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The Dom Feliciano Belt of southern Brazil and Uruguay represents part of a larger Neoproterozoic orogenic system formed during the amalgamation of Western Gondwana. The hinterland and foreland domains in parts of the belt preserve deformation structures and metamorphic assemblages that developed during early crustal thickening from c. 650 Ma. However, the metamorphic history of the southern foreland, in Uruguay, and its relationship with the hinterland, is not so well understood. We show that metamorphism in the southern hinterland is characterized by near-isothermal decompression from ~10 kbar (~770°C) down to ~6 kbar, reflecting exhumation from depths of ~40 km during convergent thrusting and crustal thickening. This metamorphic event and associated magmatism is constrained by garnet Lu–Hf and zircon U–Pb dating to c. 655–640 Ma, supporting age and P–T constraints from previous studies. In contrast, prograde metamorphism in the foreland supracrustal rocks reached maximum lower-amphibolite facies conditions (~6–7 kbar and ~550–570°C) and is constrained by garnet Lu–Hf dating to 582 ± 23 Ma. An exposed sheet of imbricated foreland basement rocks reached partial melting at upper-amphibolite facies conditions, and metamorphism is similarly constrained to c. 585–570 Ma by monazite U–Pb dating. The data indicate that metamorphism in the foreland occurred during a sinistral transpressional event c. 55–85 Ma after the start of crustal thickening recorded in the hinterland, whereby strain partitioning during sinistral transpression led to imbrication in the foreland and oblique thrusting of the basement over more distal supracrustal rocks. This event is coeval with transpressional deformation in the Kaoko and Gariep belts, indicating a distinct two-stage tectonic history driven by the three-way convergence between the Congo, Kalahari, and South American cratons.

The Balsfjord Series in Troms and Finnmark, N-Norway is part of a thrust-related nappe stack emplaced during the Ordovician–Silurian Caledonian orogeny. It overlies the Lyngen Magmatic Complex and Reisa Nappe Complex in the E and is overlain by the Nakkedal and Tromsø nappes in the W. Past research on the geological history of the Balsfjord Series was only undertaken locally and the tectonic meaning of this unit is still poorly understood. This is especially evident considering its role as a low–medium grade unit situated between two high grade complexes with diachronous evolution.The structural evolution of the Balsfjord Series is characterized by three sets of deformation structures. In low-grade areas, the original bedding S0 was affected by boudinage with generally WSW-ENE-oriented stretching axes. In higher-grade regions, the S0 was folded by tight–isoclinal F1 folds showing flat axial surfaces parallel to the surrounding penetrative metamorphic foliation S1. The FA1 fold axes are parallel with the stretching lineation Ls1, and both show considerable rotation from a NW–SE orientation in the NW towards E–W and ENE–WSW in the SE of the area. The F1 folding was syn-metamorphic as it folded the bedding and simultaneously developed the peak metamorphic assemblage in the S1 fabric. A second deformation phase locally folds the metamorphic fabric S1 and Ls1. It is represented by open–tight F2 folds with flat–moderately dipping fold axial surfaces in higher-grade areas, or by development of deformation bands in low-grade rocks. The latest set of structures is represented by steep F3 folds and associated axial planar cleavage S3. The F3 folding and cleavage development becomes increasingly accentuated closer to the contact of the Balsfjord Series with the Lyngen Gabbro.Mineral assemblages and P-T estimates show that the Balsfjord Series features an inverse metamorphic gradient with conditions increasing from the SE into higher tectonostratigraphic levels towards the W and NW. Thermodynamic modelling revealed maximum P-T conditions of ~450°C and 6.5 kbar in the garnet-zone of the unit, increasing up to 600 °C and 8 kbar in the staurolite-bearing uppermost levels. U–Pb dating of monazite associated with the peak mineral assemblage yielded ages between ca. 425–435 Ma, coeval with localized deformation of the basement rocks.Our observations together with published data from the surrounding units suggest a tectonic scenario, which involves two suture/thrust zones. The uppermost Tromsø and Nakkedal nappes reached their metamorphic peak at ca. 450 Ma. Their exhumation to upper crustal levels likely occurred soon after that and there these units remained tectonically dormant. At ~440 Ma, the Nordmannvik Nappe of the Reisa Nappe Complex reached its peak metamorphism as a part of the eastern subduction channel. Final exhumation of the Nordmannvik Nappe and closure of the eastern suture took place at ~430 Ma. This was accompanied/followed by underthrusting of the Balsfjord+Lyngen nappe assembly in the west under the Tromsø+Nakkedal+nappe assembly causing the deeper burial and peak metamorphism of the Balsfjord Series at around the same time.

Ediacaran syn-tectonic plutonic rocks (amphibole gabbros, quartz diorites/tonalites to biotite- and muscovite-bearing granites) of the Angra Fria Magmatic Complex (Kaoko Belt, north-western Namibia) belong to two compositionally similar, magnesian, transitional tholeiitic–calc-alkaline suites, the Older (∼625–620 Ma) and the Younger (∼585–575 Ma). Both have counterparts in the broadly contemporaneous Florianópolis Batholith (southern Brazil), from which they were separated during the Cretaceous opening of the southern Atlantic. In the Angra Fria Magmatic Complex, the only unequivocal mantle contributions are identified in mingling zones of the Younger Suite and hybrid mafic–intermediate dykes of uncertain age. Previously published Hf-in-zircon isotopic data, together with new whole-rock geochemical and Sr–Nd isotopic signatures, underline an important role of crustal anatexis of a material with late Palaeoproterozoic to early Mesoproterozoic mean crustal residence (1.9–1.5 Ga). This interval resembles some of the published Nd model ages for Tonian ‘Adamastor Rift’-related felsic magmatic rocks in the Namibian Coastal and Uruguayan Punta del Este terranes. In detail, the Older Suite probably originated mainly by fluid-present melting of metabasalts and metatonalites, followed by (near) closed-system fractional crystallization (with or without accumulation) of amphibole ± plagioclase. For the Younger Suite, the principal process was the dehydration melting of relatively felsic lower crustal protoliths (metagreywackes or intermediate–acid orthogneisses >> metapelites), leaving garnet in the residue. Based on the geological context, the conspicuous enrichment of hydrous-fluid-mobile large ion lithophile over the conservative high field strength elements is not interpreted through a classic model of oceanic plate subduction, devolatilization, and fluxed-melting of the overriding mantle wedge. Instead, it is thought to reflect high-grade metamorphism of deeply buried continental crust and attendant water-fluxed melting of the overlying crustal lithologies, connected with inversion of the Tonian ‘Adamastor Rift’.

The Dom Feliciano Belt in southern Brazil and Uruguay represents the western half of a Neoproterozoic orogenic belt located in the southern portion of the South Atlantic Neoproterozoic Orogenic System. Current interpretations are divided as to the nature of orogenesis in this belt, in part owing to lacking geochronological constraints. Metamorphosed and deformed supracrustal sequences of the Brusque Complex in the northern Dom Feliciano Belt, representing part of the orogenic foreland, record the onset and duration of crustal thickening. Structural analysis and pressure–temperature estimates indicate that the complex reached peak regional metamorphic conditions of 540–570°C and 5.5–6.7 kbar during thrusting and burial, consistent with orogenic metamorphism and early crustal thickening. Garnet–whole rock Lu–Hf and Sm–Nd isochron ages date this event to between circa 660–650 Ma. Ar–Ar dating of mica suggests thrust-controlled exhumation and partial cooling by circa 635 Ma, and that localised deformation occurred into the late Ediacaran. Our results show that the orogenic foreland reached metamorphic conditions typical for crustal thickening 20–30 million years prior to the onset of massive magmatic activity in the hinterland. Such a delay is typical of hot, internal parts of orogens, which supports interpretations that hinterland magmatism in the northern Dom Feliciano Belt represents post-collisional magmatism and not arc magmatism above a subduction zone. Instead, we suggest that orogenesis in the northern Dom Feliciano Belt was initiated by rift-basin inversion driven by far-field forces transmitted through the crust in an intracontinental rift or back-arc rift setting.

