Zircon dating and trace element content of transparent heavy minerals in sandstones from the NE Alps and Outer Dinarides flysch basins

Il flysch \ue8 un particolare tipo di successione torbiditica che \ue8 generalmente considerata come un deposito marino sinorogenico di margini passivi. Generalmente \ue8 formato da un'alternanza di arenarie e marne/siltiti in strati sub-paralleli che possono essere pi\uf9 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'et\ue0 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 \ue8 stato suggerito che i bacini si siano riempiti in sequenza, dal pi\uf9 vecchio (JB), seguito da BK fino al pi\uf9 giovane (IB), mentre la dinamica di ZB non era stata presa in considerazione. Questa teoria suggerisce che il bacino pi\uf9 "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 \ue8 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 et\ue0 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 \ue8 stata localizzata all\u2019interno delle Dinaridi. Questo \ue8 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\u2019area Alpina. Inoltre, queste correnti potrebbero aver connesso i bacini creando un riciclo di sedimento lungo tutta la stratigrafia o per un breve periodo. L\u2019evidenza 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\u2019eventuale 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

Coexistence of MORB- and OIB-like dolerite intrusions in the Purang ultramafic massif, SW Tibet: a paradigm of plume-influenced MOR-type magmatism prior to subduction initiation in the Neo-Tethyan lithospheric mantle

The Yarlung Zangbo Suture Zone (YZSZ) of South Tibet is divided by the Zhongba-Zhada terrane into two subparallel ophiolitic belts in its western end. The peridotite massifs of the southern belt tectonically overlie the Tethyan Himalaya sequence. The Purang peridotite body in this belt is intruded by two groups of dolerite dikes, providing significant compositional, geochronological, and isotopic information about the melting history of the Neo-Tethyan mantle. U-Pb ages of zircons separated from dolerites show that peridotites of West Purang were intruded by an early generation of dikes at 138.5 \ub1 2.0 Ma (Valanginian). These dolerites show ocean island basalt (OIB)-type normalized multi-elemental profiles and Sr-Nd isotopic signatures [(La/Yb)N = 13\u201316], high initial 87Sr/86Sr ratios (0.70598\u20130.70765), and low \u3b5Nd(t) values (\u20132.6 to \u20132.3). Zircons separated from this group of dolerites have slightly radiogenic \u3b5Hf(t) values (+2.6 to +4.6). The next generation of dolerite dikes intruded the East Purang peridotites between 124.5 \ub1 2.5 Ma and 124.4 \ub1 3.2 Ma (Aptian). These East Purang dolerites show normal mid-ocean ridge basalt (N-MORB)-type normalized multi-element patterns [(La/Yb)N = 0.6\u20130.9] with noticeable negative Nb and Th (\ub1Ti) anomalies, and have high 87Sr/86Sr(i) (0.70295\u20130.70618) and high \u3b5Nd(t) values (+7.7 to +9.2). Zircons separated from the East Purang dolerites show strongly radiogenic \u3b5Hf(t) values (+3.5 to +17.0). Semiquantitative geochemical modeling demonstrates that the parental magmas of West Purang dolerites were generated from 5%\u201310% polybaric partial melting of a deep-seated juvenile asthenospheric source enriched by plume-type components. In contrast, the parental melts of East Purang dolerites were derived from more than 20% melting of a juvenile spinel-bearing MORBtype mantle source that was modified by subduction-related melts/fluids to a minor extent. A possible tectono-magmatic model for the petrogenesis of the Purang ophiolitic massif could be linked to incipient continental rifting and subsequent oceanic seafloor spreading associated with decompression upwelling of an asthenospheric source contaminated by plume-type components. This plume-proximal seafloor spreading-system was succeeded by the initiation of Neo-Tethyan intra-oceanic subduction close to the active continental margin of Eurasia during the Early Cretaceous

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

Garnet is one of the most abundant heavy minerals present in the Cretaceous \u2013 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

The Österplana Fossil Meteorites and… What Else? Terrestrial Cr-Spinels and Zircons in the Ordovician Limestones of the Thorsberg Quarry (Sweden)

In 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

Triassic magmatism in the European Southern Alps as an early phase of Pangea break-up

