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The Kamieniec Metamorphic Belt (KMB) and the Doboszowice Metamorphic Complex (DMC) expose a fragment of the pre-Variscan volcano-sedimentary cover preserved in the Fore-Sudetic Block in the NE part of the Bohemian Massif. We present the age of detrital and magmatic zircon grains and the bulk rock chemical composition of rock samples from the KMB and the DMC to better understand the evolution of the early Palaeozoic Gondwana margin. The zircon age spectra were acquired by U–Pb LA–ICP–MS dating and represent two groups that differ by maximum depositional age (MDA). The paragneiss from the DMC displays the MDA at 456 Ma, whereas the mica shist from the KMB displays the MDA at 529 Ma. Older age peaks in both groups of samples are represented by the Neoproterozoic and less frequent the Paleoproterozoic and Archean. The data presented indicate that the rock successions were sourced from the Cadomian orogen and deposited in the basins that developed on the Gondwana margin. Our results support the suggestion that the crystalline basement in the eastern part of the Fore-Sudetic Block has an affinity to the Trans-Saharan Belt or West African Craton and was part of a Gondwana shelf. The final stage of evolution of the studied successions was related to the Variscan thermal overprint. Based on presented data, we support the idea that the suture separating the Brunovistulian domain from the rest of the Gondwana-derived terranes is not related to the closure of the Rheic Ocean and represents a local feature. [ABSTRACT FROM AUTHOR]

Results of a combined petrological, geochemical and geochronological study suggest that metasedimentary rock units in the Krndija region of the Slavonian Mountains, Croatia, were affected by at least three major tectonometamorphic imprints: during the Middle Ordovician (Sardic event), the early Carboniferous (Variscan event), and the Cretaceous (Alpine event). All three metamorphic phases are established by electron microprobe-based in-situ U–Th–Pb dating of monazite grains. The Sardic metamorphic event is additionally confirmed by a precise Lu–Hf garnet-whole-rock isochron age of 466.0 ± 2.3 Ma. Taken together, the data unveil a relatively large and well-preserved piece of the cryptic Sardic orogen in central Krndija, that we name the Kutjevo Zone. A Sardic subduction-related metamorphic event (ca. 540-580 ℃, 8–11 kbar) at ca. 466 Ma is manifested in the mineral paragenesis Ca-rich garnet plus rutile. A low degree of retrograde reequilibration suggests a subsequent fast exhumation. Low-Ca cores in some garnets and staurolite relics record a pre-HP metamorphic event that involves isobaric heating from 570 to 610 ℃ at ~ 7 kbar. We attribute this (so far undated) event to mid-crustal contact metamorphism caused by early Sardic magmatism. Southern parts of Krndija (the Gradište Zone) experienced an (additional?) clockwise PT evolution in Variscan times at ca. 350 Ma. Garnet formed with ilmenite during a PT increase from 580 ℃/5 kbar to 600 ℃/6 kbar and underwent later strong retrograde resorption. Slow Variscan exhumation resulted in andalusite formation at < 550 ℃/ < 3.8 kbar. Penetrative Alpine metamorphism was observed in low-grade phyllites in the north. The lithology and metamorphic history of the Kutjevo Zone is similar to what has been reported from the Sardic Strona-Ceneri Zone in the western Alps. Both areas expose metapelitic (metagreywacke) rocks with a pre-middle Ordovician formation age. These metasedimentary rocks are inter-layered with numerous small amphibolitic units as well as metagranitoids and were likely deposited along the active Gondwana margin, perhaps in a fore-arc position, prior to their subduction during the middle Ordovician. According to recent palaeogeographic reconstructions, both the Kutjevo Zone and the Strona-Ceneri Zone have once resided in an eastern sector of the northern Gondwana margin (i.e., in E-Armorica). We conclude that in the Middle Ordovician, important subduction activities took place in this E-Armorican segment of north Gondwana, which is today exposed in the Alps. The W-Armorican segment of north Gondwana (now exposed in the French, German, and Czech Variscides) had probably already mutated from a (Cadomian) subduction setting to an extensional (transtensional–transpressional) setting by the late Cambrian. [ABSTRACT FROM AUTHOR]

A metapelitic sequence with Silurian protolith from the Horváthertelend Unit (Tisza Mega-unit, Hungary) has a K-white mica + chlorite + quartz + albite + anatase ± tourmaline mineral assemblage. Moderately developed disjunctive and well-developed continuous foliations are present. Geochemical results reflect a dominantly felsic source of the protoliths, suggesting an intermediate to acidic volcanic arc in the provenance area. Metasandstone pebbles in the metaconglomerate indicate a recycled sedimentary source. Raman spectroscopy of carbonaceous material indicates a ~ 350‒370 °C peak metamorphic temperature. The Kübler Index (KIBasel) values of the phengitic K-white mica indicate epizonal metamorphism (0.22 ± 0.04 Δ°2θ). The Chlorite 'Crystallinity' Index (ChCCIS) suggests metamorphic alteration near to the anchizone‒epizone boundary (0.31 ± 0.06 Δ°2θ). K–Ar ages of K-white mica are interpreted as a result of Variscan metamorphism (c. > 310 Ma) and post-Variscan uplift (c. 290 Ma). The predominance of lydite and slate of Llandoverian age and the overlying coarse-grained metagreywacke and metaconglomerate beds of the Szalatnak Slate Formation show strong lithological similarities with the proximal Silurian sequences in the Małopolska Massif (Kielce Region, Holy Cross Mountains, Poland). The original position of the Horváthertelend Unit is presumably to the northeast from the Bohemian Massif, next to the Upper Silesian Block (Moravo-Silesian Zone) and the Małopolska Terrane. [ABSTRACT FROM AUTHOR]