This work investigates the pre-collisional (before ca. 650 Ma) history of the Dom Feliciano Belt in southernmost Brazil through geochronological and zircon oxygen isotope study. U–Pb SHRIMP dating of two orthogneiss samples from the Várzea do Capivarita Complex and one metarhyolite sample from the Porongos Complex yielded crystallisation ages of 786 ± 5 Ma, 780 ± 10 Ma and 787 ± 5 Ma, respectively. The mean oxygen isotope values calculated for the ca. 790 Ma zircon cores from the orthogneisses are 8.41 ± 0.13‰ and 8.68 ± 0.14‰, and 8.75 ± 0.72‰ for the metarhyolite. Such values suggest that zircon crystallised in the more evolved magmas, either from the melting of host rocks and sediments or assimilation of crustal material by mantle-derived magmas. The detrital zircon population was analysed in one additional paragneiss sample from the Várzea do Capivarita Complex, and most of the values cluster at 790–750 Ma. The data spread is centred at ca. 790 Ma, which is the crystallisation age of the interleaved orthogneisses. In our interpretation, such dataset suggests a syn-volcanic origin of the paragneiss protolith and, therefore, a volcano-sedimentary origin of the Várzea do Capivarita Complex. The correspondence of geochronological data and zircon oxygen isotope values for the studied meta-igneous samples suggests that the Várzea do Capivarita and Porongos complexes have shared the same igneous history. Therefore, the samples probably represent one magmatic event at different levels of a single basin at ca. 800–770 Ma. Such results bring first-order information about the meaning of tectonic limits in this Gondwana-related belt and implications for reconstructing the pre-collisional history of the orogen

Accepted manuscript version, licensed CC BY-NC-ND 4.0. In this study, U–Pb ages and Hf isotopic composition of detrital zircons from the Precambrian metasedimentary autochthon of the Brunovistulian Domain in the eastern Bohemian Massif were investigated to understand the pre-collisional evolution of the eastern periphery of the European Variscan belt. Detrital zircons of the Tonian sequences have mostly Paleoproterozoic to Neoproterozoic ages (c. 2.1–0.9 Ga) and are interpreted as detritus derived from the basement of either Baltica or Amazonia. The mostly positive εHf(t) values (–4 to + 16) indicate a juvenile nature of their magma sources with minor older crustal components. In contrast, the Ediacaran sequences contain dominantly Neoproterozoic zircons (c. 600 Ma) and only rare Paleo- and Mesoproterozoic ages indicate that they were sourced from the adjacent Neoproterozoic magmatic arc with very limited input of recycled cratonic detritus. The large spread of εHf(t) values (–15 to + 13) of the Neoproterozoic zircons suggests significant mixing of mantle-derived magmas with mature crustal material, typical of large continental magmatic arc systems. The zircon age patterns of the Ediacaran sequences, characterized by a dominance of the late Neoproterozoic zircons and limited Mesoproterozoic zircons, are nearly identical to those from the Teplá–Barrandian Unit and Moldanubian Zone, pointing to their similar sources. We consider such age populations as a record of sources actually exposed at the time of deposition, rather than the real provenance signature of the continental basement. The change in detrital zircon U–Pb age and Hf record of the Brunovistulian Domain took place between the early and late Neoproteorozoic, and probably reflects the plate-tectonic reconfiguration from the Rodinia formation/break-up to the evolution of the Gondwana or Baltica active margins. Our data challenge the main arguments for an existence of the Rheic oceanic suture between the Brunovistulian Domain and Moldanubian Zone and allow for an alternative pre-collisional model of the Bohemian Massif as a single Neoproterozoic crustal domain

Metamorphic minerals in the Brusque Complex of the northern Dom Feliciano Belt, Brazil, provide new insights into the timing and mode of regional convergence, challenging a long-lived subduction-collision model for orogenesis. The key evidence for subduction is an extensive linear belt of granitic rocks (the Granite Belt) that intruded the length of the hinterland of the Dom Feliciano Belt between ~630─580 Ma, and that is inferred to represent arc magmatism above the subducting Adamastor Ocean prior to continental collision. The study area comprises supracrustal units of a foreland fold-and-thrust belt outcropping along the western edge of the symmetric Kaoko─Dom Feliciano orogenic system. The integrated study of primary metamorphic mineral assemblages and associated deformation fabrics support the interpretation of a fold-and-thrust belt environment, with early tectonic movement top-to-NW away from the hinterland. P─T estimates constrained by garnet compositions indicate peak metamorphic conditions of 540─570°C and 5.5─6.5kbar, in line with typical geothermal gradients associated with orogenic metamorphism. The timing of early garnet growth, and by inference the early stages of crustal thickening in the foreland, is constrained by Lu─Hf garnet geochronology at ~660─650 Ma. The data indicate that the onset of metamorphism and deformation in the orogenic foreland occurred ~20–30 m.y. prior to intrusion of extensive granitic magmatism into the orogenic hinterland. The timing of early orogenic thickening in the foreland precludes the interpretation of the Granite Belt as an arc above a large-scale subduction zone in the lead up to orogenesis. Instead, it is interpreted to represent syn-orogenic magmatism typical for hinterland domains in other ancient and recent orogenic systems.We appreciate financial support from Diku Norway and CAPES Brazil (project UTF-2018-10004), and from the Czech Science Foundation (project no. 18-24281S). This work was partly supported by the Research Council of Norway through the funding to The Norwegian Research School on Dynamics and Evolution of Earth and Planets, project number 249040/F60.

Mafic rocks consist of strong minerals (e.g. clinopyroxene, plagioclase) that can only be deformed by crystal plastic mechanisms at high temperatures (>800°C). Yet, mafic rocks do show extensive deformation by non-brittle mechanisms when they have only reached lower temperatures (~650°C). In many of such cases, the deformation is accommodated by an interaction of deformation with simultaneous mineral reactions. Here we show that dissolution-precipitation creep plays a major role in deformation of gabbro lenses at mid and upper amphibolite facies conditions.The Kågen gabbro in the North Norwegian Caledonides intruded the Vaddas Nappe at 439 Ma at pressures of 7-9 kbar, temperatures of 650-900°C, and depths of ∼26-34 km. The Kågen gabbro on south Arnøya is comprised of undeformed gabbro lenses with sheared margins wrapping around them. This contribution analyses the evolution of the microstructures and metamorphism from the low strain gabbro lenses to high strain mylonites at margins of the lenses. Microstructural and textural data indicate that dissolution-precipitation creep is the dominant deformation mechanism, where dissolution of the gabbro took place in reacting phases of clinopyroxene and plagioclase, and precipitation took place in the form of new minerals: new plagioclase and clinopyroxene, amphibole, and garnet. Amphibole shows a strong CPO that is primarily controlled by its preferential growth in the extension direction. Synchronous deformation and mineral reactions of clinopyroxene suggests mafic rocks can become mechanically weak during the general transformation weakening process, i.e. the interaction of mineral reaction and deformation by diffusion creep. The weakening is directly connected to a fluid-assisted transformation process that facilitates diffusion creep deformation of strong minerals at far lower stresses and temperatures than dislocation creep. Initially strong lithologies can become weak, provided that reactions can proceed during deformation, the transformation process itself is an important weakening mechanism in mafic (and other) rocks, facilitating deformation at low differential stresses.

