Marcello Franceschelli

SHRIMP dating of zircons in eclogite from the Variscan basement in north-eastern Sardinia (Italy)

SHRIMP (Sensitive High-Resolution Ion Microprobe) U-Pb ages of zircons from a single sample of mafic eclogite (Punta de li Tulchi, Sardinia, Italy) are reported. The study under cathodoluminescence (CL) reveals two groupsof metamorphic zircons and the SHRIMP analyses allow recognition of three ages: 1) 453 ′ 14 Ma; 2) 400 ′ 10 Ma, and 3) 327 ′ 7 Ma. The age of 453 ′ 14 Ma could be that of the magmatic protolith and an age of 327 ′ 7 Ma can reasonably be attributed to the main Variscan collisional event in Sardinia, which produced Barrovian-type metamorphism, and retrogression of eclogite under amphibolite-facies metamorphism. The intermediate age 400 ′ 10 Ma is difficult to interpret and it could represent either the age of the eclogite facies metamorphism or it is a result of Pb-loss during the main Variscan event at 327 ′ 7 Ma.

Fine-scale chlorite-muscovite association in low-grade metapelites from Nurra (NW Sardinia), and the possible misidentification of metamorphic vermiculite

Chlorite-muscovite porphyroblasts occur in the low-grade metapelites from Nurra (NW Sardinia). Although actually heterogeneous and built up by two different intermixed minerals, the porphyroblasts strongly simulate optical homogeneity and produce microprobe analyses that are typical for metamorphic vermiculite. Transmission and scanning electron microscopy, combined with analytical facilities, leads to a deeper characterization of the porphyroblasts, that actually consist of a random association of chlorite and muscovite.As a further result, the present study also shows that the existing criteria used to identify metamorphic vermiculite, namely optical features and microprobe composition do not warrant the correct identification.Finally, the present results once more confirm the importance of back-scattered electron imaging in the SEM as a powerful tool for the routine petrographic study of sedimentary and low-grade metamorphic rocks.

Transformation of cumulate mafic rocks to granulite and re-equilibration in amphibolite and greenschist facies in NE Sardinia, Italy

Abstract The ultramafic amphibolites hosted in the Hercynian migmatite of NE Sardinia consist of three main compositional layers (A, B, C) from a few metres to a few decametres thick. Layer A is made up of coarse-grained olivine, chlorite, amphibole, spinel, minor pyroxene, garnet and, rarely, plagioclase. Layer B is made up of coarse-grained plagioclase, olivine, pyroxene, spinel, garnet and amphibole. Layer C consists mainly of porphyroblastic garnet, pyroxene, large amphibole grains (up to 5 cm) and minor plagioclase. On the basis of mineral assemblages and microstructures, three stages of mineralogical re-equilibration can be recognised: granulite, amphibolite and greenschist. Primary igneous olivine and anorthite reacted under granulite conditions to produce coronas consisting of orthopyroxene, clinopyroxene, green spinel and garnet. The amphibolite stage is characterised by the formation of brown and green clinoamphiboles (between pyroxene and corona garnet), anthophyllite, talc, Mg-rich chlorite, plagioclase and Cr-bearing spinel. Greenschist stage minerals, mostly replacing the mafic minerals, consist of tremolite, fayalite, epidote, albite, calcite, dolomite and serpentine. The history of the ultramafic amphibolites started with igneous crystallisation and continued through granulite ( T =700–740 °C, P =∼8–10 kbar), amphibolite ( T =580–640 °C, P =4–6 kbar) and greenschist facies ( T =∼330–400 °C, P Evaluation of P – T conditions indicated a P – T path from the protolith to granulite stage, characterised by an increase in pressure and temperature, and then from the granulite facies through amphibolite to the greenschist stage, characterised by a decrease in pressure and temperature. The petrological evolution of the ultramafic amphibolites and the P – T time path is discussed in the context of the Hercynian orogeny in Sardinia.

Sudoite, a rock-forming mineral in Verrucano of the Northern Apennines (Italy) and the sudoite-chloritoid-pyrophyllite assemblage in prograde metamorphism

Sudoite, the di-trioctahedral chlorite with ideal composition (Mg2Al)(Al2)(Si3Al)O10(OH)8 is a widespread rock-forming mineral in meta-siltstones and psammites of the Verrucano sequence of the Northern Apennines. Sub-ellipsoidal aggregates, probably derived from muscovite clasts, consisting of sudoite, pyrophyllite and muscovite, are common; sudoite may also occur as thin blades in the rock matrix. The co-existence of sudoite, Ferich chloritoid and pyrophyllite, reported here for the first time, has been observed in specimens from the M. Argentario and Monticiano-Roccastrada areas. This three-phase assemblage, diagnostic of a specific metamorphic facies, may be a tool for detailed zonation of low-grade terranes.

The East Variscan Shear Zone: new insights into its role in the Late Carboniferous collision in southern Europe

A comparison of the petro-tectonic features recorded in the Variscan Massifs scattered throughout the Alps, the Corsica-Sardinia-Maures-Tanneron Massif, the Calabria-Peloritani Arc, and the Northern Apennines, has allowed us to propose that they belonged to the same geodynamic realm until Late Carboniferous time. In the interval 330–300 Ma, the development of a regional dextral strike–slip shear zone, the East Variscan Shear Zone (EVSZ), affected all the massifs, leading to their spatial separation. The EVSZ developed, together with numerous regional shear zones, under a transpressional tectonic regime deriving from the Late Carboniferous collision between Gondwana, peri-Gondwana microcontinents (Armorica and Avalonia), and Laurussia plates. The EVSZ evidently played a key role in the evolution of the subsequent Alpine and Apenninic cycles, acting as a pre-existing tectonic barrier. Our proposed geodynamic reconstruction does not reflect the acquisition of new data, but is based on the analysis and review of the recent geological literature.

