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Zircon dating and trace element content of transparent heavy minerals in sandstones from the NE Alps and Outer Dinarides flysch basins
Full text linkIl flysch è un particolare tipo di successione torbiditica che è generalmente considerata come un deposito marino sinorogenico di margini passivi. Generalmente è formato da un'alternanza di arenarie e marne/siltiti in strati sub-paralleli che possono essere più o meno ricchi di materiale carbonatico e fossili. Gli strati possono avere spessori variabili che dipendono fortemente dal tipo di sedimento e dallo stadio di sedimentazione. I bacini flyshoidi situati nella parte nordorientale della Placca Adria sono una tipica espressione di questo genere di corpi sedimentari presupponendo che si siano formati durante la fase orogenica Dinarica. I bacini studiati si compongono del Bacino Giulio (JB), Bacino Birchini (BK), Bacino Istriano (IB) e il Bacino Dalmato Settentrionale (ZB), mostrando un'età che varia da Fine Cretacico (Maastrichtiano) fino alla fine dell'Eocene (Isola di Pag, IB). Tutti i bacini mostrano una forma allungata NW-SE e sono paralleli alla catena Dinarica. In passato è stato suggerito che i bacini si siano riempiti in sequenza, dal più vecchio (JB), seguito da BK fino al più giovane (IB), mentre la dinamica di ZB non era stata presa in considerazione. Questa teoria suggerisce che il bacino più "nuovo" si riempisse solo quando quello precedente si fosse riempito completamente.
Il bacino Giulio e quello Dalmato hanno una sequenza stratigrafica completa e ben documentata, mentre i bacini Birchini e Istriano risultano ancora mancanti da questo punto di vista, dato che spesso sono stati studiati membro per membro a causa degli effetti orogenici che li hanno smembrati e deformati. In particolare IB è comunemente diviso in tre parti, il Bacino Triestino, il Bacino di Pazin e quello delle Isole del Quarnero. In questo lavoro tratteremo tutti e tre i bacini come fossero uno, data la stessa età di formazione e le strutture tettoniche simili che suggeriscono un'evoluzione comune. In passato, numerosi autori hanno suggerito che la frazione carbonatica sia stata prevalentemente fornita dalla disgregazione della Piattaforma Carbonatica Adriatica posta direttamente a Sud-Est rispetto ai bacini, mentre la sorgente primaria della frazione silicoclastica è stata localizzata all’interno delle Dinaridi. Questo è stato suggerito anche da recenti studi di provenienza improntati su Cr-spinello, granati, clinopirosseni e anfiboli detritici, che posizionano la loro sorgente nelle Dinaridi Interne ed Esterne. In accordo con questa ipotesi, il rapporto tra Cr-Spinelli magmatici e peridotitici segue il cambiamento litologico delle ofioliti che affiorano vicino le ipotetiche aree di sorgenza.
Numerosi autori hanno suggerito che le paleocorrenti durante il periodo di deposizione avessero una direzione NW-SE, perpendicolare alla direzione principale di derivazione del materiale silicoclastico, e parallelo alla direzione della catena Dinarica. Queste sono state interpretate come sorgenti secondarie di materiale silicoclastico che potrebbe derivare anche dall’area Alpina. Inoltre, queste correnti potrebbero aver connesso i bacini creando un riciclo di sedimento lungo tutta la stratigrafia o per un breve periodo. L’evidenza potrebbe essere suggerita da un netto cambio di mineralogia/granulometria/chimica.
In questo lavoro di tesi, sono stati analizzati gli elementi in traccia di granati, rutili e zirconi assieme alla geocronologia U/Pb di questi ultimi. I dati verranno usati per localizzare la zona di provenienza della frazione silicoclastica dei quattro bacini e verificare un’eventuale connessione tra loro.The Flysch is a particular turbiditic succession that generally is considered as a synorogenic marine deposit in passive margins. Generally, it is composed by an alternation of sandstones and mudstones/siltstones in sup-parallel strata that can be more or lesser rich in carbonate material and fossils. These could have different thickness that strongly depend on the sediment supply and the stage of sedimentation. The Flysch Basins located in the North Eastern Part of the Adria Plate are typical expression of this kind of sedimentary body being supposed as derived during the Dinaric orogenic phase.
The basin investigated are the Julian (JB), Brkini (BK), Istrian (IB) and North Dalmatian Flysch Basins (ZB) and show an age between the Maastrichtian and the Lower Eocene. All the basins show elongated form with NW-SE direction being almost parallel to the Dinarides. In the past it has been suggested that the basins were filled in sequence, starting from the oldest one (JB) and following with BK, IB and finally ZB. This theory suggested that the new basin will open only when the previous one was filled. The Julian and North Dalmatian Basins show a complete and well documented stratigraphy, while Brkini and Istrian ones are still incomplete and often studied separating their member, due to the extreme tilting, faulting and overthrusting they have suffered during the Dinaric Phase. In particular, IB is commonly divided in three part, Trieste, Pazin and Kvarner Islands basins. In this work, it will be considered as one since all the three are coeval and their structures seem to be similar suggesting a common evolution.
