Laura Gaggero

The Variscan post-collisional volcanism in Late Carboniferous–Permian sequences of Ligurian Alps, Southern Alps and Sardinia (Italy): a synthesis

Abstract In the considered wide sector of the West-Mediterranean southern Europe, the collisional phase of the Variscan orogeny during Late Carboniferous and Permian times was followed by magmatic intrusive and effusive activity and sedimentation into intracontinental, alluvial to lacustrine basins originated by wrench- to normal-fault systems. The first volcanic cycle (generally Late Carboniferous-Early Permian in age) is represented by early calc-alkaline andesites and rhyolites, in variable amounts, and by following large volume of rhyolites, and by dacites. Both andesites and rhyolites show K-normal and high-K calc-alkaline character. The origin of the liquids of the first cycle is ascribed to partial melting processes at the mantle–crust interface telescoped within a thickened crust. The melting is considered as the consequence of thermal re-equilibration following isostatic disequilibrium and the subsequent collapse of the orogenic belt; the ascent of liquids occurred in a (trans-)tensional regime. The second magmatic cycle is represented by alkaline magmatism, and exhibits typical anorogenic features consistent with a rifting regime. This event was no more related with the collapse of the Variscan belt, but rather to the post-Variscan global re-organization of plates that evolved during Late Triassic times to the neo-Tethyan rifting. In both cycles, important differences in timing, areal distribution and outpoured volumes arise.

Age, origin and geodynamic significance of plagiogranites in lherzolites and gabbros of the Piedmont-Ligurian ocean basin

Abstract U-Pb zircon dating, Sr-Nd isotope tracing and major/trace/RE element analyses were performed to constrain the age, origin and geodynamic significance of plagiogranites that intrude lherzolites and gabbros in the Ligurian Alps and the Northern Apennines. In addition, a host Fe-diorite was investigated. Samples from the Ligurian Alps were collected from the Voltri Group and the Sestri-Voltaggio Zone, whereas the plagiogranites from the Northern Apennines were taken in the Bracco unit. All these units have been affected by Alpine metamorphism reaching eclogite facies in the Voltri Group, blueschist degree in the Sestri Voltaggio samples, and prehnite-pumpellyite facies in the Bracco Unit, which has additionally been affected by rodingitization. U-Pb zircon ages of 150 ± 1, 153 ± 1 and ≈ 156 Ma were obtained, respectively, for two plagiogranites and the host Fe-diorite in the Ligurian Alps, and an age of 153 ± 1 Ma was determined for the plagiogranite in Northern Apennines. Inherited components in zircon and initial Pb in plagioclase indicate mixing of variously differentiated basaltic magmas with small amounts of roughly 1.7–2.1 Ga old continental crust material. REE patterns in both the plagiogranites and the host diorite are characterized by high REE abundance, and moderate LREE enrichment. Nd isotopic compositions lie in the range of N-MORB sources, yielding initial epsilon Nd values between + 8.8 and + 9.7, whereas Sr is isotopically heterogeneous. The geochemical pattern of the plagiogranites and the host Fe-diorite requires melting of a MORB-type mantle source that experienced LREE enrichment shortly before melting. The most likely explanation for such enrichment is the injection of melts derived by small degrees of melting from an adjacent mantle region. The basaltic, LREE-enriched parent magmas generated from this enriched domain have probably undergone up to about 72% of low-pressure fractional crystallization prior to their emplacement into the gabbro-peridotite complex. The 156–150 Ma magmatism occurred in close relation to normal faulting, sedimentation of breccias, and detachment of the mantle complex from its overlying continental crust, followed by exposure on the ocean floor. This tectono-magmatic event in the Ligurian Alps and the Northern Apennines reflects rifting of the Adriatic-Iberian continental plate segment, preceding wider opening of the Piedmont-Ligurian ocean basin and pillow basalt deposition.