Deciphering the structural and metamorphic history of the Balsfjord Series in the Upper Allochthon of the Scandinavian Caledonides in northern NorwayHöpfl Stephan1, Konopásek Jiří1, Stünitz Holger1,2 Bergh G., Steffen1UiT Norges arktiske universitet, Institutt for geovitenskap, stephan.m.hopfl@uit.no 1Department of Geosciences, UiT The Arctic University of Norway, Tromsø 9037, Norway2Institut des Sciences de la Terre (ISTO), Université d’Orleans, Orleans 45100, France The Balsfjord Series is located in the central part of Troms–Finnmark County, northern Norway, and is part of the upper allochthon of the Scandinavian Caledonides. It consists of an Ordovician–Silurian metsedimentary sequence lying on top of the mostly gabbroic Lyngen Magmatic Complex (LMC). The unit exhibits an inverted metamorphic gradient, where the metamorphic conditions increase from the base to the top, from very low grade in the southeast to medium grade in the west and northwest. The Balsfjord Series is sandwiched between two high-grade units, the Nakkedal + Tromsø Nappe Complex in the hanging wall and the Nordmannvik Nappe as the top part of the Reisa Nappe Complex (RNC) in the footwall. The Nakkedal + Tromsø Nappe Complex features metamorphic peak ages of ca. 455–450 Ma and the Nordmannvik Nappe of ca. 430 Ma. The peak metamorphism of the Balsfjord Series has never been dated and the role of the inverted metamorphic gradient is not yet understood. One of the main motivations in this project is to resolve the Caledonian deformation history in the Balsfjord Series, ideally leading to a regional tectonic model explaining the tectonostratigraphic and metamorphic relationships between the abovementioned units.The Balsfjord Series features two main discernible folding phases. The earlier phase displays tight to isoclinal folds with flat lying axial surfaces parallel to the penetrative foliation. Observed fold axes are parallel with the stretching lineation. These folds are best preserved in the northwestern, upper part of the unit and are syn-metamorphic in certain areas, as they fold original bedding (transposed foliation). A later folding phase is represented by mainly open folds with inclined to steep axial surfaces. Their fold axes are gently plunging with a predominant NE–SW orientation. We interpret these two folding events to be genetically related but slightly diachronous. The earlier folding phase with flat lying axial surfaces was likely generated during nappe thrusting and peak metamorphism of the Balsfjord Series. The subsequent open folding with inclined to steep axial surfaces is explained as a result of continued shearing and shortening of the weaker metapelitic Balsfjord Series against the more rigid gabbroic part of the LMC during the late stages of the Caledonian nappe thrusting. Observed thrust kinematics and penetrative retrogression at the bottom of the Nakkedal + Tromsø Nappe Complex suggest that its final exhumation took place during prograde metamorphism of the underlying Balsfjord Series. The ongoing dating of the prograde metamorphism in the Balsfjord series will provide important information about a possible continuity between the timing of peak metamorphism in the Nakkedal + Tromsø Nappe Complex, the Balsfjord series and the underlying RNC.

Deformation of natural mafic rocks by viscous deformation mechanisms can occur even at low temperature conditions. In such instances, crystal plastic mechanisms are not operative, as their activity is restricted to very high temperatures for amphiboles, pyroxenes, and plagioclase. Instead, simultaneous mineral reactions may facilitate deformation at low temperature conditions. The gabbro from the Lyngen Magmatic Complex (LMC) constitutes a good example of such processes, because it has experienced deformation at low temperatures of greenschist to lower amphibolite-facies conditions, and the rock has been transformed from gabbro to greenschist. This study focuses on detailed analysis of deformation processes, metamorphic reactions and fabric development in the LMC gabbro. Most samples are overprinted by epidote amphibolite and greenschist-facies mineral assemblages. Preliminary observations distinguish two different types of amphiboles, which have been interpreted as different generations. The predominant type defines the stretching lineation and shows long prismatic habits whereas the less abundant type crystallized in a sub- to anhedral manner. The metamorphic conditions of growth for each amphibole type is yet not well constrained. However, we initially interpret the former to grow during epidote amphibolite- or greenschist facies-conditions, whereas the latter could represent relict grains from the original magmatic assemblage or products generated at amphibolite- or epidote amphibolite-facies conditions. Further analysis will determine the orientation, geochemistry and metamorphic conditions during growth for both amphibole types. A recent model proposed for eclogites suggests that simultaneous mineral growth and deformation can result in new products growing in a preferred direction. Such preferential growth can generate a shape preferred orientation parallel to the lineation, which results in the formation of crystal preferred orientations (CPO). We aim to test if similar microstructural observations can be translated to the amphiboles of the LMC gabbro. In such case, amphibole CPO’s would not be the product of crystal plasticity but of preferential growth. The large scale deformation of the LMC emphasizes the relevance of these results, as it would demonstrate that the interaction between mineral reactions and deformation can play a major role on regional deformation of large mafic bodies, such as the ocean floor.

The Encruzilhada Block lies between the eastern hinterland and western foreland of the Dom Feliciano Belt (southern Brazil) and its tectonic meaning in the orogenic evolution of the belt is still poorly understood. Its low magnetic signal is similar to that of the foreland supracrustal sequences, whereas the high gammaspectrometric signal appears due to the hinterland post-collisional granitoids. Satellite image lineament analysis points to tectonic similarities between the Encruzilhada Block and the foreland, as the structures in both domains show predominantly NNW and NNE trends, while the hinterland mostly presents NE-trending structures. The pre-orogenic connection between the Encruzilhada Block and the foreland is indicated by the correlation between the Varzea do Capivarita and Cerro da Arvore complexes. Both contain 800 – 780 Ma metaigneous rocks with arc-like geochemistry, and parametamorphic rocks with similar sedimentary provenance. Common syn-orogenic deformation history of both complexes is suggested by progressive to P-T o-th E-W transpressive deformation and similar peak metamorphic age (660 – 640 Ma). PT-paths suggest exhumation of the hinterland Varzea do Capivarita Complex from high-T/low-P conditions of 790–820 °C/4.4–4.8 kbar to 660–720 °C/2.5–3.4 kbar at ca. 630 Ma. On the other hand, the foreland Cerro da Arvore Complex was subject to progressive metamorphism from 555 to 565 °C/5.4–5.7 kbar to 560–580 °C/5.8–6.3 kbar, which is interpreted as a result of orogenic thickening. Evidence suggests that both complexes have originated in a similar, if not a single, basin on an attenuated lithosphere with high geothermal gradient, possibly a (back-arc?) rift. Oblique collision has caused basin inversion and thrusting of the lower crust over the rift margin, which may have been accompanied by lithosphere delamination. Progressive transpressional deformation and voluminous post-collisional magmatism (640 – 578 Ma) controlled by major lineaments have blurred the original tectonic contact and finally caused thermal metamorphism in both complexes at ca. 3 kbar, when the autochthonous Encruzilhada Block was stabilized in its present geological configuration. The intracontinental character of major shear zones limiting hinterland and foreland requires a review of the existing evolutionary model for the Dom Feliciano Belt in the context of Western Gondwana amalgamation.

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The Dom Feliciano Belt (DFB), part of the Mantiqueira Province in southern Brazil and Uruguay, results from multiple Neoproterozoic collisions leading to the construction of the Gondwana supercontinent. Therefore, complex structural relations are expected, as it is the case for the Porongos Metamorphic Complex (PMC) and Varzea do Capivarita Complex (VCC). Several papers have described structural features from both complexes, and some evolution models were proposed for them. However, very few papers have tried to correlate them, and recent geochronological and provenance studies carried out in both complexes pointed out some inconsistencies in these models. Such studies demonstrate that part of the protoliths of metasedimentary and metaigneous rocks of PMC and VCC, of different metamorphic grades, could have been deposited in the same sedimentary environment. The geological meaning of such data is still poorly explained and requires a better understanding of the original geometrical relations between PMC and VCC. This paper focuses on the metavolcanic rocks from two subareas near the northeastern tip of the Porongos Metamorphic Complex area, and on the previously published data for the VCC. In order to investigate the structural and kinematic history of this part of PMC, detailed structural mapping, petrography, whole-rock and mineral chemistry studies were carried out in selected subareas. Metavolcanic rocks from Subarea 1 and Subarea 2 are metadacites and metarhyolites bearing a geochemical imprint of arc magmatism at ca. 790 Ma. Structural investigation of the area reveals that the main deformation phase D1 is mostly compressive and reached the metamorphic peak at amphibolite facies conditions. D2 and D3 are late deformation phases which develop open folds and axial plane cleavages. The most conspicuous D1 structure is an alternation of non-mylonitic S1 and mylonitic-S1, developed over an originally horizontal foliation, which suggests strain concentration in sub-horizontal shear zones interpreted as related to a fold and thrust belt evolution. The scattering of stretching lineation (L1) measurements along a great circle in stereoplots is interpreted to result from shearing along S1 planes, similar to what is described for the progressive deformation of VCC nearby. The present dataset supports the interpretation that PMC and VCC have shared a single volcano-sedimentary basin which subsequently achieved different crustal levels. A W-directed collisional event at 650 Ma metamorphosed the rocks and generated thrust-folds in both complexes. As indicated by recent studies in the local literature, deformed metavolcanics rocks of ca. 578 Ma magmatic age form concordant sheets in the PMC. Such data suggest a metamorphic-deformational event younger than ca. 578 Ma, which, in our view, is probably related to the reactivation of the contractional structures generated at ca. 650 Ma. This younger event must have folded the eastern part of the PMC generating shallow-level folds, and thrusted the rocks onto the eastern side to place them on top of the PMC western-side rocks. Thus, it is possible that PMC rocks register two thrusting episodes, one during peak metamorphic conditions (ca. 650 Ma) and one under retrograde metamorphic conditions (ca. 578 Ma).