In the past, several authors suggested that the main carbonate fraction supply could derive from the disgregation of the Adriatic Carbonatic Platform located southwest regarding the basins, while the primary siliciclastic sources were located in the Dinarides. This is also suggested by recent provenance studies focused on detrital Cr-spinel, garnet, clinopyroxene and amphibole, which place the main source of these minerals in both the Inner and External Dinarides. In agreement with this thesis, the distribution of the Cr-spinels ratio between peridotitic and magmatic crystals in the basins follows the changing of the ophiolites affinity that outcrops in the suggested source areas.
Several authors stated that paleocurrents have been also present flowing from northwest to south east, perpendicular to the suggested main supply flows and parallel to the elongation of the basin bodies. These are interpreted as second suppliers of siliciclastic sediments that could derive from the Alpine Area. Furthermore, the currents could have connected the basins creating a recycle of the old sediments along all the stratigraphy or just for brief moments testified by sudden changes in mineralogy/granulometry/chemistry.
In this work trace elements of garnet, rutile and zircon together with the U-Pb geochronology of the zircon are used to locate the provenance area of the siliciclastic fraction of the four flysch basins and to verify possible connection among them. Furthermore, the data will be also useful for better understanding the geodynamic Cretacic-Eocenic evolution of the region
Caratterizzazione mineralogica di due filoni bauxitici rilevati in prossimità di Baška (Isola di Krk- Croazia)
Full text linkSono stati analizzati i materiali provenienti da due filoni bauxitici rilevati in prossimità di Baška (Isola di Krk- Croazia). Le analisi mineralogiche hanno permesso di stabilire la presenza di boehmite, goethite ed ematite. Nonostante la boehmite sia il tipico minerale finale derivante dall’alterazione di rocce allumosilicatiche in un clima mediterraneo, la presenza di gibbsite indicante la non completa deidratazione di tale materiale in boehmite
Depositi di riempimento di cavità presso Sella Prevala (Monte Canin, Alpi Giulie - NE Italia)
No full textSono stati analizzati campioni di sedimenti semiconsolidati raccolti all’interno di alcune cavità di piccole dimensioni nel massiccio carsico del Monte Canin (Alpi Giulie, Udine). Le analisi hanno evidenziato la presenza di depositi ben classati prevalentemente carbonatici ma con presenza di granuli di quarzo, miche e alcuni minerali pesanti quali anfiboli ferro-magnesiaci, epidoti e tormaline. Dalla natura del sedimento e dalla paragenesi dei minerali si può ipotizzare un’origine
eolica dei granuli di alcuni di questi depositi. La provenienza possibile è dalle zone delle Alpi orientali, ove affiorano rocce metamorfiche in facies di scisti verdi
Is 600° C enough to produce air oxidation in Cr-spinels?
No full textEleven natural crystals of Cr-bearing spinels containing Cr 0.1-1.5 and total iron 0.2 – 1.0 atoms per formula unit, have been studied by X-ray single crystal diffraction at ambient conditions and after heating at 600°C for 7 and 28 days in order to verify if they suffered oxidation. SEM-BSD analyses revealed the absence of zoning as a possible indication of oxidation. Structural analyses show that no vacancies are present after heating. In crystals with low Fetot and Cr# the main effects of heating are related to Mg and Al exchange among the two sites; in spinels with 0.75 0.45 a.p.f.u. and Cr > 1.2 a.p.f.u. show very limited inversion degree due to the high Cr content. In this case, movement of ferric iron among the sites seems to be dependent upon its initial content in the site. Air oxidation seems to be absent in the studied natural spinels annealed at 600°C
Structural parameters of Cr-bearing spinels and pleonaste from the Cuillin Igneous Complex (Isle of Skye, Scotland): Implications for metamorphic and cooling history
No full textThe Outer Layered Suite of the Cuillin Igneous Complex (Isle of Skye,NWScotland) comprises a Peridotite Series and a younger Allivalite Series (the latter comprising troctolites, eucrites and gabbros). Close to the junction between the Peridotite and the Allivalite Series (but wholly contained within the latter), an ultramafic breccia unit containing abundant peridotite xenoliths crops out. In the Peridotite Series, reddish brown Cr-bearing spinels are present as disseminated crystals in the peridotite and also as chromitite seams, while in the peridotite xenoliths of the breccia unit, green pleonaste occurs in both of these modes of textural occurrence. Optical absorption spectroscopy reveals that the colour difference between the two spinel phases is related mainly to variable Al, Cr and Fe contents, while crystal structural analysis shows that the cooling rate calculated utilizing the oxygen positional parameter is comparable for all samples. The intracrystalline closure temperature for the Cr-spinel in the Peridotite Series is different for the disseminated and seam textural occurrences of the spinels, while the temperatures yielded by pleonaste in the peridotite xenoliths are the same for both textural occurrences. Our dataset suggests that the pleonaste in the peridotite xenoliths has been heated and equilibrated under subsolidus conditions, probably during breccia formation.
