Littérature scientifique sur le sujet « Zirconi detritici »

The bottom sediments of the Caraguatatuba bay, localized in the coast of São Paulo State, were collected for a qualitative study of the heavy minerals in the sandy fraction between 0,125 and 0,062 mm. The mineralogical study allowed the recognition of three groups of heavy minerals: group I – opaque minerals, with magnetite, ilmenite and pyrite; group II – transparent minerals, with amphibole, pyroxene, epidote, biotite, muscovite, zircon, tourmaline, titanite, rutile and apatite; and group III – carbonates, represented by bioclasts and aragonite needles. The source areas of these heavy detritic minerals are related with rocks of the coastal complex, of syntectonic granites and of the alkaline rocks from São Sebastião island. The average content of heavy detritic minerals in the Caraguatatuba bay is 0,77%. Besides biotite and bioclast material, magnetite and ilmenite, are the most abundant phases with an average percentage of 12,9% in the 0,125/0,062 mm fraction.

The U–Pb dating and Hf isotope systematics of detrital zircons from a sandstone interbed in the section of the upper conglomerate sequence of the Mt. South Demerdzhi were carried out. The dominant populations of detrital zircons in the studied sample characterize episodes of magmatic activity within the source of the Upper Jurassic conglomerates. Magmatism was manifested in the Vendian-Cambrian, Carbon-Triassic and Late Jurassic. The åHf values of detrital zircons of these ages indicate the insignificant role of the ancient (Archean–Early Proterozoic) continental crust in the protolith of magmatic chambers. The similarity of the detrital zircons age distribution from the Middle Jurassic and Upper Jurassic conglomerate strata suggests that they are molasses of the Cimmerian orogen. The absence of products of Middle Jurassic magmatism in molasses of the Cimmerian orogen, which we fixed, limits position of the Cimmerian orogen in the southern part of the Scythian plate. It is shown that the primary source of the Precambrian detrital zircons were mobilized within the Cimmerian orogen the crustal fragments of the Peri-Gondwanan origin, rather than the basement complexes of the East European Platform, similar to the complexes of the Ukrainian shield. The reconstruction of the main stages of the accumulation of the coarse-grained strata of the Mountaineous Crimea in the context of the tectonic evolution of the southern margin of Laurasia during the Mesozoic is presented.

The sediments analysed are generally coarse grained, sand gravels to gravel sands. The silt and clay content of the samples is low and their textural study revealed a fluvial deposition environment. The amounts of quartz, feldspars and lithic fragments range from 33% to 56%, 8% to 18% and 28% to 52%, respectively. The samples studied may be classified as garnet-mica bearing litharenites. Quartz, plagioclases and micas are the predominant constituents. Detritai calcite and pyroxenes are present in small amounts. The samples are texturally and mineralogically immature. Detailed polarized microscopy revealed the presence of garnet, staurolite, kyanite, zoisite, routile, epidote, zircon, apatite and tourmaline, as well as of opaque minerals. These accessory minerals are considered indexes of metamorphic provenance. Geochemically the samples are classified as sublitharenites and subarkoses, all being rich in Si02- The overall percentages of quartz, feldspars and rock fragments designate intense physical weathering and rapid transport and deposition in the Herso Basin which is part of a passive continental margin (recycled orogen).

Abstract The Ibar Basin was formed during Miocene large scale extension in the NE Dinaride segment of the Alpine- Carpathian-Dinaride system. The Miocene extension led to exhumation of deep seated core-complexes (e.g. Studenica and Kopaonik core-complex) as well as to the formation of extensional basins in the hanging wall (Ibar Basin). Sediments of the Ibar Basin were studied by apatite and zircon fission track and vitrinite reflectance in order to define thermal events during basin evolution. Vitrinite reflectance (VR) data (0.63-0.90 %Rr) indicate a bituminous stage for the organic matter that experienced maximal temperatures of around 120-130 °C. Zircon fission track (ZFT) ages indicate provenance ages. The apatite fission track (AFT) single grain ages (45-6.7 Ma) and bimodal track lengths distribution indicate partial annealing of the detrital apatites. Both vitrinite reflectance and apatite fission track data of the studied sediments imply post-depositional thermal overprint in the Ibar Basin. Thermal history models of the detritial apatites reveal a heating episode prior to cooling that began at around 10 Ma. The heating episode started around 17 Ma and lasted 10-8 Ma reaching the maximum temperatures between 100-130 °C. We correlate this event with the domal uplift of the Studenica and Kopaonik cores where heat was transferred from the rising warm footwall to the adjacent colder hanging wall. The cooling episode is related to basin inversion and erosion. The apatite fission track data indicate local thermal perturbations, detected in the SE part of the Ibar basin (Piskanja deposit) with the time frame ~7.1 Ma, which may correspond to the youngest volcanic phase in the region.