Magmatic rocks from the Dolomites, Carnic and Julian Alps, Italy, have been sampled to inves- tigate the origin and geodynamic setting of Triassic magmatism in the Southern Alps. Basaltic, gabbroic and lamprophyric samples have been characterized for their petrography, mineral chemistry, whole-rock major and trace elements, and Sr, Nd and Pb isotopic compositions. Geothermobarometric estimates suggest that the basaltic magmas crystallized mostly at depths of 14–20km. Isotopic data show variable degrees of crustal contamination decreasing westwards, probably reflecting a progressively more restitic nature of the crust, which has been variably affected by melting during the Permian period. Geochemical and isotopic data suggest that the mantle source was metasomatized by slab-derived fluids. In agreement with previous studies and based on geological evidence, we argue that this metasomatism was not contemporaneous with the Ladinian–Carnian magmatism but was related to previous subduction episodes. The lamprophyres, which likely originated some 20 Ma later by lower degrees of melt- ing and at higher pressures with respect to the basaltic suite, suggest that the mantle source regions of Triassic magmatism in the Dolomites was both laterally and vertically heterogeneous. We conclude that the orogenic signatures of the magmas do not imply any coeval subduction in the surrounding of Adria. We rather suggest that this magmatism is related to the Triassic rifting episodes that affected the western Mediterranean region and that were ultimately con- nected to the rifting events that caused the break-up of Pangea during the Late Triassic – Early Jurassic period

Euganean trachytic grinding stones in the Caput Adriae from the Iron Age to the Roman period: Reassessment of the Protohistoric quarries

A group of Euganean trachytic grinding stones from Slovenia and the available data about the same type of artefacts in the whole Caput Adriae are presented. The occurrence of Iron Age saddle querns in Karst and Istria is confirmed, but our study suggests a likely provenance from Mts Cero/Murale instead of Mts Altore/Rocca Pendice, as previously suggested. Este, an important Venetic centre, is just south of Mts Cero/Murale, suggesting its central role in the production and distribution of saddle querns. During the Roman period Euganean trachytes are still used for rotary millstones, but new quarries (Mt Rosso), relatively close to ancient Padua, are exploited. Such shift in the position of millstone quarries most probably reflects the changed geopolitical framewor

Mineralogy and geochemistry of a base metal sulfide-bearing magnetitite body from the Eretria mine, East Othris massif, Greece: insights into an ancient seafloor hydrothermal system

Magnetite deposits comprise a very small volume of a serpentinized peridotite body that constitutes the Eretria chrome mine of the East Othris ophiolite in central Greece. Magnetite deposits have been examined to aid determination of their mode of occurrence, mineralogy and origin. Research attention has been focused on an ore body that consists of a ~30 cm-thick pod of semi-massive to massive magnetite (~75%\u201385% modal) hosted in a serpentinite shear zone. The silicate matrix of magnetitite (~10%\u201315% modal) is composed of unstrained serpentine, chlorite and subordinate amounts of andradite and actinolite. Base metal sulfides (BMS; ~5%\u201310% modal) and phosphates are dispersed between the magnetite grains. Variably altered Cr-spinel crystals are poikilitically enclosed within magnetite. The investigated ore samples show great variations in the Cu (890\u20139530 ppm), Ni (190\u20133260 ppm), Co (120\u20132180 ppm) and Zn (20\u20131240 ppm) contents. The \u3b418\u39fSMOW value for magnetite from a magnetitite sample is 2.2\u2030. The undeformed nature of the silicate minerals in magnetitite points toward crystal growth from a postmagmatic fluid. The low Ni, Mn, Mg and high Si contents in magnetite suggest that it was not derived from fluids produced concurrent to serpentinization. The presence of phosphates, BMS and hydrosilicates suggest that the ore forming fluids were acidic and halogen-bearing. Our preferred hypothesis for the formation of magnetitite is that mixing of upwelling, metal-rich brines along the dilational segments of the host shear zone caused saturation of the ensuing fluids with Fe, which eventually prompted magnetite precipitation. The T estimates deduced from mineralogy, with an uppermost limit of ~460 \u25e6C required, and cessation of mineralization at ~150\u2013100 \u25e6C, are consistent with the ambient T present within a cooling oceanic lithospheric slab. In analogy with some modern serpentinite-hosted BMS deposits at rifted settings, the BMS-bearing magnetitite of Eretria may represent part of an ancient seafloor hydrothermal system. Our study provides important insights into the initiation and episodicity of hydrothermal activity in oceanic settings