During this heating, homogenization of the closure temperatures of pleonaste spinels occurred
The Triassic sills of the Costabella Crest (Moena, TN-Italy) in comparison with coeval Dolomitic magmatites
No full textThe trigger of the Triassic magmatism in the Eastern Alps (about 235 Ma) is still highly debated, and both extensional and compressional theories are invoked. The first is mainly supported by the related extensional geologic structures, while the second family of theories is supported by the chemistry of the better exposed lithotypes and the high sedimentation rate, typical of a back-arc environment, whose arc is supposed to be located east or south of the Adria Plate. We support a not orogenic genesis for the dolomitic Triassic magmatism due to the presence of coeval magmatic products showing not orogenic features, by the position of such magmatic province during Triassic times (edge of the Intra Pangea Dextral Shear; IPDS) and by the sedimentation rate which appears comparable with that of the North America Atlantic coast located along the IPDS. It is interesting to notice that the IPDS, which probably begun to move in early Permian, started its extensional movement 235 My ago. In this work we mainly attempt to evaluate possible interaction of the magmatic products with the carbonate (host rocks) through the study of the sills located in the Costabella Crest (Moena, TN Italy), and with the comparison with other dolomitic Triassic products, then, to give a small contribute about the geodynamic discussion.Generally, the samples show ipoialine to strong porphyric textures and a variable grade of alteration mainly due to disequilibrium between olivine and the melts in the magmatic chamber. In the Total Alkali Silica diagram the sills fall in the alkaline field (from basanite to tefriphonolite), but this partially contrasts with their paragenesis. Indeed, among the phenocrystals the plagioclase appears very abundant and show a bimodal composition (labradorite and anorthite). Carbonates can be present with primary textures to suggest the presence of small amount of carbonatic melts in the magma chamber possibly explaining the isotopic features found in the dolomitic flows where a strong isotopic Sr variability is associated to small Pb and Nd isotopic ones. Apart the strong interaction with the host rocks and the post-emplacement alterations, three groups with different La/Yb ratio can be recognized suggesting a heterogenic source. Finally, to evaluate the supposed subduction-related chemical features, as the negative Nb anomaly, the Triassic samples have been compared with other not arc related magmatic products placed along the IPDS of the Central Atlantic Magmatic Province emplaced at the TJ boundary. Despite the very different grade of alkalinity, locally their incompatible element pattern shows strong similitudes that we relate to past subductions
The Österplana Fossil Meteorites and… What Else? Terrestrial Cr-Spinels and Zircons in the Ordovician Limestones of the Thorsberg Quarry (Sweden)
Full text linkIn the Ordovician limestone of the Thorsberg quarry (South Sweden), about 130 meteorites have been found. Among the extraterrestrial material, several terrestrial Cr-spinels and zircons have been found too. In particular, in the interval 416–447 cm above the Arkeologen bed, terrestrial Cr-spinels, compositionally different from previous studied Cr-spinels of the same sequence, are present. Previous studies on zircon provided depositional ages that range from 464.22 ± 0.37 Ma to 465.01 ± 0.26 Ma. The trace element content of zircons suggests different possible source rocks. In fact, zircons from the oldest ash layer resemble those from dolerite, while those in the youngest layers are similar to zircons commonly found in granitoids, with more than 65% wt. SiO2. The chemistry of Cr-spinels suggests a strong alteration, so that it is difficult to assign them to a specific area, however they recall the chemistry of altered spinels from ophiolitic occurrences (among other possibilities). The geological setting of the Laurentia and Baltica areas, including the description of basalts to rhyolite association and the presence of ophiolitic slices, makes us confident about the derivation of these zircons and Cr-spinels from those areas
Trace and Rare Earth Elements chemistry of detrital garnets in the SE Alps and Outer Dinarides flysch basins: an important tool to better define the source areas of sandstones
No full textGarnet is one of the most abundant heavy minerals present in the Cretaceous – Eocene flysch deposits of the Southeastern Alps and Outer Dinarides (Julian, Brkini and Istrian basins). About 300 detrital garnets from the Cretaceous-Eocene flysch deposits of the Southeastern Alps have been analysed by means of electron microprobe and LA-ICP-MS. In the Julian and Istrian basins, supplies are from amphibolite-facies rocks and mafic and ultramafic metamorphic rocks, while in the Brkini basin the latter are almost missing. Moreover, in the Julian and Istrian basins, supplies from skarns, very low-grade metabasites, or from ultra-high temperature metamorphosed calc-silicate granulites are present. Among these different groups, LA-ICP-MS analyses showed that trace element content can be very different in almandine-rich garnets from the different sources. In particular, the source that supplied the Julian Basin is significantly different from that of the Istrian Basin. From the Cretaceous to the Palaeocene the main supplies of Bi-type garnets derived from an area where feldspar-free garnet-bearing rocks were exposed. Successively, Bi-type garnets were supplied from an area where feldspar-garnet-bearing rocks were exposed. The presence of garnets from feldspar-free garnet-bearing rocks in the Brkini and Istrian basins can be ascribed to both recycling of material from the Julian Basin as well as direct input from the same areas that supplied the Julian Basi
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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