Our study was focused on low to medium radioactive sandstones of the Godula Formation of the external fl ysch zone of the Western Carpathians. It is widely known that concentrations of radioactive elements (K, U and Th ) in siliciclastic sediments are primary controlled by facies and mineral and chemical composition, which is largely influenced by their provenance. In the Godula Formation, K was found to be predominantly incorporated in some framework and accessory detritic minerals (K-feldspars, albite, mica, glauconite, illite). Main sources of U and Th were identified in heavy minerals, such as monazite and zircon. Variations of K, U and Th concentrations correspond to changes in modal composition of sandstones and indicate uplift and subsequent erosion of structurally deeper crustal parts of source area (Silesian Ridge). Significant increase of K, U and Th concentrations was observed near the boundary between the Middle and Upper Godula beds. Gamma-ray spectrometry could be used as good supplementary technique for discrimination of sandstones of these lithostratigraphical parts of the Godula Formation.

Questa tesi presenta lo studio della composizione delle sabbie dei più grandi deserti africani e dei fiumi limitrofi al fine di illustrare gli effetti dell'interazione tra processi fluviali ed eolici sul trasporto di sedimenti in ambiente arido. Le sabbie del Sahara, del Kalahari e dello Zambesi sono state analizzate mediante petrografia, mineralogia della frazione pesante e geocronologia U-Pb di zirconi detritici. Per il caso di studio dello Zambesi sono stati analizzati anche la geochimica elementare, gli isotopi del Nd e i minerali delle argille. I campi di dune del Sahara sono, con poche eccezioni, composti da pura sabbia quarzosa con suite di minerali pesanti molto impoverite, dominate da minerali ultrastabili. La composizione varia solamente lungo la Valle del Nilo, in prossimità della catena dell'Anti-Atlante e alla provincia vulcanica libica. La sabbia delle dune del Kalahari è costituita principalmente da quarzo associato a minerali pesanti ultrastabili. La composizione varia solo ai margini occidentali e orientali del deserto, riflettendo in parte l’apporto fluviale di primo ciclo dai basamenti cristallini nella Namibia centrale, nello Zimbabwe occidentale e nelle dune vicino alle cascate Vittoria dove i sedimenti sono erosi dalle lave del Karoo. La morfologia segmentata del fiume Zambesi si riflette nella sua mineralogia e geochimica. La sabbia quarzosa erosa dalle dune del deserto del Kalahari viene progressivamente arricchita in frammenti litici basaltici e clinopirosseno. Successivamente nuovo apporto di sedimenti avviene a valle del lago Kariba, documentando una graduale diminuzione del quarzo e dei minerali ultrastabili. La composizione diventa quarzo-feldspatica nel tratto finale. L'abbondanza di feldspato nella sabbia del basso Zambesi non ha equivalenti tra i grandi fiumi sulla Terra e supera di gran lunga quella nei sedimenti del delta e della piattaforma, rivelando che il segnale di provenienza dell’alto Zambezi ha cessato di essere propagato a valle dopo la chiusura delle grandi dighe. La smectite, dominante nei fanghi generati dai basalti del Karoo o nel clima equatoriale delle pianure mozambicane, prevale su illite e kaolinite. La geochimica indica: l'aggiunta di quarzo per riciclo; l'erosione dei basalti del Karoo; l’erosione dei basamenti precambriani. Lo studio di Sahara e Kalahari consente di mettere a confronto deserti dominati dai processi eolici e deserti caratterizzati da una consistente interazione fluvio-eolica. Nel Sahara, la maggior parte della sabbia sembra essere riciclata da arenarie quarzose e il principale meccanismo erosivo e di trasporto è il vento. Nel Kalahari, i sedimenti sono trasportati dai fiumi che, erodendo gli orogeni ai fianchi del deserto, accumulano e omogeneizzano il detrito al centro del bacino grazie all’azione eolica. Lo studio permette di riconsiderare criticamente diversi dogmi della sedimentologia, come il presunto aumento della “maturità” mineralogica durante il trasporto fluviale: i sedimenti dell’alto Zambezi, erosi dalle dune del Kalahari e ricchi di quarzo, vengono progressivamente diluiti da sedimenti lito-feldspatici lungo il corso del fiume. Gli indici geochimici e la composizione dei fanghi sembrano indicare maggiore alterazione chimica nell'arido bacino dell’alto Zambesi rispetto al più umido Zambesi centrale e inferiore, testimoniando che il segnale di “paleo-alterazione del Kalahari” registrato nei sedimenti delle dune viene trasportato a valle fino alla foce.