The Mouriscas Complex is a deformed and metamorphosed predominantly mafic igneous complex of Ediacaran and Ordovician age and crops out at the Ossa-Morena/Central Iberian zone boundary in the Iberian Massif, Central Portugal. It comprises amphibolite with Neoproterozoic protoliths (ca. 544 Ma), protomylonitic felsic dykes derived from younger trondhjemitic protoliths (ca. 483 Ma) and garnet amphibolite derived of even younger dioritic protoliths (ca. 477 Ma). The protoliths of the Neoproterozoic amphibolites are calc-alkaline magmas of basic to intermediate compositions with intraplate and active continental margin affinities and are considered to represent the final phase of the Cadomian arc magmatism. They are interpreted to have originated as coarse-grained intrusions, likely gabbro or diorite and generated from the partial melting of meta-igneous lower crust and mantle. Their emplacement occurred near the Cadomian metamorphic event dated at ca. 540 Ma ( P = 7–8 kbar and T = 640–660 °C) which is interpreted to represent a continental collision. During the Late Cambrian-Early Ordovician an extensional episode occurred in the central-southern Iberian Massif and was also observed in other areas of the Variscan Orogen. It led to mantle upwelling and to the development of an aborted intracratonic rift located at the Ossa-Morena/Central Iberian zone boundary and to the opening of the Rheic Ocean to the south of the area studied in present coordinates (i.e., between the Ossa-Morena and South Portuguese Zones). This event has been dated at ca. 477 Ma and was responsible for the melting of deep ancient mafic crust and mantle with formation of bimodal magmatism in an intra-plate setting, as indicated by the protoliths of the protomylonitic felsic dykes with trondhjemitic composition and of the garnet amphibolite. Subsequent Variscan metamorphism took place under amphibolite facies conditions ( P = 4–5.5 kbar; T = 600–625 °C) at lower P - T conditions than the Cadomian metamorphic event. It was followed by greenschist retrogression as suggested by the appearance of actinolite rims and formation of chlorite and epidote. [ABSTRACT FROM AUTHOR]

Two garnet-chloritoid bearing metapelites from the northern Malpica-Tuy unit (MTU) and a staurolite-andalusite bearing metapelite from the underlying parautochthon were studied to decipher their pressure–temperature (P–T) evolution, mainly based on chemically zoned minerals, such as phengite, using P–T, P–O 2 , and T–O 2 pseudosections. In addition, monazite ages were determined with the electron microprobe in the metapelite from the parautochthon. The chloritoid schists from the MTU experienced peak P–T conditions around 21 kbar at 570 °C and a subsequent isothermal exhumation. Thus, two high pressure rock types can be distinguished in the MTU: orthogneiss (peak pressures: 13–16 kbar) and eclogite + metapelite (peak pressures: 20–23 kbar). The latter rock types were exhumed in a subduction channel environment. The staurolite-andalusite bearing schist from the parautochthon experienced only peak pressures around 8 kbar at a temperature around 570 °C since garnet and potassic white-mica with Si contents above 3.2 per formula unit are lacking. This rock represents sediments that were originally at the surface of the downgoing continental plate and metamorphosed in early Variscan times (around 364 Ma) similar to the MTU rocks. Our mean monazite ages of 339 and 320 Ma might be related to backthrusting of the MTU rocks from deeper crustal levels onto the parautochthon. [ABSTRACT FROM AUTHOR]

Coronitic metagabbros occur as small isolated bodies along the contact between the Mariánské Lázně and the Teplá Crystalline complexes in the northwestern part of the Teplá–Barrandian Unit in the Bohemian Massif. Metagabbros show variable metamorphic and textural characteristics with respect to their magmatic mineral assemblage and degree of metamorphism. The aim of this study has been to characterize the mineralogical, chemical, and microstructural changes related to metamorphism in selected samples. Magmatic mineral assemblages in the metagabbros represented by plagioclase, orthopyroxene, clinopyroxene, amphibole, biotite, ilmenite and occasionally involving olivine, spinel or quartz are generally well preserved. Metamorphism is mainly reflected by the formation of single or multiple coronas at the contact of plagioclase with the other primary minerals. The coronas typically consist of amphibole, garnet and orthopyroxene. The progressive breakdown of magmatic plagioclase is reflected by the formation of a mixture of An40 and An90 plagioclase associated with spinel, corundum and locally also kyanite. The calculated P–T conditions show an increase in metamorphic grade towards the structural footwall, i.e. from the east–southeast (~600 ± 50 °C; 10 ± 1.5 kbar) to the west–northwest (~700 ± 50 °C; 13.5 ± 1.5 kbar), which corresponds to the previously reported Variscan metamorphic field gradient in this area. Since there is no gap in P–T conditions between metagabbros included in MLC and TCC and showing similar age and geochemical signature, it is concluded that both complexes were brought together before the gabbro intrusion at ~500 Ma. In addition, the maximum pressure of ~14 kbar estimated for the metamorphism of the gabbro occurring in the eclogite-bearing Mariánské Lázně Complex suggests that the eclogite-facies metamorphism might have been pre-Variscan. The chemistry of the studied metagabbros corresponds to subalkaline basalts with traceelement signatures characteristic of E-MORB, which is consistent with an interpretation that the intrusion of these rocks was related to an intracontinental rifting of the Teplá–Barrandian Unit during Late Cambrian and was not connected to any subduction processes. [ABSTRACT FROM AUTHOR]