumulated radiation damage through time is suggested by low percentages of grains yielding Archean U-Pb ages despite the basin being surrounded by Archean cratonic blocks. Intense weathering is documented by the lack of carbonate grains in sand and by dominant kaolinite and geochemical signatures in mud. In sand, composed almost entirely of SiO2, the weathering effect is masked by massive addition of quartz grains recycled during multiple events of basin inversion since the Proterozoic. Changes in mineralogical, geochemical, and geochronological signatures across Bas-Congo concur to suggest that approximately 10% of the sand supplied to the Atlantic Ocean is generated by rapid fluvial incision into the recently uplifted Atlantic Rise. The Congo River connects with a huge canyon similar to 30 km upstream of the mouth, and pure quartzose sand is thus funnelled directly toward the deep-sea to feed a huge turbidite fan. Offshore sediments on both sides of the canyon are not derived from the Congo River. They reflect mixed provenance, including illite-rich dust wind-blown from the arid Sahel and augite, hypersthene, and smectite ejected from volcanic centres probably situated along the Cameroon Line in the north. Because mixing of detritus from diverse sources and supply of polycyclic grains almost invariably occurs in the terminal lowland tract of a sediment-routing-system, no ancient sandstone can be safely considered as entirely first-cycle. Moreover, the abundance of pure quartzarenite in the rock record can hardly be explained by chemical weathering or physical recycling alone. The final cleansing of minerals other than quartz, zircon, tourmaline, and rutile requires one or more cycles of chemical dissolution during diagenesis, which operates at higher temperatures and over longer periods than weathering at the Earth's surface

Teoria histórico-cultural e teoria do ensino desenvolvimental: bases para uma epistemologia psicológico-didática do ensino

Resumo: O artigo apresenta considerações de ordem epistemológica sobre as contribuições da teoria histórico-cultural e da teoria do ensino desenvolvimental para o ensino na Educação Básica. Defende-se a tese de que estas duas teorias formam uma epistemologia psicológico-didática que articula dialeticamente ensino e aprendizagem. Apresenta-se conceitos da teoria histórico-cultural e da teoria do ensino desenvolvimental, entendidos como nucleares para a compreensão destas teorias como uma epistemologia, buscando articulá-los ao ensino escolar, com destaque para: o papel da educação escolar no processo geral de desenvolvimento humano; a lei genética geral do desenvolvimento dos processos psíquicos superiores; a zona de desenvolvimento próximo; a mediação; o pensamento teórico e a atividade de estudo. Palavras-chave: Teoria histórico-cultural. Teoria do ensino desenvolvimental. Atividade de estudo. Mediação pedagógica

Teoria histórico-cultural e teoria do ensino desenvolvimental: bases para uma epistemologia psicológico-didática do ensino

Resumo: O artigo apresenta considerações de ordem epistemológica sobre as contribuições da teoria histórico-cultural e da teoria do ensino desenvolvimental para o ensino na Educação Básica. Defende-se a tese de que estas duas teorias formam uma epistemologia psicológico-didática que articula dialeticamente ensino e aprendizagem. Apresenta-se conceitos da teoria histórico-cultural e da teoria do ensino desenvolvimental, entendidos como nucleares para a compreensão destas teorias como uma epistemologia, buscando articulá-los ao ensino escolar, com destaque para: o papel da educação escolar no processo geral de desenvolvimento humano; a lei genética geral do desenvolvimento dos processos psíquicos superiores; a zona de desenvolvimento próximo; a mediação; o pensamento teórico e a atividade de estudo.\ud Palavras-chave: Teoria histórico-cultural. Teoria do ensino desenvolvimental. Atividade de estudo. Mediação pedagógica

ENSINO DESENVOLVIMENTAL E LITERATURA NOS ANOS INICIAIS DO ENSINO FUNDAMENTAL: CONTRIBUIÇÕES PARA O DESENVOLVIMENTO LINGUAGEM ESCRITA

