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Metagabbros, meta-monzogabbros and metabasites from Serre Massif (Calabria-Italy): evidence of a Neoproterozoic magmatism at crust-mantle transition
No full textDifferent types of Variscan mafic granulites characterize the lower crust of the Serre Massif (Calabria,
Southern Italy). They consist of: layered metagabbros (Pl+Opx+Amph±Cpx±Bt±Qtz±Grt), metric
bodies of meta-monzogabbros (Pl+K-feld+Opx+Cpx+Bt±Qtz±Grt) and lenses or layers of metabasites
(Pl+Opx+Cpx±Bt±Amph±Qtz±Grt) interleaved the overlying migmatitic metapelites and felsic granulites.
The Variscan metamorphic events developed in high grade metamorphic conditions in which partial melting
and fluid interaction were effective (Fornelli et al., 2011, 2014). However, the whole chemical composition
of mafic granulites seems unmodified because migrations of melts at large scale have not been documented.
Therefore, the whole rock compositions of granulite mega-samples can give suggestions about the magmatic
history of their protoliths.
Geochemical data indicate a mantle origin for the Neoproterozoic magmatic protoliths (~570 Ma, Fornelli
et al, 2011). Metagabbros and metabasites show sub-alkaline features (Na2O+K2O=1-5%) with K2O<1%,
whereas meta-monzogabbros display alkaline character (Na2O+K2O=5-7%) with K2O around 3% testified by
Bt and K-feld. In metagabbros, the 87Sr/86Sr(570 Ma) ratio ranges from 0.703 to 0.708 with low values of 87Rb/86Sr
(from 0.001 to 0.024). Metabasites have higher 87Sr/86Sr(570 Ma) values (0.707-0.710) with 87Rb/86Sr ratio from
0.025 to 0.237 Meta-monzogabbros show 87Sr/86Sr(570 Ma) ratio around 0.705 with high 87Rb/86Sr (0.489-0.544).
The εNd(570 Ma) varies from 4.902 to -0.843 in metagabbros, from 0.741 to -2.388 in metabasites, whereas in
meta-monzogabbros assume the values varying from -1.078 to -1.253.
The meta-monzogabbros show a strong mantle affinity but are enriched in incompatible elements, their
origin seems linked to small degree of enriched mantle melting. The metagabbros and metabasites show
crust-mantle interaction for their initial 87Sr/86Sr and εNd(570 Ma) values. We hypothesize that subsequent and
incremental partial melting events (after the production of gabbroic magma) produced mantle melts with
tholeiitic and calc-alkaline affinities migrating in the overlying metasedimentary crust; the stay in hot lower
metasedimentary crust induces a certain grade of crust contamination charged by crustal fluids or melts,
evidenced by the increase of initial 87Sr/86Sr ratio and by decrease of εNd(570 Ma) values at least in protoliths of
metabasites. On the other hand, the placement of metabasites within metasedimentary crust constrains this
suggestion
Tectonothermal history of the Hercynian continental crust of the Serre (Southern Calabria, Italy) monitored by Rb-Sr biotite resetting
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PER UN' ARCHEOLOGIA FORESTALE IN PIEMONTE. IL GIACIMENTO VILLAFRANCHIANO DELLA STURA DI LANZO (VILLANOVA NOLE CANAVESE).
No full textLe indagini microxilotomiche condotte sui tronchi rinvenuti nella Stura di Lanzo nel sud del Piemonte, in terreni villafranchiani, hanno mostrato l'appartenenza di questi ad entità arboree della famiglia delle Taxodiaceae
Building an orogen: Review of U-Pb zircon ages from the calabria-peloritani terrane to constrain the timing of the southern variscan belt
Full text linkThe application of zircon dating to the reconstruction of orogenic systems is invaluable since time constraints of the geological evolution of orogens are crucial for the proposal of geodynamic and paleogeographic models. Zircon is one of the most promising accessory minerals in geochronology of crystalline basements because of its high-closure temperature. Moreover, U-Pb data of relict and recrystallized grains indicate the maximum sedimentation age as well as the timing of metamorphism in metasediments. In addition, the U-Pb ages of magmatic zircons constrain the timescale of magmatism. The Calabria-Peloritani terrane (CPT) represents a key area in the Southern Variscan Belt, whose reconstruction is still unresolved. Therefore, a review of literature zircon age data accompanied with new data from six samples of orthogneisses, paragneisses, amphibolites, and actinolite schists, helps to constrain the evolution of this Cadomian fragment, affected by metamorphic and magmatic Variscan events. A revisiting of the timing of the geological events from Paleo-proterozoic to Permian is revealed by comparing the internal textures of zircons and their U-Pb age clusters. The detected age peaks at 2500 Ma, 1600 Ma, and 1000 Ma in the CPT were related to a provenance from West and East Gondwana realms. A sedimentation age around 630 Ma emerges for the middle-deep crust terranes of the CPT, affected by Ediacaran (579- 540 Ma) intrusions, accompanied by metamorphism dated at 556-509 Ma in the host metasediments. In the following, during Ordovician-Silurian extensional tectonics, the former Cadomian terranes were at least locally affected by fluid-assisted metamorphism (around 450 Ma) whereas the upper extensional basins that formed, were infilled by sediments along with interspersed volcanic to subvolcanic products. All these pre-Silurian terranes were involved in the subduction process of the Palaeotethys-Gondwana margin beneath Laurussia. The compressive phase began around 347 Ma, with under-thrusting of the formerly Gondwana substrate that was subjected to middle-high grade metamorphism, while the Ordovician-Silurian sediments were scraped off along the front of the Southern Variscan Belt and affected by low-grade metamorphism. Decompression of the whole Variscan orogenic system started around 320 Ma, together with uplifting of the chain and emplacement of widespread granitic intrusions which ended around 280 Ma and completed the Variscan orogenic cycle in the CPT