U–Pb zircon ages for post-Variscan volcanism in the Ligurian Alps(Northern Italy)

Abstract: Ages of Permian volcanic rocks from the Ligurian Briançonnais domain (Western Italian Alps) have been determined by laser ablation inductively coupled plasma mass spectrometry U–Pb dating of zircon. Three major volcanic units yielded zircons that were dated: calc-alkaline rhyolites (285.6 ± 2.6 Ma), andesites (with inherited cores yielded ages c. 476 Ma and older) and voluminous rhyodacites–rhyolites (272.7 ± 2.2 Ma). Following an amagmatic, sediment-starved time gap of c. 14 Ma, alkaline volcanic activity is recorded, at the top of the sequence, by K-alkaline rhyolites dated at 258.5 ± 2.8 Ma. The Ligurian segment of the Southern Variscan belt records transtensional and then extensional tectonics associated with the volcanic activity. The switch from calc-alkaline to alkaline activity corresponds to the transition from a post-orogenic to an anorogenic setting in the Southern Variscides; it may represent progressive and increasing delamination of the continental lithosphere, accompanied by partial melting of the lithospheric mantle. Supplementary material: Technical and data acquisition parameters, U–Th–Pb isotope analyses and calculated ages of zircons from samples and standard, and trace element compositions of selected zircons are available at: http://www.geolsoc.org.uk/SUP18322.

Rifting and Arc-Related Early Paleozoic Volcanism along the North Gondwana Margin: Geochemical and Geological Evidence from Sardinia (Italy)

Three series of volcanic rocks accumulated during the Cambrian to Silurian in the metasediment-dominated Variscan basement of Sardinia. They provide a record of the changing geodynamic setting of the North Gondwana margin between Upper Cambrian and earliest Silurian. A continuous Upper Cambrian–Lower Ordovician succession of felsic submarine and subaerial rocks, dominantly transitional alkaline in character (ca. 492–480 Ma), is present throughout the Variscan nappes. Trace element data, together with Nd isotope data that point to a depleted mantle source, indicate an ensialic environment. A Middle Ordovician (ca. 465 Ma) calc-alkaline bimodal suite, restricted to the external Variscan nappes, overlies the Sardic Unconformity. Negative εNdi values (−3.03 to −5.75) indicate that the suite is a product of arc volcanism from a variably enriched mantle. A Late Ordovician–Early Silurian (ca. 440 Ma) volcano-sedimentary cycle consists of an alkalic mafic suite in a post-Caradocian transgressive sequence. Feeder dykes cut the pre-Sardic sequence. The alkali basalts are enriched in Nb-Ta and have Zr/Nb ratios in the range 4.20–30.90 (typical of a rift environment) and positive εNdi values that indicate a depleted mantle source. Trachyandesite lavas have trace element contents characteristic of within-plate basalt differentiates, with evidence of minor crustal contamination.

Geochemical investigation of the igneous rocks from the Nurali ophiolite mélange zone, Southern Urals

Abstract The Nurali massif is a small ophiolite located along the Main Uralian Fault in the Southern Urals. It includes a mantle sequence, a transition zone and a melange zone, with an intrusive, gabbro to oxide-diorite, unit interposed between the transition zone and the melange. Various igneous rocks occur as exotic blocks within the Nurali melange, including tholeiitic (gabbro to diorite) and calc-alkaline (diorite, tonalite, granodiorite and granite) intrusives, tholeiitic basalt and calc-alkaline basaltic andesite to dacite. The Poljakovka Complex basalts of Silurian age are similar to the tholeiitic effusives and are also intruded by gabbros similar to those in the melange zone calc-alkaline intrusives. The rocks of tholeiitic affinity in the tectonic melange are the remnants of an oceanic stage. Both intrusive and effusive calc-alkaline products can likely represent a subduction-related magmatism developed in an arc environment (intra-oceanic arc or continental margin). They are analogous to those from the Kraka massif which occur west of the Main Uralian Fault.

High-Mg potassic rocks in the Balkan segment of the Variscan belt (Bulgaria): implications for the genesis of orogenic lamproite magmas

Ultrapotassic plutons from several domains of the Variscan orogenic belt have been in turn interpreted as syn- to post-orogenic due to their age spread, but assessment of their geodynamic setting and source regions is still open to interpretation. In the Svoge region (Bulgaria), at the southern margin of the Balkan orogen, peralkalic plutons are hosted within Ordovician pelites. The main intrusion, with lamproitic affinity, which hosts monzodiorite xenoliths and a polyphase syenite suite, was emplaced at a shallow level. 40 Ar– 39 Ar dating by step-heating of amphibole and biotite yielded a Early Carboniferous intrusion age for the main body (337 ± 4 and 339.1 ± 1.6 Ma). The lamproite intrusion is silica-rich compared with bona fide lamproites and characterized by moderate LILE and La N /Yb N enrichments. Sr and Nd isotopic data (initial ϵ Nd in the range −4.87 to −5.88) suggest an origin in a depleted lithospheric mantle, possibly refertilized by eo-Variscan subduction. The high-K syn-tectonic plutonism in several zones of the Variscan orogen (Bohemian, Austro-Alpine, Vosges, French and Corsica domains) is consistent with a derivation of high-K magmatism from partial melting of metasomatized mantle following the subduction along the collision front between Gondwana and Laurasia.