U–Pb zircon dating and nature of metavolcanics and metarkoses from the Monte Grighini Unit: new insights on Late Ordovician magmatism in the Variscan belt in Sardinia, Italy

In the external units of the Sardinian Variscides Nappe Zone, volcanic and volcanoclastic successions of Middle Ordovician age follow Lower Paleozoic calc-alkaline magmatism developed at the northern Gondwana margin. We present geochemical and zircon U–Pb isotopic data for the Truzzulla Formation, a low-to-medium-grade metamorphic volcanic–volcanoclastic succession belonging to the Monte Grighini Unit, the deepest unit in the Nappe Zone. Geochemical and radiometric data allow us to define a Late Ordovician (Katian) magmatic (volcanic) event of calc-alkaline affinity. These new data, in conjunction with previously published data, indicate that in the Sardinian Variscides, the age of Lower Paleozoic Andean-type calc-alkaline magmatism spans from Middle to Late Ordovician. Moreover, the age distribution of calc-alkaline volcanics and volcanoclastic rocks in the Nappe Zone is consistent with a diachronous development of Middle–Late Ordovician Andean-type magmatic arc through the portion of the northern Gondwanian margin now represented by the Sardinian Variscides. This reconstruction of the Sardinian Variscides reflects the complex magmatic and tectonic evolution of the northern margin of Gondwana in the Lower Paleozoic.

Li, B-rich Rhaetian metabauxite, Tuscany, Italy: reworking of older bauxites and igneous rocks

Abstract Li, B-rich metabauxite layers occur in two outcrops of talus metabreccias intercalated between Norian and Liassic marbles in the Alpi Apuane, Tuscany, Italy. Field evidence, petrographical, geochemical, and paleogeographic, paleoclimatologic considerations suggest that bauxitic material was derived from a multistage reworking of Late Carboniferous–early Autunian primary deposits now completely removed. The identified lithotypes are: cookeite-rich metabauxite, cookeite-bearing metabreccia, chloritoid schist and chloritoid phyllite. The Mg content of calcite, and the Si content of muscovite and mineral assemblages indicate temperatures of 350°–380°C, pressures of f O 2 -values of 10 −30 to 10 −24 for the Alpine metamorphism. In comparison with the average Mesozoic bauxite of Bronevoy et al. (1985) the metabauxite is: strongly enriched in Si, K, Ca, Mg, Li, B, Be, Ni and Ce; it shows comparable contents of Co, La, Eu, Tb, Ta and Th; it is depleted in Ga, Cr, V, Zn, Mn, Hf, U; and is strongly depleted in Sr, F, Cu. A metabauxite layer shows decreasing abundance of Fe and Ga from bottom to top with a pattern opposite to that of primary bauxite deposits (inverted profile). The strong enrichment of Li and B is not related to evaporitic conditions, since the MgO, Na 2 O and Sr contents are too low, and indicator minerals such as dravitic tourmaline, albite and scapolite are missing. During the Permian, volcanic products similar to the coeval peralkaline–peraluminous rocks of Corsica–Sardinia were widespread in Tuscany. B, Li-rich minerals were produced by circulation of hot hydrothermal fluids within the Permian volcanic rocks during and soon after their emplacement. The erosion of such hydrothermal products supplied detrital material to the talus metabreccia.

Ca distribution between almandine-rich garnet and plagioclase in pelitic and psammitic schists from the metamorphic basement of North-Eastern Sardinia

AbstractGarnet and plagioclase pairs from fourteen selected samples, from garnet to sillimanite zones, collected along a NS traverse throughout the metamorphic basement of NE Sardinia, have been analyzed by microprobe.Beyond the garnet isograd, plagioclase has albitic composition and the garnet (a Ca-rich almandine) shows Ca/ Ca+Mg+Fe ratios of about 0.35–0.30, fairly constant from core to rim.Towards the North, still in the garnet zone, when on the large albitic core of plagioclase a thin and discontinuous oligoclasic rim (An22–An18) formed, we observe in the garnet edge an abrupt decrease of the Ca/Ca+Mg+Fe ratio (0.27–0.16).In the staurolite and sillimanite zones garnet does not show significant Ca-zoning and it is characterized by low Ca content (Ca/Ca+Mg+Fe<0.1); the coexisting plagioclase has oligoclasic (An16–An27) composition.The chemical data and the microstructural evidence on growth time indicate that the garnet and plagioclase had a strong mutual interference in determining the relative Ca distribution.The most relevant reactions are discussed and, in particular, the antipathetical Ca-zoning, recorded by garnet and plagioclase in the garnet zone, is considered as the evidence of temperature increase during growth of the two minerals. It is also suggested that the sharp variation of Ca content at the garnet edge was controlled by the discontinuous nature of plagioclase solid solution in the “peristeritic” range.The order of appearance of garnet and oligoclase in the basement of NE Sardinia is also discussed in comparison with other well known metamorphic sequence (Vermont, New Zealand and Dalradian). It is concluded that the different order of appearance is controlled other than the different nature of the calcic phases in the lower grade zones also by the

Geothermobarometry on anatectic melts – a high-pressure Variscan migmatite from northeast Sardinia

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