Preliminary U-Pb Detrital Zircon Ages from Tufiti di Tusa Formation (Lucanian Apennines, Southern Italy): Evidence of Rupelian Volcaniclastic Supply
Full text linkU-Pb spot ages have been determined on detrital zircons from two samples of volcaniclastic arenites belonging to the Tufiti di Tusa Formation (TTF) outcropping in the Lucanian Apennines (Southern Italy). Many petrographic and geochemical studies have been performed on these sandstones with the aim of defining their detritus source. A new and precise evaluation of the mineralogical composition of metamorphic lithic fragments, together with U-Pb detrital zircon ages, helps to clarify the deposition age of these syn-sedimentary volcaniclastic sandstones and constrains their source areas. Volcaniclastic arenites consist of andesitic fragments and single minerals of plagioclases, clinopyroxenes, and hornblendes, while the metamorphic lithics are mainly fragments of blue amphibole-bearing micaschists, serpentinites, ophicalcites, phyllites, and medium-grade micaschists. Phaneritic plutonic fragments consist of quartz, feldspar, and micas. Carbonate components include biomicritic and biosparitic fragments. Eighty age data collected from 56 zircons reveal a wide age spectrum, ranging from Neoarchean to Rupelian (from 2712 ± 25 to 30 ± 1 Millions of years (Ma)). The age data show that in volcaniclastic sandstones there is evidence of ancient crystalline basements involved in Cadomian and Variscan orogenesis (ages from 2712 ± 25 Ma to ≈260 Ma), whereas the measured ages of 157 Ma testify the events of Pangea fragmentation and the ages between 78 and 67 Ma are related to subduction metamorphism connected to the Alpine orogenesis. Fifty percent of the estimated detrital zircon ages show a mean concordant age of 33 ± 1 Ma, they have been measured on idiomorphic crystals with undisturbed magmatic oscillatory zoning. These data reveal the true sedimentation age of Tufiti di Tusa sandstones at least at the sampled levels, coeval with that recorded in other Rupelian volcaniclastic successions outcropping in the Northern Apennines and in the Western Alps (e.g., Val d’Aveto-Petrignacola Formation, Ranzano Formation and Taveyannaz Sandstone). These data represent preliminary suggestions of the sedimentation age of the Tufiti di Tusa Formation and Rupelian paleogeography in the Western Mediterranean area. During Priabonian-Rupelian times, the volcanic calc-alkaline detritus linked to a widespread syn-sedimentary igneous activity on the hinterland terranes of the foredeep basins in the Apennine-Maghrebian orogen extended from north to south in the Western-Central Mediterranean area
Ductile Shearing and Focussed Rejuvenation: Records of High-P (eo-)Alpine Metamorphism in the Variscan Lower Crust (Serre Massif, Calabria—Southern Italy)
Full text linkIn the present study, we unveil the real significance of mylonitic reworking of the polymeta-morphic crystalline basement in the Serre Massif of Calabria (Southern Italy). We use a multidisci-plinary approach to comprehend the structural, microstructural and petrologic changes that occurred along a, so far, not much considered shear zone affecting the Variscan lower crustal rocks. It was never before studied in detail, although some late Cretaceous ages were reported for these mylonites, sug-gesting that this shear zone is of prime importance. Our observations reveal now that the formation of the new structural fabric within the shear zone was accompanied by changes in mineral assem-blages, in a dominant compressive tectonic regime. During this tectono-metamorphic event, high-P mylonitic mineral assemblages were stabilized, consisting of chloritoid, kyanite, staurolite, garnet and paragonite, whereas plagioclase became unstable. Average peak P–T conditions of 1.26–1.1 GPa and 572–626◦C were obtained using THERMOCALC software. These data question (i) that the Serre Massif represents an undisturbed continuous section of the Variscan crust, as generally suggested in the literature, and (ii) highlight the role of (eo-)Alpine high-P tectonics in the Serre Massif, recorded within mylonite zones, where the Variscan basement was completely rejuvenated. © 2022 by the authors. Licensee MDPI, Basel, Switzerland
Studio Petrografico e Mineralogico dei materiali utilizzati nella costruzione del Castello di Lagopesole (Pz)
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