Crystal-rich mass flow deposits related to the eruption of a sublacustrine silicic cryptodome (Early Permian Collio Basin, Italian Alps).

Abstract Sedimentological, granulometric and petrographic data are presented from a detailed study on a crystal-rich mass flow deposit, which is presumably related to the eruption of a sublacustrine cryptodome. The deposit forms a prominent intercalation in the Lower Permian Collio Formation in the Italian Alps north of Brescia. Outcrops of the 10–20-m-thick volcaniclastic deposit (Dasdana I Beds, DB) can be traced over 12 km from east to west. The DB consists of a thick, crystal-rich, sandy–gravelly lower subunit representing a sequence of amalgamated Bouma-a(b) divisions overlain by a thin, well-bedded, sandy–muddy subunit that is rich in outsize porphyritic silicic fragments. Modal and computer-aided image analyses reveal that the crystal-rich lower subunit contains up to 80% of volcanogenic crystals. Some samples contain up to 60% of porphyritic fragments, which have a phenocryst content of about 20%. The wide textural range from cryptocrystalline, poikilomosaic, to rarer medium-grained granophyric groundmass, the irregular to lensoid shapes of the porphyritic fragments, and the presence of basement and sedimentary clasts suggest that the DB originated from a sublacustrine eruption of a partially extrusive cryptodome (ca. 1.6 km3). Two other porphyritic felsic cryptodomes (Dosso dei Lupi, Dosso del Bue), described briefly here, emplaced into the Collio Formation sometime after the DB event, and expose flat bases and tilted sediments at their sides. Textures observed in these domes are comparable to those found in the DB porphyritic fragments.

Nature of the transition zone in the Nurali ophiolite, southern Urals

Abstract The Moho transition zone above residual peridotites of the Nurali ophiolite is represented by a layered dunite-wehrlite-clinopyroxenite sequence with an enstatite-olivine upper part. The absence of deformational fabric patterns of solid-state flow sets up a problem of the magmatic nature of the Nurali transition zone ultramafics. It is interpreted to be the product of a multistage magmatic crystallisation in narrow chambers at low-pressure (plagioclase-peridotite facies) conditions. Systematic chemical trends across the transition zone show gradual increase of Mg# and depletion in compatible elements in the magma upsection, and the formation of the high-Mg enstatite-olivine cumulates at the top. This model suggests high degree of melting of the mantle source. The crystal fractionation of limited magma portions did not yield any regular chemical trends at the scale of the whole transition zone section, but resulted in rhythmic variations of mineral composition. Such a series of magma injections is thought to occur within a rifting zone. Metasomatic influence of gabbroic magma led to only local compositional variations. The gabbroids (hornblende gabbro to diorite) are the products of a later magmatic process with more significant crystal fractionation.

On the conservation of architectural artistic handwork of the ‘Pietra di Finale’

Abstract This paper focuses on the characteristics of alteration of the Pietra di Finale, a stone with long tradition in Ligurian regional architecture. The Pietra di Finale is quarried near Finale Ligure in western Liguria, and widely used in the ancient buildings (e.g. by the architect Galeazzo Alessi). Three main types are exploited: white, pale pink and dark pink, and all were the object of the present study. The stone was exposed to atmospheric agents, to investigate weathering processes and eventual suitable protection against the Ligurian climate. In particular we studied a laboratory procedure of ageing by exposing the stone to an artificial acid rain whose composition is the same as the most polluted rains fallen on Ligurian region. Consequently we observed the alterations that occurred on stone after exposure to the same amount of artificial and natural rains. The same procedure was applied to samples protected by a fluorinated copolymer such as Akeogard LTX (Syremont, Italy).

Substratum microtexture affects the boring pattern of Cliona albimarginata (Clionaidae, Demospongiae)

This work focuses on the erosion pattern morphology of the Indonesian excavating sponge Cliona albimarginatain three different calcareous substrates (Carrara marble, branches of the coral Acropora, and the umbo of the bivalve Hippopus). The experimental data demonstrate that the sponge produced galleries that followed the preferred orientation of fibrous aragonite crystals (Acropora), or the parallel lamination within the Hippopusshell. Conversely, the homogeneous granoblastic, mosaic texture of Carrara marble was etched without preferred directions. SEM analysis showed that pit size also varies between the three substrates in relation to the respective substrate porosity.