The article presents a synthesis and main findings of a doctoral research developed between 2018 and 2019 on the pedagogical work with literature in the Elementary School Early Years based on the theory of developmental teaching, par­ticularly in the work of Davydov (1982, 1987, 1988). The objective of the investiga­tion was to understand the relationship between pedagogical work with literature and the development of written language in the Elementary School Early Years. We seek to show, from the pedagogical-didactic assumptions of developmental teaching, how the pedagogical work with literature can be organized in order to boost the de­velopment of written language and also the imagination and creativity of students. To achieve these objectives, we carried out a didactic intervention in the fourth early year class of a municipal school in the city of Goiânia, Goiás. The analysis of the study tasks proposed to the students throughout the intervention allows us to affirm that the pedagogical work with the literary text, organized from the psychological and didactic premises of developmental teaching, allowed most students to develop more conscious reasons for the performance of reading and literary production, as well as signs that the tasks performed were related to the development and/or im­provement of certain reading and writing skills were identified and analyzed.O artigo apresenta uma síntese e os principais achados de uma pesquisa de doutorado desenvolvida entre 2018 e 2019 sobre o trabalho pedagógico com a literatura nos Anos Iniciais do Ensino Fundamental fundamentado na perspectiva da teoria do ensino desenvolvimental, particularmente na obra de Davydov (1982; 1987; 1988). O objetivo da investigação foi compreender a relação entre o trabalho pedagógico com a literatura e o desenvolvimento da linguagem escrita nos Anos Iniciais do Ensino Fundamental. Buscamos evidenciar, a partir dos pressupostos pedagógico-didáticos do ensino desenvolvimental, como o trabalho pedagógico com a literatura pode ser organizado de forma a impulsionar o desenvolvimento da linguagem escrita e também da imaginação e da criatividade dos estudantes. Para alcançar esses objetivos, realizamos uma intervenção didática numa turma do quarto ano de uma escola municipal da cidade de Goiânia, Goiás. A análise das tarefas de estudo propostas para os estudantes ao longo da intervenção nos permite afirmar que o trabalho pedagógico com o texto literário, organizado a partir das premissas psicológicas e didáticas do ensino desenvolvimental, possibilitou que a maioria dos estudantes desenvolvesse motivos mais conscientes para a realização da leitura e da produção literária, bem como foram identificados e analisados indícios de que as tarefas realizadas incidiram no desenvolvimento e/ou aprimoramento de determinadas habilidades de leitura e escrita

PROVENANCE OF KARAKUM DESERT SAND (TURKMENISTAN): LITHIC-RICH OROGENIC SIGNATURE OF CENTRAL ASIAN DUNE FIELDS

Major deserts do not necessarily consist of eolian dunes as quartz-rich as those of the Great Nafud, Rub’ al Khali, Sahara, and Mega-Kalahari sand seas accumulated in anorogenic settings of Arabia and Africa. The Karakum dune field of Turkmenistan is one of the several examples of central Asian deserts bound by recent or recently reactivated orogenic belts where eolian sand includes abundant sedimentary and subordinately metamorphic and volcanic lithic fragments. Feldspatho-litho-quartzose detrital modes and epidote-amphibole-garnet heavy-mineral suites of southern Karakum dune sand compare well with those in mountain branches of the Amu Darya, indicating provenance from the western Pamir mountains of Tajikistan in the east. Dunes closer to the Caspian Sea in the west contain additional carbonate or felsic volcanic grains which, together with decreasing heavy-mineral concentration and increasing ZTR indices, reveals local recycling of cover strata exposed in the Kopeh-Dagh and Balkhan zone. Our data suggest that the huge Amu Darya River, which in Plio-Pleistocene to historical times has repeatedly changed its course across Turkmenistan from westward toward the Caspian Sea to northward toward the Aral Sea, represents the major sediment source for the Karakum Desert. Together with the Taklamakan sand sea of the Tarim basin, the Ordos and adjacent deserts of northern China, and the Thal and Thar deserts of the western Himalayan foreland basin, the Karakum would thus represent another example of large dune field principally fed by one major fluvial feeder system

Diagenetic control on mineralogical suites in sand, silt, and mud (Cenozoic Nile Delta): Implications for provenance reconstructions

