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Lineamenti geologico-petrografici delle formazioni affioranti metamorfiche paleozoiche affioranti nell'area di Boccheggiano
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The b0 spacing of muscovite and assemblage in slates from Mt. Argentario (southern Tuscany)
No full text40Ar-39Ar results on amphibole and biotite of an anatectic amphibole-bearing migmatite from the Variscan basement of NE Sardinia, Italy
No full textAmphibole-bearing migmatite from NE Sa rdinia (Italy) and its constraints on Variscan partial melting
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Li-and B-rich cookeite-bearing metabauxite within late Triassic talus breccia, Alpi Apuane, Italy
No full textGeochemistry and mineralogy of detritic rocks from Verrucano type-sequences of Northern Apennines (Monti Pisani and Punta Bianca), Italy
No full textContinuous vs. discontinuous garnet growth in mylonitic micaschists from northeastern Sardinia, Italy: Evidence from LA-ICPMS trace element mapping
Full text linkGarnet with complex, discontinuous zoning is a common occurrence in metamorphic terrains, and the rela tionship between major and trace element zoning can provide insight into the metamorphic evolution of the host rock. Mylonitic micaschists along the Posada-Asinara Shear Zone in the Axial Zone of the Sardinia Variscan chain contain garnet porphyroblasts, enveloped by the S2 schistosity, with distinct core and rim domains. A large garnet porphyroblasts was investigated by laser ablation-inductively coupled plasma-mass spectrometry (LA ICPMS) mapping. The major element compositional variation follows a bell-shaped zoning, with Ca and Mn contents progressively decreasing, and Fe and Mg increasing, from the core to the outer rim. LA-ICPMS mapping revealed a thin and sharp annular enrichment zone in Y, Sc, Dy, Ho, Er, Tm at the mantle-rim boundary. The trace element (TE) compositional profiles show a central enrichment area for HREE (Tm, Yb, Lu). This enrich ment decreases progressively, as a function of atomic number, for Er, Ho and Dy. Elements with even lower atomic number (Tb, Gd, Eu and Sm), are depleted in this central domain, but their content increases in broad shoulders towards the garnet rim. The position of these lateral shoulders migrates progressively rimwards with decreasing atomic number. The REE distribution, trend and behavior in the growth zones of the garnet is an example of TE control during a continuous growth ruled by diffusion-limited REE uptake. The Y + HREE annular enrichment zone, interpreted as resulting from a decrease in the garnet growth rate, reflects a short-lived episode in the garnet growth history
Variscan metamorphism in Sardinia, Italy: review and discussion
No full textThe Sardinian segment of the Variscan chain is divided into four tectono-metamorphic zones from NE to SW: 1) Inner zone, with medium- to high-grade metamorphic rocks and migmatites; 2) Internal Nappe zone, with low- to medium-grade rocks; 3) External Nappe zone, with low-grade metamorphic rocks; 4) External zone, with very low- to low-grade rocks. Metabasite lenses, with eclogite and granulite relics, occur in the migmatites.
Pre-Variscan ages are attributed to the E-W trending folds below the Sardic unconformity (SW Sardinia). Five Variscan deformation phases have been distinguished, the principal ones being: D1, compressional, with SW-vergent folds and S1 schistosity; D2, extensional, with S2 schistosity transposing S1. Later, a composite network of post-D3, syn-D4 shear zones was active from an older HT/LP shear event to a younger MT-LT/LP one. The late D5 phase produced kilometric flexures with an axis parallel to the orogenic trend.
For Inner zone sequences, a complete prograde sequence has been recognised, from the chlorite zone to the sillimanite + K-feldspar one. The Nappe zone shows decreasing metamorphic grade from bottom to top of the Nappe pile, as well as in each single unit. The Variscan P-T-t path is the clockwise loop typical of continental collision. For the Barrovian stage, thermobarometers yielded a Ppeak of 1. 2 GPa (end of D1) and Tpeak of 750°C (early D2). For HP/HT metamorphic relics, calculations indicated: eclogite stage, T 550°-700°C, P 1.3-1.7 GPa; granulite stage, T 650°-900°C, P 0.8-1.2 GPa; amphibolite stage: T 550°-650°C, P 0.3-0.7 GPa. Pre-Variscan igneous rocks consist of Lower to Middle Paleozoic metabasites with MORB or WPB affinity and Middle Ordovician calc-alkaline intrusive and effusive rocks.
The Corsica-Sardinia microplate belongs to the southern passive margin of the Hun Superterrane of the Swiss authors. The beginning of Variscan continental collision is marked by a probable eclogite-producing HP event. Ages of 355-335 Ma and 335-320 Ma for the first two Variscan metamorphic phases and 310-280 Ma and 290-280Ma for late-post orogenic Variscan magmatism in Sardinia match those of the same events in the northern Hun Superterrane active leading margin.
The Variscan Corsica-Sardinia batholith is made up of late-orogenic calc-alkaline intrusive rocks and post orogenic leucogranites. The greater volumes (surface area 6000 Km2) and long gestation times of the Corsica-Sardinia batholith as compared to those of the northern Hun Superterrane batholiths is attributed to the collision between the southern Hun Superterrane margin, including Corsica-Sardinia, and the huge Gondwana Supercontinent
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