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 Nd i values (Ϫ3.03 to Ϫ5.75) 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 Nd i 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.
Eclogitic rocks were sampled from two zones in the basement of the Sredna Gora terrane (central western Bulgaria): (1) partially retrogressed eclogites and amphibolites embedded in sillimanite-bearing garnet-micaschists with kyanite relics and migmatites and (2) banded amphibolites associated with muscovite-bearing metagranites within two-mica paragneisses. Rutile relics and oligoclase + green hornblende + epidote ± biotite pseudomorphs after garnet suggest an eclogite facies event. A tholeiitic, transitional affinity was determined for the protoliths, suggesting a continental rift environment, consistent with several eclogite-bearing complexes in the eastern segments of the Variscan belt that arose from the Cambro-Ordovician Gondwana break-up. Decreasing pressure after the eclogite overprint was demonstrated by (a) diopside-albite symplectite, and (b) plagioclase + red-brown to green amphibole kelyphite. The early static re-equilibration, dated to 398 ± 5.2 Ma by 40 Ar-39 Ar technique, was followed by an amphibolite facies foliation, which was pervasive in amphibolites, gneisses and micaschists, and poorly developed in eclogites. The lithospheric PT paths corresponding to higher and lower metamorphic gradients reflect the juxtaposition of crustal and lithospheric mantle units, respectively. In the build-up of the basement of the Balkan orogen, the physical properties of the lithological complexes might have influenced the collisional pattern of involved microplates.
In the post-Variscan Early Permian deposits of southwestern Europe, andesites belong to successions characterized by the constant occurrence of a basal rhyolite ignimbrite, followed by andesites, and by abundant dacite-rhyodacite products, interbedded with lacustrine sediments. The subalkaline andesite to K-andesite volcanism developed within intramontane basins following the collapse of the Variscan orogen. The compositional features, including trace element ratios and initial Sr (and Nd) of post-Variscan intermediate products allow excluding continental or island arc settings. The andesite composition of Lower Permian lavas was reproduced starting from two potential primary magmas: picrobasaltic and enriched MORB compositions. In particular, the picrobasalt was modified, by adding incremental amounts of a felsic granulite, from the lower continental crust. Mixing and fractional crystallization (MFC) were modelled by the MELTS software in a closed system under isobaric, isenthalpic conditions (P = 0.3 GPa). The computed residual liquid evolves to match closely the composition of Lower Permian andesite lavas after ~34 % crystallization by adding ~27 % of contaminant to the parental magma.
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.40Ar–39Ar 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 LaN/YbNenrichments. Sr and Nd isotopic data (initial ϵNdin 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.
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