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Geochemical characterization of recent volcanism from the Tuscan Magmatic Province (Central Italy): the Roccastrada and San Vincenzo centers
No full textThe aim of this paper is to give a contribution to a general picture of the main processes governing recent magmatism in Tuscany, by studying the petrogenesis of the Roccastrada and S. Vincenzo volcanic centers in depth. Geochemical (major elements and Rb, Sr, Ba, Y, Nb, Zr, Hf, Th, Ta, Cr, Co, Sc, and REE) and mineral chemistry (two feldspars, biotite and cordierite) data on the Roccastrada and S. Vincenzo rhyolites, as well as Sr isotopic data on Paleozoic metasedimentary formations belonging to the Tuscan basement , are therefore presented. At Roccastrada only purely anatectic melts were observed . The crystallization temperature, pressure and water content for the phenocryst assemblages are estimated to be about 700°C, < 1 Kb and 2.5%. The rocks that crop out East and West of M. Alto represent two magma batches derived from the same source rock, by differing degrees of partial melting leaving differing residual mineralogies. The Paleozoic garnet-bearing micaschists could, on the basis of their geochemical and isotopic characteristics, be suitable source rocks . The S. Vincenzo volcanites are considerably enriched in Sr, Ba , Y, and REE in some samples. They have been interpreted to be the result of an interaction between a crustal anatectic melt and a mantle-derived melt. Linear a rrays on the A/B vs. 1/B diagrams allowed the extrapolation of the composition o f the mantle-derived magma. It exhibits strong enrichments in incompatible elements, and his pattern in a multielemental plot is very similar to those of some alkalic magmas cropping out in Southern Italy
Source contamination and mantle heterogeneity in the genesis of Italian potassic and ultrapotassic volcanic rocks: Sr-Nd-Pb isotope data from Roman Province and Southern Tuscany
No full textThe Tyrrhenian border of the Italian peninsula has been the site of intense magmatism from Pliocene to recent times. Although calc-alkaline, potassic and ultrapotassic volcanism overlaps in space and time, a decrease of alkaline character in time and space (southward) is observed. Alkaline ultrapotassic and potassic volcanic rocks are characterised by variable enrichment in K and incompatible elements, coupled with consistently high LILE/HFSE values, similar to those of calc-alkaline volcanic rocks from the nearby Aeolian arc. On the basis of mineralogy and major and trace element chemistry two different arrays can be recognised among primitive rocks; a silica saturated trend, which resulted in formation of leucite-free mafic rocks, and a silica undersaturated trend, charactrerised by leucite-bearing rocks.
Initial 87Sr/86Sr and 143Nd/144Nd values of Italian ultrapotassic and potassic mafic rocks range from 0.70506 to 0.71672 and from 0.51173 to 0.51273, respectively. 206Pb/204Pb values range between 18.50 and 19.15, 207Pb/204Pb values range between 15.63 and 15.70, and 208Pb/204Pb values range between 38.35 and 39.20. The general εSr vs. εNd array, along with crustal lead isotopic values, clearly indicates that a continental crustal component has played an important role in the genesis of these magmas. The main question is where this continental crustal component has been acquired by the magmas. Volcanological and petrologic data indicate continental crustal contamination to be a leading process along with fractional crystallisation and magma mixing. Considering, however, only the samples thought to represent primary magmas, which have been in equilibrium with their mantle source, a clearer picture emerges. A large variation of εSr vs. εNd is still observed, with εSr from −2 to +180 and εNd from + 2 to −12. A bifurcation of this array is observed in the samples that plot in the lower right quadrant, with mafic leucite-bearing Roman Province rocks buffered at εSr = + 100 whereas the mafic leucite-free potassic and ultrapotassic rocks point to strongly radiogenic Sr compositions. We may argue that mafic leucite-bearing Roman Province rocks point to εSr and εNd values similar to those of Miocene carbonate sediments whereas mafic leucite-free potassic and ultrapotassic rocks point to a silicate upper crust end-member. Lead isotopes plot well inside the field of island arcs, overlapping the values of pelagic sediments as well, but bifurcation between the samples north and south of Rome is observed.
The main characteristic for the mantle source of Italian potassic and ultrapotassic magmas is the clear upper crustal signature acquired prior to partial melting through metasomatic agents released by the subducted slab. In addition, one lithospheric mantle source in the north and an asthenospheric mantle source, pointing to an HIMU reservoir, in the south were recognised. The chemical and isotopic differences observed between the northern and southern sectors of the magmatic region were possibly due to the presence of a carbonate-rich component in the crustal enriching agent in the south. One crustal component might have been generated by melting of silicate metasedimentary rocks or sediments from an ancient subducted slab. The second one might reflect the activity of mostly CO2-rich fluid released more recently by the incipient subduction of carbonate sedimentary rocks
Rb-Sr dating and Sr-Nd isotopic characteristics of Hercynian granites from the western Central System, Iberian Massif.