This Nile Delta case study provides quantitative information on a process that we must understand and consider in full before attempting provenance interpretation of ancient clastic wedges. Petrographic and heavy-mineral data on partly lithified sand, silt, and mud samples cored from the up to 8.5 km-thick post-Eocene succession of the offshore Nile Delta document systematic unidirectional trends. With increasing age and burial depth, quartz increases at the expense of feldspars and especially of mafic volcanic rock fragments. Heavy-mineral concentration decreases drastically, transparent heavy minerals represent progressively lower percentages of the heavy fraction, and zircon, tourmaline, rutile, apatite, monazite, and Cr-spinel relatively increase at the expense mainly of amphibole in Pliocene sediments and of epidote in Miocene sediments. Recent studies have shown that the entire succession of the Nile Delta was deposited by a long drainage system connected with the Ethiopian volcanic highlands similar to the modern Nile since the lower Oligocene. The original mineralogy should thus have resembled that of modern Delta sand much more closely than the present quartzose residue containing only chemically durable heavy minerals. Stratigraphic compositional trends, although controlled by a complex interplay of different factors, document a selective exponential decay of non-durable species through the cored succession that explains up to 95% of the observed mineralogical variability. Our calculations suggest that heavy minerals may not represent >20% of the original assemblage in sediments buried less than ~1.5 km, >5% in sediments buried between 1.5 and 2.5 km, and >1% for sediments buried >4.5 km. No remarkable difference is detected in the intensity of mineral dissolution in mud, silt, and sand samples, which argues against the widely held idea that unstable minerals are prone to be preserved better in finer-grained and therefore presumably less permeable layers. Intrastratal dissolution, acting through long periods of time at the progressively higher temperatures reached during burial, can modify very drastically the relative abundance of detrital components in sedimentary rocks. Failure to recognize such a fundamental diagenetic bias leads to grossly mistaken paleogeographic reconstructions, as documented paradigmatically by previous provenance studies of ancient Nile sediments

Insights into the provenance of the Chinese Loess Plateau from joint zircon U-Pb and garnet geochemical analysis of last glacial loess

The Chinese Loess Plateau, the world’s largest and oldest loess record, preserves evidence of Asia’s long-term dust source dynamics, but there is uncertainty over the source of the deposits. Recent single-grain detrital zircon U-Pb age analysis has progressed this issue, but debates remain about source changes, and the generation and interpretation of zircon data. To address this, we analyze different groupings of new and existing datasets from the Loess Plateau and potential sources. We also present the results of a first high resolution sampling, multi-proxy provenance analysis of Beiguoyuan loess using U-Pb dating of detrital zircons and detrital garnet geochemistry. The data shows that some small source differences seem to exist between different areas on the Loess Plateau. However, sediment source appears to be unchanging between loess and palaeosols, supporting a recent material recycling hypothesis. Our zircon and garnet data demonstrates, however, that Beiguoyuan experienced a temporary, abrupt source shift during the last glacial maximum, implying that local dust sources became periodically active during the Quaternary. Our results highlight that grouping data to achieve bigger datasets could cause identification of misleading trends. Additionally, we suggest that multi-proxy single-grain approaches are required to gain further insight into Chinese Loess Plateau dust sources

The exhumation of the Indo-Burman Ranges, Myanmar

The Indo-Burman Ranges (IBR) are a mountain range comprised of Mesozoic-Cenozoic rocks which run the length of Western Myanmar, extending into India and Bangladesh; to the west lies the Indian Ocean, and to the east lies the Central Myanmar Basin (CMB) along which the Irrawaddy River flows. The IBR are considered to be an accretionary prism, developed at the juncture of the Indian and Sunda plates, and a number of hypotheses have been proposed for their evolution. However, in order for these hypotheses to be evaluated, the timing of IBR evolution needs to be determined. We undertook a two-pronged approach to determining the timing of uplift of the IBR. (1) We present the first low-temperature thermochronological age elevation profiles of the IBR using ZFT, AFT and ZHe techniques. Our data show: a major period of exhumation occurred around the time of the Oligo-Miocene boundary; we tentatively suggest, subject to further verification, an additional period of exhumation at or before the late Eocene. (2) We carried out a detailed multi-technique provenance study of the sedimentary rocks of the IBR and Arakan Coastal region to their west, and compared data to coeval rocks of the CMB. We determined that during Eocene times, rocks of the CMB and IBR were derived from similar local provenance, that of the Myanmar arc to the east. Therefore at this time there was an open connection from arc to ocean. By contrast, by Miocene times, provenance diverged. Rocks of the CMB were deposited by a through-flowing Irrawaddy River, with detritus derived from its upland source region of the Mogok Metamorphic Belt and Cretaceous-Paleogene granites to the north. Such a provenance is not recorded in coeval rocks of the IBR, indicating that the IBR had uplifted by this time, providing a barrier to transport of material to the west. To the previously published list of viable proposals to explain the exhumation of the range, we add a new suggestion: the period of exhumation around the time of the Oligo-Miocene boundary could have been governed by a change to wedge dynamics instigated by a major increase in the thickness of the incoming Bengal Fan sediment pile