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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]

Abstract: 40Ar/39Ar dating of white mica in metapelites, thermobarometric calculations and new structural and geological mapping have been carried out in the Somosierra region (Iberian Central System, ICS) to investigate the Variscan thickening–exhumation processes and the evolution of the paleo-thermal structure in a key region of the Central Iberian Zone. The rocks underwent an intermediate P/T event (M1) during a D1 stage of crustal thickening initiated between 354 and 347Ma. This event was probably related to a first S to SE-directed emplacement of the NW Iberian allochthon, and during the initial development of a mid-crustal subhorizontal shear zone accommodating the ductile flow of thermally weakened crust parallel to the main orogenic trend. Syn-convergent extension of the upper levels occurred by 335–327Ma, when tectonic unroofing led to D2 domal uplift of the orogenic roots and to recrystallization of rocks under intermediate to low P/T conditions (M2). Individualized gneiss domes developed along the orogenic trend, whereas some post-M2 thrusts dated at c. 319Ma reflect lateral flow related to limited gravitational spreading of the domes. Younger upright to SW verging folding of the ICS dome during D3 progressive renewal of contractive deformation took place after 316Ma. Final D4 gravitational collapse occurred under low P/T conditions (M3) at c. 300Ma, accompanied by the intrusion of large volumes of anatectic granitoids. [Copyright &y& Elsevier]

Abstract: The Fe–Mn-rich garnetites (coticules) associated with banded iron formations occur in the Neoproterozoic metamorphic complex of the Desná Unit. Their mineral assemblage consists of almandine-spessartine garnet, quartz, magnetite, epidote and sporadically biotite, chlorite, amphibole, plagioclase, calcite, ilmenite and apatite. The mineralogical, petrological and geochemical results demonstrate that the protolith of coticules and associated BIFs was formed during periods of submarine hydrothermal activity in the Late Proterozoic by differential precipitation of Fe and Mn from low-temperature solutions. The Fe–Mn-rich precipitates mixed to a varied degree with the detritic siliciclastic material. Subsequently the protolith underwent polyphase metamorphism reaching the amphibolite facies grade during the pre-Variscan (M1) and Variscan periods. The P–T conditions assumed for the peak Variscan metamorphism (M2) are T ∼540–580°C and P ∼600MPa, the M3 is characterized by an upper pressure limit 400MPa at a maximum temperature of 600°C and a metamorphic event M4 related to the hydration reactions which occurred during the late-stage of the Variscan orogenesis is characterized by T ∼150–300°C at P <200MPa. [Copyright &y& Elsevier]

Three crystalline complexes were newly described in the area of the junction between the Mongolian Altay and Gobi Altay in the surroundings of Chandman Sum, SW Mongolia. Khan Khayrkhan, Chandman Khayrkhan and Unegt Uul crystalline complexes were distinguished on the basis of their geological position, distinct petrography, metamorphic style and contrasting geochronological data. These units are situated along the northern margin of the Gobi-Altay Terrane and at first sight seem to have a very simple and uniform history. However, detailed studies reveal a more complex and varied evolution. The Unegt Uul Crystalline Complex is a tectonic mélange of leucogranites, amphibolites and mica schists exposed between the branch of the Bogd fault in the north and Lower Palaeozoic sediments in the south. Metamorphic rocks suffered a prograde metamorphic event reaching temperatures of c. 650 °C and pressures of 6-7.5 kbar estimated from amphibolites. This Complex represents the oldest of the three units; the age of leucogranite formation corresponds to Cambrian (518 ± 5 Ma). Chandman Khayrkhan Crystalline Complex is also restricted by Bogd fault from the north but southern boundary is limited by an intrusive contact with the Chandman Massif. It is composed of orthogneisses and migmatites with amphibolite and calc-silicate lenses. The Complex was affected by a HT, likely periplutonic, metamorphic event and subsequent retrogression. While the rocks of the Chandman Khayrkhan Crystalline Complex remain undated, the age of the granitic rocks in the Chandman Massif itself are Early Carboniferous (345 ± 2 Ma). The Khan Khayrkhan Crystalline Complex has tectonic contacts with the surrounding Palaeozoic volcanosedimentary units. It is built by orthogneisses, amphibolites, paragneisses and mica schists. Metamorphic style corresponds to a pro-grade event with peak at c. 670 °C and 7-10 kbar. The crystallization age for the granitic protolith to the orthogneisses was the latest Devonian (363 ± 3 Ma). [ABSTRACT FROM AUTHOR]