Metamorphic–deformation paths and interpretations of regional structures are essential for the understanding of how orogens evolve. While in active orogens, structures can be easily mapped for hundreds of kilometres, regional structures in ancient mountain belts may appear only as subsurface geophysical data. In this paper, we constrain the metamorphic–deformation history of the southeastern part of the Passo Feio Complex located in the Neoproterozoic Dom Feliciano Belt in southernmost Brazil. The complex is crosscut by a block-limiting structure defined by magnetometric data, the Cacapava Lineament. Our petrographic interpretations and thermodynamic modelling of pelitic schists of the Passo Feio Complex indicate the beginning of garnet growth at ca. 530–550 °C and 3–4.3 kbar during D1, and metamorphic peak at ca. 560–570 °C and 5–5.5 kbar (M1), during progression to D2. After M1 (~17–19 km depth), the complex was exhumed to at least 14 km (~4 kbar) depth, as the contact metamorphism by the Cacapava Granite (ca. 562 Ma) produced andalusite. The correlation of D3 structures of the Passo Feio Complex (S3 and L3), magmatic structures in the Cacapava Granite, orientation of geophysical anomalies in the region and kinematics of S3 corroborate the interpretation of the Cacapava Lineament as a dextral shear zone. Due to the location of anomalies and previous three-dimensional modelling of the granitic body, we interpret that a curved ENE to NS branch of the NE-striking Cacapava Shear Zone is responsible for the accommodation of the Cacapava Granite. Lastly, we discuss the regional implications of such interpretation, which brings up the question of block-limiting character of the Cacapava Shear Zone and the geotectonic position of the Passo Feio Complex.

Early Neoproterozoic metaigneous rocks occur in the central part of the Kaoko–Dom Feliciano–Gariep orogenic system along the coasts of the southern Atlantic Ocean. In the Coastal Terrane (Kaoko Belt, Namibia), the bimodal character of the ca. 820–785 Ma magmatic suite and associated sedimentation sourced in the neighbouring pre-Neoproterozoic crust are taken as evidence that the Coastal Terrane formed as the shallow part of a developing back arc/rift. The arc-like chemistry of the bimodal magmas is interpreted as inherited from crustal and/or lithospheric mantle sources that have retained geochemical signature acquired during an older (Mesoproterozoic) subduction-related episode. In contrast, the mantle contribution was small in ca. 800–770 Ma plutonic suites in the Punta del Este Terrane (Dom Feliciano Belt, Uruguay) and in southern Brazil; still, the arc-like geochemistry of the prevalent felsic rocks seems inherited from their crustal sources. The within-plate geochemistry of a subsequent, ca. 740–710 Ma syn-sedimentary volcanism reflects the ongoing crustal stretching and sedimentation on top of the Congo and Kalahari cratons. The Punta del Este–Coastal Terrane is interpreted as an axial part of a Neoproterozoic “Adamastor Rift”. Its opening started in a back-arc position of a long-lasting subduction system at the edge of a continent that fragmented into the Nico Perez–Luis Alves Terrane and the Congo and Kalahari cratons. The continent had to be facing an open ocean and consequently could not be located in the interior of the Rodinia. Nevertheless, the early opening of the Adamastor Rift coincided with the lifetime of the circum-Rodinia subduction system.

Accepted manuscript version, licensed CC BY-NC-ND 4.0. A composite cross section from the Florianópolis Batholith towards the Tijucas Fold Belt in the northern Dom Feliciano Belt (southern Brazil) is divided in three structural domains: the Major Gercino Shear Zone, the suprastructural Brusque Complex and the infrastructural Camboriú Complex. A kinematic correlation among the structural domains is based on structural and petrological data integrated with geochronology. An oblique collisional event at 650-645 Ma affected all structural domains and is best recorded in the Porto Belo Complex, which shows migmatization (700 °C/4.3 kbar) and top-to-the-NNW+dextral shear along the Major Gercino Shear Zone. Subsequent strain partitioning led to progressive tangential movement recorded in the Quatro Ilhas Granitoids (625-615 Ma) followed by later granitic intrusions (after 615 Ma) controlled by dextral strike-slip. Meanwhile, the contractional component was absorbed by the Tijucas Fold Belt infrastructure, causing exhumation of the Camboriú Complex migmatites (from 5 to 3.4 kbar) and unroofing of the suprastructural Brusque Complex (around 635 Ma). Tectonic juxtaposition occurred along a dextral+normal detatchment zone between the complexes. As a consequence, heating of the Brusque Complex locally reached amphibolite-facies conditions and suprastructure thrusting inverted to extension, which is recorded in discrete shear structures with normal kinematics. The sequence of events and their age suggest that the hinterland Porto Belo Complex and the foreland Tijucas Fold Belt were juxtaposed already at ca. 650–645 Ma, which questions the validity of the subduction-related tectonic models in the northern Dom Feliciano Belt.

Existing models of tectonic evolution of the Neoproterozoic orogenic system rimming the shores of the South Atlantic Ocean (the Araçuaí–Ribeira–Congo and Dom Feliciano–Kaoko–Gariep belts) interpret the belts as subduction-related orogens and emphasize the role of the “Adamastor Ocean” in their pre-collisional evolution. A critical problem in such an interpretation is the confined nature of the northern termination of the orogenic system, as well as a very short time span between the end of rifting and onset of convergence recognized in its southern part. In this contribution, we review the data for the pre- and synorogenic evolution of this system of orogens (here collectively called the South Atlantic Neoproterozoic Orogenic System) and show that the data speak against the presence of a large oceanic domain before the onset of its orogenic evolution. We propose a new and simple intracontinental model, suggesting that Neoproterozoic oceanic crust played only a minor role in the development of the South Atlantic Neoproterozoic Orogenic System and that its overall architecture and thermal evolution is the result of inversion of large-scale rift structures with a protracted, and probably episodic, extensional history. True oceanic crust probably developed only in the southern part of the rift system, but it must have been narrow, akin to the Red Sea–Gulf of Aden stage of the “Adamastor Rift” evolution just before the onset of convergent thickening. publishedVersion

International audience; The deformation of eclogites and the driving forces for their fabric development are an important topic, potentially allowing to determine deformation rates and stresses in subduction zones, where the greatest number of large earthquakes occurs. Here, fabric studies of grain size and shape, texture, and chemical composition from two locations of Variscan and Alpine eclogites are presented. All samples show a well-developed crystallographic preferred orientation (CPO) of omphacite with a strong maximum of [001] in the lineation direction and a weaker maximum of poles to (010) normal to foliation. Garnet shows no systematic CPO. Anisotropic chemical zoning developed in omphacite and garnet during growth together with elongated grain shapes and can be related to a prograde (in terms of pressure change) P,T-path. The individual chemically zoned and elongated grains orientated in the stretching direction are single crystals without major internal mis-orientations. Chemical, microstructural, and CPO data indicate that the deformation microstructure and texture were produced by preferential crystal growth of garnet and omphacite grains in the extension direction. Dislocation creep can be excluded as a possible fabric formation process by the systematic and oriented chemical zonation of single crystals and absence of dynamic recrystallization microstructures. The dominant deformation is inferred to be diffusion creep, where dissolution of material took place in reacting mafic phases (plagioclase, pyroxene) and precipitation took place in the form of new eclogite facies minerals (omphacite, garnet, zoisite). This type of diffusion creep deformation represents a transformation process involving both, deformation and metamorphic reactions. It is emphasized that the weakening is directly connected to the transformation and therefore transient. The weakening facilitates diffusion creep deformation of otherwise strong minerals (pyroxene, garnet, zoisite) at far lower stresses than dislocation creep. The results imply low stresses during the deformation of eclogite blocks in subduction zones. These results can be applied to other rock types, too.