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Lead isotope composition of the sublimates from the fumaroles of Vulcano (Aeolian Islands, Italy): inferences on the deep fluid circulation
No full textIndagini sulle reliquie attribuite a "San Luca Evangelista", Basilica di Santa Giustina in Padova: studio isotopico e microchimico del piombo, della bara e delle epigrafi
No full textQuesto lavoro espone i risultati della studio geochimico-isotopico del materiale plumbeo proveniente dalla Basilica di Santa Giustina. Il materiale analizzato e’ rappresentativo della cassa e del coperchio della bara di San Luca; della cassa di san Mattia; delle cupole e delle epigrafi. E’ stato inoltre studiato un campione di carbonato di piombo tolto da incrostazioni dello scheletro. La composizione isotopica del piombo, determinata mediante spettrometria di massa, delle reliquie conservate in Santa Giustina indica che i campioni della cassa e il carbonato di piombo adeso alle ossa del bacino sono uguali e nettamente diversi dai campioni del caperchio.
Questi ultimi risultano invece simili ai frammenti della cassa di san
Mattia, ritenuta di fattura cinquecentesca. I risultati ottenuti sui
campioni delle cupole e delle epigrafi indicano che essi costituiscono un gruppo piuttosto eterogeneo di materiale lavorato e posto in opera in epoca moderna e ottenuto da miscelazione di piombo proveniente da più fonti. I manufatti antichi, rappresentati dalla cassa di san Luca e dal carbonato di piombo di incrostazione dello scheletro, sono riferibili al I-IV secolo d.C. La composiziane isotopica del piombo di tali manufatti nan é riconducibile alla produzione di specifici giacimenti, ed é pertanto interpretabile come effetto del riciclaggio di metallo o rottami metallici provenienti dalle varie miniere dell’area mediterranea, un processo molto diffuso durante il periodo imperiale
Sr, Nd,and Pb isotope evidence of open system evolution at Vulcano (Aeolian Arc)
No full textIn this paper isotopicSr, Nd, and Pb.and geochemical data are discussed with the aim of developing a model for the
evolution of the magmatic system of VulcanoAeolian Arc, Italy.. The magmatism at Vulcano is shoshonitic in affinity. The
87 86 Vulcano products display, throughout the stratigraphic sequence, repeated variations in the isotope ratios Srr Srs
0.70426–0.70524; 143Ndr144Nds0.51254–0.51273; 206Pbr204Pbs19.287–19.548; 207Pbr204Pbs15.616–15.865 and
208Pbr204Pbs39.132–39.818.. The isotopic variations are correlated with the degree of evolution of the rocks, the most
crustal compositions occurring in the most differentiated rocks, and are interpreted as produced by AFC assimilationq
fractional crystallization.. The chemical and isotopic data suggest that in the evolution of the volcanic rocks older than 25 ka
Vulcano Primordiale and opening phases of ‘Caldera della Fossa’., basaltic melts underwent fractional crystallization and
were contaminated by crust material that was isotopically similar to that of the lower crust. This occurred in different
shallow magma chambers, and the melts evolved to mainly shoshonitic compositions. The shoshonitic magmas further
evolved to rhyolites through AFC within chambers in the upper crust, which fed the Mastro Minico and Lentia complexes
25–15 ka.. In the most recent phase of activity, after the collapse of the Lentia complex post-15 ka., shoshonitic and latitic
magmas were enriched in alkalies and hygromagmatophile elements with respect to the older rocks. This change in
composition may be interpreted as due to a change in the primary magmas, as a result of a lower degree of partial melting of
the mantle source compared with the older products. Alternatively, this may occur through magma evolution by AFCB
processes assimilation during fractional crystallizationqtappingqbatch replenishment. in a shallow magma chamber.
Shoshonites and latites evolved further through AFC to rhyolitic compositions, in small, very shallow magma traps emplaced
in upper crust material. Isotopic modelling indicates that syneruptive mixing, together with AFC, determines the isotopic
complexity of the La Fossa products
LOWER CRUSTAL GRANITE GENESIS CONNECTED WITH CHEMICAL FRACTIONATION IN THE CONTINENTAL-CRUST OF CALABRIA (SOUTHERN ITALY) RID A-1229-2011
No full textGranulite-facies rocks, of metasedimentary and metaigneous type are exposed in the Serre (Southern Calabria). They are representative of the intermediate - lower crust remaining after the Hercynian orogeny and exhumed during the Alpine orogeny. Huge masses of calc-alkaline and peraluminous granitoids intrude both the lower crust and the overlying shallower crustal levels. In this paper, we deal with the composition of the lower crust and its relationship with granitoids along the Serre section, on the basis of major, minor, trace elements, REE and Sr, Pb, Nd isotope systematics. The metapelites, which make up a large part of the lower crust section, have a restite character due to the extraction both of in situ crystallized leucosomes and of some of the late-Hercynian peraluminous granites which intrude the upper crust. The dominant calc-alkaline granitoids seem to require the contribution of hydrated basic lower crust and/or mantle-derived magmas for their genesis. As a result it appears that the melting processes which affected the lower crust about 300 Ma ago, at the same time as decompression, produced a crustal differentiation contributing to the generation of granitoids which migrated toward high crustal levels. The occurrence of some gabbroic rocks and microgranular mafic enclaves associated with the calc-alkaline plutonic rocks indicates that the mantle contributed both thermally and chemically to granitoid genesis. Nd model ages in granitoids (1.0 - 2.4 Ga) indicate significant addition of juvenile material in late Proterozoic - early Palaeozoic times
Contrasting Fe-enrichment trends and redox paths in calc-alkaline granitoids from Cima d'Asta, southern Alps, Italy: the control of PH2O. Mineral
No full textLower crust composition and evolution: geochemical and isotopic evidence from the Serre granulites (Calabria, southern Italy)
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