Abstract: Garnet–cordierite–orthoamphibole rocks crop out in centimetric lenses within the pre-Caradocian metapelites of the Osor complex in the sillimanite metamorphic zone of the HT-LP Variscan sequence from the Guilleries Massif in the Catalan Coastal Ranges (NE Iberia). The P–T path followed by the metapelites shows increasing T and P to a peak of 660°C–5kbar, then isobaric cooling and finally cooling and decompression. Reaction textures on amphibolite lenses show evidence of resorption of garnet and gedrite and growth of cordierite, anthophyllite and chlorite, which has led to the stable sub-assemblages: garnet+cordierite and gedrite+anthophyllite+cordierite±chlorite. Petrography, mineral chemistry, and modelling of the low-Ca amphibolite reaction assemblages using a pseudosection approach in MnNCFMASH also suggest a decompressional path when generation of two effective bulk compositional microdomains during cordierite moat formation is taken into account. [Copyright &y& Elsevier]

The Austroalpine basement to the south of the Tauern Window once was part of the northern margin of Gondwana. It includes the “Altkristallin” and the phyllitic Thurntaler Complex. In the Altkristallin (AMU, MPU), suites of arc-related metamafic sequences occur together with calc-alkaline metagranite. SHRIMP U–Pb dating of zircon from calc-alkaline metagranite associated with an eclogitic amphibolite give an age of 470 ± 3 Ma interpreted as the age of protolith emplacement. In the Thurntaler Complex, metaporphyroids occur together with tholeiitic as well as alkaline within-plate basalt-type metabasite. The metaryholites of this association give a crystallization SHRIMP age of 477 ± 4 Ma, which suggests contemporaneity of arc-related and extensional settings in the Austroalpine basement units. The age data demonstrate widespread magmatic activity associated with the Early-Ordovician amalgamation at the end of the 550–470 Ma subduction–accretion–collision cycle. The Pb–Pb and U–Pb systematics of step-wise leached staurolite and kyanite from the peak-metamorphic assemblage of the Altkristallin indicate that (1) step-wise leaching of staurolite and kyanite yields the age of inclusions rather than the host; (2) zircon inclusions in staurolite suggest an Ordovician or older age for the precursor of the staurolite-schists; (3) the weighted average of the 206Pb/238U data of the various leaching steps yields a Variscan age for the inclusions (ilmenite, biotite, and andesine). Since these inclusions are part of the metamorphic mineral assemblage, this age provides a minimum estimate for staurolite growth, i.e., metamorphism. Thus, the Pb–Pb and U–Pb systematics of staurolite provide evidence for a Variscan metamorphism of the Austroalpine basement, e.g., MPU, AMU and Thurntaler Complex, to the south of the Tauern Window. [ABSTRACT FROM AUTHOR]

The Mondon˜edo thrust sheet has been studied to investigate the complex dynamic relationships that may be involved in the development of low- and medium-P metamorphic domains. This unit underwent an initial medium-P event during the initial stages of Variscan convergence, related to crustal thickening. Subsequently, the thrust sheet evolved to a low-P baric type of metamorphism, related to syn-convergence thinning and exhumation. Its footwall, cropping out in two tectonic windows, registered a different evolution, with a low-P history that evolved from low- to high-T under a high geothermal gradient. Several different P–T paths of the Mondon˜edo thrust sheet and its relative autochthon are traced and interpreted according to the structural evolution of the area. Following the initial crustal thickening, two main syn-convergence extensional shear zones developed. One of them occurs in the hangingwall, whereas the other affects the footwall unit. Both extensional shear zones were contemporaneous with ductile thrusting in the inner parts of the thrust sheet, and their activity is viewed as a consequence of the need for gravitational re-equilibration within the orogenic wedge.The most commonly accepted models of tectonothermal evolution in regions of thickened continental crust assume that low-P metamorphism is essentially a late phenomenon, and is linked to late-orogenic tectonic activity. In the Mondon˜edo thrust sheet, our conclusions indicate that low-P metamorphism may also develop during convergence, and that this may occur in at least two cases. One is tectonic denudation of an allochthonous unit during its emplacement, and the other, thinning and extension at the footwall unit of an advancing thrust sheet. As a consequence, the low-P evolution may show different characteristics in different units of an orogenic nappe pile. [Copyright &y& Elsevier]