New geochronological, isotopic and geochemical data were obtained from the late Paleoproterozoic and Mesoproterozoic magmatic units of the Nico Perez Terrane (Uruguay). A U-Pb LA-ICP-MS zircon age of 1768 ± 11 Ma confirms the Statherian age for the Illescas rapakivi intrusion, being thus comparable with the age of the Campanero Unit felsic orthogneisses. Though both the Illescas and Campanero intrusions exhibit dominantly granitic, meta- to peraluminous compositions, the first shows a dominant shoshonitic and ferroan composition, whereas high-K calc-alkaline and magnesian to ferroan compositions characterize orthogneisses of the Campanero Unit. Additionally, Sm/Yb and LaN/YbN ratios indicate that both suites were emplaced in a thickened crust, whereas Lu-Hf zircon data of the Illescas granite together with available Sm-Nd data of the Campanero Unit point to significant recycling of an older, Archean and/or Paleoproterozoic crust of the Nico Perez Terrane. Results suggest a common origin for both the Illescas and Campanero magmatism, probably related to a post-collisional/post-orogenic setting. On the other hand, metagabbros of the Zanja del Tigre Complex present U-Pb LA-ICP-MS zircon ages of 1479 ± 4 and 1482 ± 6 Ma and tholeiitic gabbroic compositions. Geochemical and Sm-Nd isotopic data show similarities with LIP-related intracontinental mafic magmatism, suggesting a mixed magma source derived from depleted asthenosphere and fertile subcontinental lithospheric mantle contributions. Emplacement of the metagabbros was related to lithospheric extension in a continental rifting setting and, therefore, fertilization of the lithospheric mantle probably took place during older Paleoproterozoic subduction-related events. Coeval late Paleoproterozoic and early Mesoproterozoic magmatism is also recorded in the northern Nico Perez Terrane of southernmost Brazil, and in basement inliers of the Kaoko Belt and the Angolan Shield (southwestern Congo Craton), thus suggesting a common evolution of these blocks prior to the Neoproterozoic. Fil: Oriolo, Sebastián. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Geociencias Básicas, Aplicadas y Ambientales de Buenos Aires. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Geociencias Básicas, Aplicadas y Ambientales de Buenos Aires; Argentina Fil: Oyhantçabal, Pedro. Universidad de la República. Facultad de Ciencias; Uruguay Fil: Konopásek, Jirí. Czech Academy of Sciences. Institute of Geology; República Checa Fil: Stipp Basei, Miguel Angelo. Universidade de Sao Paulo; Brasil Fil: Frei, Robert. Universidad de Copenhagen; Dinamarca Fil: Sláma, Jirí. Czech Academy of Sciences. Institute of Geology; República Checa Fil: Wemmer, Klaus. Universität Göttingen; Alemania Fil: Siegesmund, Siegfried. Universität Göttingen; Alemania

This is a post-peer-review, pre-copyedit version of an article published in the International journal of earth sciences. The final authenticated version is available online at https://doi.org/10.1007/s00531-016-1416-y. Cadomian magmatic complexes of the Brunovistulian Domain crop out at the eastern termination of the Bohemian Massif. However, the age, nature and geotectonic affinity of some of pre-Variscan (meta-)igneous rock complexes from this domain are still unknown. Geochronological and geochemical study of the granitic rocks across the Brunovistulian Domain reveals new information about the timing and nature of this magmatic activity originally situated along the northern margin of Gondwana. Zircon U–Pb data (601 ± 3 Ma, Brno Massif; 634 ± 6 Ma, paraautochtonous core of the Svratka Dome; 568 ± 3 Ma, Bíteš orthogneiss) from the allochtonous Moravicum indicate the prolonged magmatic activity within the Brunovistulian Domain during the Ediacaran. The major- and trace-element and Sr–Nd isotopic signatures show heterogeneous geochemical characteristics of the granitic rocks and suggest a magmatic-arc geotectonic setting. The two-stage Depleted Mantle Nd model ages (c. 1.3–2.0 Ga) indicate derivation of the granitic rocks from a relatively primitive crustal source, as well as from an ancient and evolved continental crust of the Brunovistulian Domain. These results constrain the magmatic-arc activity to c. 635–570 Ma and provide a further evidence for a long-lived (at least c. 65 Myr) and likely episodic subduction-related magmatism at the northern margin of Gondwana. The presence of granitic intrusions derived from variously mature crustal sources at different times suggests heterogeneous crustal segments to having been involved in the magmatic-arc system during its multistage evolution.

The Neoproterozoic Araçuaí-West Congo orogen in Brazil and Congo represents a branch of the Brasiliano/Pan-African orogenic system that is considered to terminate northward into a confined cratonic environment defined by the horseshoe-shaped pre-Atlantic São Francisco-Congo craton. The prevailing interpretation is that this orogen formed as a result of 50 m.y. of subduction leading to a classical continent–continent collision. This subduction-collision model hinges on interpretation of 630–580 Ma granitoids in the core of the orogen as being arc-related, and of locally exposed mafic and ultramafic metamorphic rocks as being ophiolitic. We show that when tested beyond geochemical signature, the model has fundamental problems that cannot be accounted for. In particular, there is an insurmountable oceanic space problem in this confined setting that is overlooked in most of the current literature. There are also problems with subduction initiation, lack of unambiguous evidence for oceanic crust, no trace of any high-P metamorphism and the abrupt termination of an ocean with no realistic way to transfer the large amount of oceanic opening displacement and subsequent convergence required by the model. We conclude that the prevailing subduction-collision model cannot possibly work and argue that the existing data are more consistent with hot intracontinental orogeny. We stress the importance of building tectonic models on more than one type of data, making realistic restorations and palinspastic reconstructions, and taking into account modern geotectonic knowledge. publishedVersion

This study investigates the Caledonian metamorphic and tectonic evolution in northern Norway, examining the structure and tectonostratigraphy of the Reisa Nappe Complex (RNC; from bottom to top, Vaddas, Kåfjord and Nordmannvik nappes). Structural data, phase equilibrium modelling, and U-Pb zircon and titanite geochronology are used to constrain the timing and P-T conditions of deformation and metamorphism that formed the nappes and facilitated crustal thickening during continental collision. Five samples taken from different parts of the RNC reveal an anticlockwise P-T path attributed to the effects of early Silurian heating followed by thrusting. An early Caledonian S1 foliation in the Nordmannvik Nappe records kyanite-grade partial melting at ~ 760–790 °C and ~ 9.4–11 kbar. Leucosomes formed at 439 ± 2 Ma (U-Pb zircon) in fold axial planes in the Nordmannvik Nappe indicate that compressional deformation initiated while the rocks were still partially molten. This stage was followed by pervasive solid-state shearing as the rocks cooled and solidified, forming the S2 foliation at 680–730 °C and 9.5–10.9 kbar. Multistage titanite growth in the Nordmannvik Nappe records this extended metamorphism between 444 and 427 Ma. In the underlying Kåfjord Nappe, garnet cores record lower P-T (590–610 °C and 5.5–6.8 kbar) but a similar geothermal gradient as the S1 migmatitic event in the Nordmannvik Nappe, indicating formation at a higher relative position in the crust. S2 shearing in the Kåfjord Nappe occurred at 580–605 °C and 9.2–10.1 kbar, indicating a considerable pressure increase during nappe stacking. Gabbro intruded in the Vaddas Nappe at 439 ± 1 Ma, synchronously with migmatization in the Nordmannvik Nappe. In the Vaddas Nappe S2 shearing occurred at 630–640 ºC and 11.7–13 kbar. Titanite growth along the lower RNC boundary records S2-shearing at 432 ± 6 Ma. It emerges that early Silurian heating (~ 440 Ma), probably resulting from large-scale magma underplating, initiated partial melting that weakened the lower crust, which facilitated dismembering of the crust into individual nappe units. This tectonic style contrasts subduction of mechanically strong continental crust to great depths.