This thesis presents a study of the composition of sand from desert dunes and adjacent rivers across the African continent to illustrate the effects of the interplay between fluvial and aeolian processes on sediment transport in desertic environments. The Sahara, Kalahari and Zambezi samples were analyzed by bulk-petrography, heavy-mineral, and detrital-zircon U–Pb geochronology. For the Zambezi case study, elemental geochemistry, Nd isotopes and clay minerals were also analyzed. Saharan dune fields are generally composed of pure quartzose sand with very poor heavy-mineral suites dominated by ultrastable minerals. Relatively varied compositions characterize sand along the Nile Valley, the southern front of the Anti-Atlas belt and near a basaltic field in Libya. Kalahari dune sand mostly consists of monocrystalline quartz associated with durable heavy. Composition varies only at the western and eastern edges of the desert, reflecting partly first-cycle fluvial supply eroded from crystalline basements of Cambrian to Archean age in central Namibia and western Zimbabwe. Basaltic detritus from Jurassic Karoo lavas is dominant in dunes near Victoria Falls. The segmented morphology of Zambezi River is reflected by its mineralogy and geochemistry. Pure quartzose sand recycled from Kalahari Desert dunes in the uppermost tract is next progressively enriched in basaltic rock fragments and clinopyroxene. Sediment load is renewed first downstream of Lake Kariba, documenting a stepwise decrease in quartz and durable heavy minerals. Composition becomes quartzo-feldspathic in the lower tract. Feldspar abundance in Lower Zambezi sand has no equivalent among big rivers on Earth and far exceeds that in sediments of the northern delta, shelf, and slope, revealing that provenance signals from the upper reaches have ceased to be transmitted across the routing system after closure of the big dams. Irumide ages predominate over Pan-African, Eburnean, and Neoarchean ages. Smectite, dominant in mud generated from Karoo basalts or in the equatorial climate of the Mozambican lowlands, prevails over illite and kaolinite. Elemental geochemistry reflects quartz addition by recycling, supply from Karoo basalts, and first-cycle provenance from Precambrian basements. Sahara and Kalahari case studies allow to study in situ sand generation by wind erosion versus external fluvial supply in arid environment. In the Sahara, most sand appears to be recycled from rocks with high sand-generation potential, and the main transport mechanism is the wind saltation and dune movement. In Kalahari, sediments are fed by rivers by first cycle erosion of exposed orogens at the flanks of the desert and therein homogenised. The contrasting effect of strong recycling by wind and fresh supply from rivers are the key factor for most deserts studied in literature and their identification in terms of mineralogy and provenance is proved to be precious for present and past climatic debate. In addition, evaluating the results from the Kalahari and Zambezi studies allows to critically reconsider several dogmas, such as the supposed increase of mineralogical “maturity” during long-distance fluvial transport. This is strongly affected by provenance factors: quartz-rich recycled Kalahari dune sand is progressively diluted along the Zambezi River by sediment supplied by different crustal domains. Inheritance of the “Kalahari paleo-weathering signal” by Zambezi River is highlighted also by geochemical indexes and mud composition which appear to be oddly more affected by weathering in the arid Uppermost Zambezi catchment than in the wetter Middle and Lower Zambezi.