Petrographic and geochemical analyses of partly and completely serpentinized and carbonated peridotites within the ultramafic section of the Leka Ophiolite Complex have been used to elucidate the evolution of alterations and identify possible fluid sources. The alterations show no evidence for any major deformation and are located along low-angle structures that were formed in a late stage of the structural evolution of the ophiolite complex. Modeling of mineral equilibria in the SiO 2 –MgO–FeO–Fe 2 O 3 –CaO–H 2 O ± CO 2 system has been utilized to constrain the conditions during serpentinization and carbonation. The partly altered peridotites consist of the mineral assemblage olivine–clinopyroxene–serpentine–magnetite–brucite and formed at temperatures CO2 (≤ 0.05) by the breakdown of the minerals in the partly altered peridotites. Talc–carbonate rocks formed by the breakdown of the serpentine in the previously formed serpentinite rock at temperatures CO2 , which resulted in the stabilization of the assemblage talc–magnesite–magnetite–dolomite. Carbon isotope values determined for dolomite from crosscutting carbonate lenses within the talc–carbonate rock yield δ 13 C values of ~− 5 indicative of a mantle source for the carbon required for the carbonation. Oxygen isotope values δ SMOW 18 O of ~ 10.8–11.3‰ together with initial 87 Sr/ 86 Sr i = 400Ma values of 0.7029 and 0.7063, suggest dehydration of rocks with mantle affinity as a source for the fluids. Based on analytical results and field observations we propose that the formation of the talc- and carbonate-bearing alteration zones is caused by the focused infiltration of fluids that originated at the bottom of already partly serpentinized ophiolite complex during extension-driven burial at the late stage of the Caledonian orogeny.

Accepted manuscript. Final version published in Journal of the Geological Society, 176(3), 492-504, is available at https://doi.org/10.1144/jgs2018-173. Isotopic dating of monazite and garnet from high-pressure metamorphic rocks exposed in the northern part of the Saxothuringian palaeo-suture in the Bohemian Massif revealed a diachronous metamorphism of various rock types that are now closely associated within allochthonous units representing the Devonian–Carboniferous subduction–accretionary complex. Mafic blueschists of the middle unit yielded a Lu–Hf garnet age of 363.9 ± 1.3 Ma. The blueschists occur within high-pressure, garnet-free phyllites. Monazite extracted from this rock type yielded a U–Pb isotope dilution thermal ionization mass spectrometry age of 336.5 ± 0.5 Ma. Garnet-bearing micaschist of the lower unit contains monazite with a U–Pb secondary ion mass spectrometry age of 341 ± 3 Ma, consistent with Lu–Hf garnet–whole-rock ages of 344.5 ± 1.3 and 342 ± 7 Ma obtained from the same rock type. Existing tectonic models of the Bohemian Massif, and particularly of its northern part, assume that the period of oceanic subduction was terminated at c. 380–375 Ma by the arrival of an attenuated Saxothuringian continental crust, which was partly subducted and partly relaminated underneath the overriding Teplá–Barrandian Domain. However, our data, as well as data from mafic high-pressure rocks in the southern part of the Saxothuringian Domain, suggest that the initial collision was probably caused by the arrival of a smaller crustal block present within the Saxothuringian oceanic domain. After its subduction and relamination, the oceanic subduction was re-established and terminated by continental subduction and later collision of the Saxothuringian passive margin at c. 345–335 Ma

Accepted manuscript version. Published version available at https://doi.org/10.1016/j.precamres.2017.06.017. Detrital zircon provenance study of a metamorphosed sedimentary succession in the eastern part of the Kaoko Belt in Namibia has revealed two distinct sources for the Neoproterozoic sedimentation along the southwestern Congo Craton margin. The lower part of the succession shows detrital zircon ages consistent with erosion of Paleoproterozoic basement of the Congo Craton with an inferred Mesoproterozoic volcano-sedimentary cover. Within the middle part of the succession, which includes glaciogenic sediments correlated with the Sturtian (717–660 Ma) glaciation, the Mesoproterozoic zircon grains disappear and the signal is dominated by ages known from the Congo Craton basement. The sedimentation in these parts of the succession is interpreted as related to the early Neoproterozoic rifting. The sedimentary rocks in the top part of the profile contain only subordinate proportion of the Paleoproterozoic–Archaean zircon grains and the populations are dominated by three age groups of ca. 1.0–1.2 Ga, ca. 800–750 Ma and ca. 650 Ma, consistent with erosion of the Punta del Este–Coastal Terrane exposed in the centre of the Kaoko–Dom Feliciano–Gariep orogen. An associated glaciogenic horizon interpreted as reflecting the Marinoan (645–635 Ma) glaciation constrains the sedimentation in the upper part of the succession and suggests a short time span between the high-grade metamorphism/magmatism in the Punta del Este–Coastal Terrane and its exhumation. Sedimentary rocks with such detrital zircon pattern appear also in the Damara and Gariep belts. Their source in the western part of the Kaoko–Dom Feliciano–Gariep orogen suggests that they represent an early orogenic flysch that originated during early collision in the western part of the orogen. The short time span between the metamorphism/magmatism in the Punta del Este–Coastal Terrane and deposition of the early orogenic flysch derived from it suggests that the Coastal Terrane was never separated from the Congo Craton by an oceanic domain. The estimated time span between the end of lithospheric stretching and sedimentation of the early flysch suggests that the hypothetical Adamastor Ocean separating the western and eastern forelands of the Kaoko–Dom Feliciano–Gariep orogen must have been small.

The Eger Complex in the northwestern Bohemian Massif consists mainly of amphibolite facies granitic gneisses containing a subordinate volume of felsic granulites. Microstructural changes and modelling of metamorphic conditions for both rock types suggest a short-lived static heating from ~760 to ~850 °C at a constant pressure of ~16 kbar, which led to the partial granulitization of the granitoid rocks. Detailed study of the protolith zircon modifications and modelling of the Zr re-distribution during the transition from amphibolite to granulite facies suggests that the development of c. 340 Ma old zircon rims in the granulite facies sample is the result of recrystallization of older (c. 475 Ma) protolith zircon. This study suggests that the partial granulitization is a result of a short exposure of the Eger Complex metagranitoids to a temperature of ~850 °C at the base of an arc/fore-arc domain and their subsequent rapid exhumation during the Lower Carboniferous collision along the western margin of the Bohemian Massif.

Isotopic dating of detrital zircon populations from metasediments and of magmatic zircon from intercalated metavolcanic layers in the medium- to high-grade part of the Kaoko Belt in Namibia provides first robust constraints on the age of Neoproterozoic sedimentation along the southwestern margin of the Congo Craton. Dating of detrital zircons from metasediments directly overlying the cratonic basement shows maximum sedimentary ages of c. 1.00 and c. 1.45 Ga, and age populations comparable with known protolith ages from the gneisses of the Congo/Kalahari cratons. Dating of zircon from associated metavolcanics constrains the age of the earliest preserved sediments at c. 740–710 Ma. Detrital zircon populations from the samples collected from the upper parts of the metasedimentary succession contain only small proportion of grains with ages similar to those from the Congo Craton. These samples show dominance of c. 1.00 Ga, c. 750 Ma and c. 650 Ma old zircon grains that are probably derived from the Punta del Este – Coastal Terrane (Dom Feliciano and Kaoko belts) that acted as an arc/back-arc domain at c. 650–630 Ma. Neodymium model ages for the samples of metasedimentary cover of the cratonic basement provide another evidence that the youngest preserved sediments could not have been derived from the Congo Craton. Recognition of the Punta del Este – Coastal Terrane crust as a source region for the youngest pre-collisional sediments of the Kaoko Belt suggests that this terrane must have been in close proximity to the Congo Craton passive margin already some time prior to their mutual collision at c. 570–550 Ma. This is in accord with an interpretation that the c. 650–630 Ma arc/back-arc Punta del Este – Coastal Terrane has developed directly on top of, or very close to, the attenuated Congo Craton passive margin.

Detrital zircon U–Pb age data from the Moldanubian part of the Bohemian Massif obtained by laser ablation inductively coupled plasma mass spectrometry show variations between the lithotectonic units and point to their formation in spatially close but separated basins that were supplied by detritus from somewhat different source areas, possibly over different periods of time. The youngest detrital zircon component in metasediments of the Monotonous and Varied Units is Early Ordovician and Mid- to Late Devonian in age, respectively, suggesting their deposition in Palaeozoic times. Reliable interpretation of the Mid- to Late Devonian zircons recovered from metasediments of the Gfohl Unit is precluded by their high metamorphic grade and presence of anatectic melt. The Early Carboniferous zircons from the Gfohl Unit are interpreted as being of metamorphic origin. Comparison of detrital zircon age spectra from the Moldanubian, Tepla–Barrandian and Moravo-Silesian metasediments suggests that these were deposited in separate basins but the overall similarity of the Neoproterozoic and Palaeoproterozoic age maxima and sparse Neo- and Mesoarchaean ages suggest that the three crustal segments must have been spatially related prior to the Variscan orogeny. Future tectonic models of the Variscan assembly of the Bohemian Massif must account for nearly synchronous evolution of the Moldanubian, Tepla–Barrandian and Moravo-Silesian Palaeozoic sedimentary basins that shared a common crust and most of their sedimentary sources, as well as for a rapid burial of the Moldanubian sediments to mid- and lower crustal levels that was followed by their rapid exhumation to the upper crust in Mid-Devonian to Early Carboniferous times. Supplementary material: Results of LA-ICP-MS U–Pb analyses of zircons extracted from the Moldanubian and Moravo-Silesian metasediments are available at www.geolsoc.org.uk/SUP18676.

Discontinuous and kinematically unrelated C-S fabrics have been recognized along the contact between the Gemeric and Veporic Units in the Western Carpathians. The formation of S and C fabrics within orthogneiss, quartzite and chloritoid-kyanite schist of the Veporic Unit is associated with Cretaceous syn-burial orogen-parallel flow and subsequent exhumational unroofing. The formation of the two fabrics characterized by distinct quartz deformation microstructure and metamorphic assemblage is separated by an inter-tectonic growth of transversal chloritoid-, kyanite-, ± monazite-bearing assemblage. The monazite U-Th-Pb concordia age of 97 ± 4 Ma was obtained by the laser ablation ICP-MS dating method. The age of this inter-tectonic metamorphic stage together with existing 40Ar/39Ar ages on exhumation of the Veporic Unit indicate that despite the similar appearance to shear bands or C-S mylonites there is a time span of at least 10 Myr between the formation of homogeneous S fabrics and superposed discrete C fabrics in the studied rocks

Accepted manuscript version. Published version at http://doi.org/10.1016/j.precamres.2016.05.011. License CC BY-NC-ND 4.0. The match of geological units of the dissected Kaoko-Dom Feliciano-Gariep orogenic system exposed along the coasts of the South Atlantic is poorly understood. Two suites of intrusive rocks crop out in the Angra Fria Bay area, a part of the Coastal Terrane of the Kaoko Belt in Namibia. U–Pb zircon dating of three samples from the younger suite provided ages of 574 ± 6, 586 ± 3 and 584 ± 7 Ma, similar to the ages of the oldest syn-collisional granitoids in the area. Three samples of the older suite gave ages of 626 ± 5, 622 ± 5 and 620 ± 6 Ma respectively, which have not previously been recorded in intrusive rocks of the Kaoko Belt, but which coincide with those from the Florianópolis Batholith in the Dom Feliciano Belt in Brazil. Zircon ages, Sr–Nd isotopic composition and tectonic position of these granitoids suggest that this magmatic complex could be a continuation of the Florianópolis Batholith on the African side of the Atlantic Ocean. Consequently, the Angra Fria intrusions and the Florianópolis Batholith may represent suitable localities for spatial reconstruction of the Gondwana supercontinent in the area, where the robust pre-Mesozoic connecting points are scarce.

Age spectra of detrital zircons from metamorphosed quartzites of the Krkonoše–Jizera Massif in the northeastern part of the Saxothuringian Domain were obtained by U–Pb laser ablation inductively coupled plasma mass spectrometry dating. The zircon ages cluster in the intervals of 450–530 Ma and 550–670 Ma, and show individual data between 1.6 and 3.1 Ga. Zircons in the analysed samples are predominantly of Cambrian–Ordovician and Neoproterozoic age, and the marked peak atc. 525–500 Ma suggests a late Cambrian maximum age for the sedimentary protolith. Detritus of the quartzites probably originated from the erosion of Cambrian–Ordovician granitoids and their Neoproterozoic (meta)sedimentary or magmatic country rocks. The lack of Neoproterozoic (meta)sedimentary rocks in the central and eastern part of the Krkonoše–Jizera Massif suggests that the country rocks to voluminous Cambrian–Ordovician magmatic bodies were largely eroded during the formation of early Palaeozoic rift basins along the southeast passive margin of the Saxothuringian Domain. The detrital zircon age spectra confirm the previous interpretation that the exposed basement, dominated by Neoproterozoic to Cambrian–Ordovician granitoids, was overthrust during Devonian–Carboniferous subduction–collision processes by nappes composed of metamorphosed equivalents of the uppermost Cambrian–Devonian passive margin sedimentary formations. Only a negligible number of Mesoproterozoic ages, typically from the Grenvillian event, supports the interpretation that the Saxothuringian Neoproterozoic basement has an affinity to the West African Craton of the northwestern margin of Gondwana.

Three structural profiles across the Coastal Terrane, the Boundary Igneous Complex and the Orogen Core have been studied in the Kaoko Belt of northwestern Namibia. The oldest known Si fabric is inherited from an older tectono-metamorphic event. It occurs in the Coastal Terrane only and the extent of its reworking increases from south to north. The S1 foliation reactivates or folds Si fabric in the Coastal Terrane and appears as an early planar fabric in granitoids of the Boundary Igneous Complex and migmatites of the Orogen Core domain. Superimposed subvertical S2 fabric corresponds to axial plane cleavage of upright close to isoclinal folds and the extent of its development also increases from south to north. Active migration of partial melt during S2 development in the Orogen Core dates the onset of this deformation at ~550 Ma. Distribution of F2 fold axes and L2 stretching lineations suggests pure shear–dominated deformation associated with development of N–S trending S2 cleavage preserved in the central profile, followed by sinistral simple shear–dominated deformation on newly developed NW–SE trending pervasive cleavage in the northern part of the area. Such spatial variation in the deformation record is attributed to the irregular shape of the Congo Craton indenter that is reflected by heterogeneous development of the S2 cleavage front in the Coastal Terrane and the Boundary Igneous Complex. Common orientation of L1 and L2 stretching lineations and solid-state reworking on both S1 and S2 planes suggest single event of sinistral transpression since 550 Ma with strain partitioning into domains of oblique thrusting (reactivated S1) and transcurrent sinistral shearing (S2 and S3). Such succession of deformation structures suggests that major subvertical shear zones in the Kaoko Belt do not correspond to early crustal discontinuities, but rather reflect late strain localization during cooling.

The metamorphic evolution of micaschists in the north-eastern part of the Saxothuringian Domain in the Central European Variscides is characterized by the early high-pressure M1 assemblage with chloritoid in cores of large garnet porphyroblasts and a Grt–Chl–Phe–Qtz ± Pg M2 assemblage in the matrix. Minerals of the M1–M2 stage were overprinted by the low-pressure M3 assemblage Ab–Chl–Ms–Qtz ± Ep. Samples with the best-preserved M1–M2 mineralogy mostly appear in domains dominated by the earlier D1 deformation phase and are only weakly affected by subsequent D2 overprint. Thermodynamic modelling suggests that mineral assemblages record peak-pressure conditions of ≥18–19 kbar at 460–520 °C (M1) followed by isothermal decompression 10.5–13.5 kbar (M2) and final decompression to

At the northeastern edge of the Moldanubian Zone, a large body of felsic to intermediate granulite with relics of high-pressure mineral assemblage is exposed within medium-grade paragneisses, micaschists and metagranites. A polyphase tectonic evolution in the study area resulted in three deformation phases (D 1 –D 3 ). Peak metamorphic conditions (860–1000 °C and 16 kbar) in granulites are interpreted as reflecting an early stage of the high-grade evolution of the orogenic lower crust. These conditions were followed by exhumation of the orogenic lower crustal block to mid-crustal levels (6–8 kbar). In contrast to this, the study of metamorphic conditions in the surrounding micaschists (660 °C and 8 kbar) shows that these rocks never experienced pressures and temperatures of the orogenic lower crust. Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) U–Pb dating of zircon from the granulite yielded a concordia age of 354 ± 7 Ma, whereas the conventional isotope dilution thermal ionization mass spectrometry (ID-TIMS) and LA-ICP-MS dating of xenotime gave identical concordia ages of 336.2 ± 1.2 and 339 ± 3 Ma, respectively. Combination of new structural and petrological results with LA-ICP-MS zircon and xenotime dating leads to development of a well-constrained exhumation model and brings new insights into the behaviour of lower crustal rocks during orogenesis.

(Fe–Zn–Mg)-spinel-bearing assemblages in the high-grade rocks from three localities in the Moldanubian domain of the Bohemian Massif (Variscan belt of Central Europe) have been studied to monitor the partition behaviour of zinc along the pressure–temperature path followed by the host rock and to discuss the role of Zn in spinel stabilization. Spinels in two studied samples are characterized by low gahnite content (max. 0.09 Zn p.f.u), they crystallized only under silica-undersaturated conditions and the presence of zinc is thus assumed to be insignificant for their stabilization. One sample contains spinels that are gahnite-rich (up to 0.58 Zn p.f.u) leading to their stabilization within a quartz-bearing matrix. All spinel-bearing domains reflect late re-equilibration corresponding to decompression. However, in samples with preserved relics of a high-pressure mineral assemblage, changes in the zinc content attributed to prograde garnet growth were observed in the garnet cores. In order to monitor the distribution of zinc along the pressure-temperature path, textural observations have been combined with electron-microprobe and laser ablation ICP-MS data. The results point to preferential partitioning of zinc into biotite during the spinel-absent early stages of decompression and suggest that biotite was the main source of zinc for spinel crystallizing at low pressures.

The geological inventory of the Variscan Bohemian Massif can be summarized as a result of Early Devonian subduction of the Saxothuringian ocean of unknown size underneath the eastern continental plate represented by the present-day Tepla-Barrandian and Moldanubian domains. During mid-Devonian, the Saxothuringian passive margin sequences and relics of Ordovician oceanic crust have been obducted over the Saxothuringian basement in conjunction with extrusion of the Tepla-Barrandian middle crust along the so-called Tepla suture zone. This event was connected with the development of the magmatic arc further east, together with a fore-arc basin on the Tepla-Barrandian crust. The back-arc region – the future Moldanubian zone – was affected by lithospheric thinning which marginally affected also the eastern Brunia continental crust. The subduction stage was followed by a collisional event caused by the arrival of the Saxothuringian continental crust that was associated with crustal thickening and the development of the orogenic root system in the magmatic arc and back-arc region of the orogen. The thickening was associated with depression of the Moho and the flux of the Saxothuringian felsic crust into the root area. Originally subhorizontal anisotropy in the root zone was subsequently folded by crustal-scale cusp folds in front of the Brunia backstop. During the Visean, the Brunia continent indented the thickened crustal root, resulting in the root's massive shortening causing vertical extrusion of the orogenic lower crust, which changed to a horizontal viscous channel flow of extruded lower crustal material in the mid- to supra-crustal levels. Hot orogenic lower crustal rocks were extruded: (1) in a narrow channel parallel to the former Tepla suture surface; (2) in the central part of the root zone in the form of large scale antiformal structure; and (3) in form of hot fold nappe over the Brunia promontory, where it produced Barrovian metamorphism and subsequent imbrications of its upper part. The extruded deeper parts of the orogenic root reached the surface, which soon thereafter resulted in the sedimentation of lower-crustal rocks pebbles in the thick foreland Culm basin on the stable part of the Brunia continent. Finally, during the Westfalian, the foreland Culm wedge was involved into imbricated nappe stack together with basement and orogenic channel flow nappes.

A large database of structural, geochronological and petrological data combined with a Bouguer anomaly map is used to develop a two-stage exhumation model of deep-seated rocks in the eastern sector of the Variscan belt. An early sub-vertical fabric developed in the orogenic lower and middle crust during intracrustal folding followed by the vertical extrusion of the lower crustal rocks. These events were responsible for exhumation of the orogenic lower crust from depths equivalent to 18−20 kbar to depths equivalent to 8−10 kbar, and for coeval burial of upper crustal rocks to depths equivalent to 8–9 kbar. Following the folding and vertical extrusion event, sub-horizontal fabrics developed at medium to low pressure in the orogenic lower and middle crust during vertical shortening. Fabrics that record the early vertical extrusion originated between 350 and 340 Ma, during building of an orogenic root in response to SE-directed Saxothuringian continental subduction. Fabrics that record the later sub-horizontal exhumation event relate to an eastern promontory of the Brunia continent indenting into the rheologically weaker rocks of the orogenic root. Indentation initiated thrusting or flow of the orogenic crust over the Brunia continent in a north-directed sub-horizontal channel. This sub-horizontal flow operated between 330 and 325 Ma, and was responsible for a heterogeneous mixing of blocks and boudins of lower and middle crustal rocks and for their progressive thermal re-equilibration. The erosion depth as well as the degree of reworking decreases from south to north, pointing to an outflow of lower crustal material to the surface, which was subsequently eroded and deposited in a foreland basin. Indentation by the Brunia continental promontory was highly noncoaxial with respect to the SE-oriented Saxothuringian continental subduction in the Early Visean, suggesting a major switch of plate configuration during the Middle to Late Visean.

Granitoid intrusions of the Boundary Igneous Complex separate segments with different ages of high-grade metamorphism in the Kaoko Belt, NW Namibia. Two granitoids of this complex were dated at 575 ± 10 Ma (secondary ionization mass spectrometry; SIMS) or 571 ± 9 Ma (laser ablation inductively coupled plasma mass spectrometry; LA-ICP-MS) and 562 ± 11 Ma (SIMS) or 572 ± 4 Ma (LA-ICP-MS), respectively. The age of granulite-facies metamorphism in the eastern part of the Western Kaoko Zone was established at 549 ± 5 Ma (SIMS) by analysing metamorphic overgrowths of older ( c . 1850–1000 Ma) zircons from melt segregations in amphibolites. The coastal part of the Western Kaoko Zone consists of horizons of migmatitic metasedimentary rocks that are intercalated with fine-grained orthogneisses and amphibolites resembling metamorphosed sequences of bimodal volcanic rocks. Zircons from felsic members of two bimodal suites have SIMS ages of 805 ± 4 Ma and 810–840 Ma, respectively, that are interpreted as dating their respective igneous protoliths. Melt segregations in the mafic member of the lower bimodal suite contain two populations of zircon dated at 650 ± 5 Ma (SIMS) or 645 ± 5 Ma (LA-ICP-MS) and 629 ± 6 Ma (SIMS) or 630 ± 5 Ma (LA-ICP-MS), respectively. The later age is indistinguishable from the age of 630 ± 4 Ma (SIMS) or 625 ± 10 Ma (LA-ICP-MS) obtained from melt patches present in overlying metagreywackes. The available age data suggest that the Boundary Igneous Complex masks the suture between the Coastal Terrane and the rest of the Kaoko Belt. Ages of granitoid intrusions in this igneous complex are indicative of magmatic activity between 580 and 550 Ma.

Source:http://dx.doi.org/10.1016/j.jsg.2016.06.008 The Krkonoše-Jizera Massif in the northern part of the Variscan Bohemian Massif provides insight into the exhumation mechanisms for subducted continental crust. The studied region exposes a relatively large portion of a flat-lying subduction-related complex that extends approximately 50 km away from the paleosuture. wide extent of HP-LT metamorphism has been confirmed by new P-T estimates indicating temperatures of 400–450 °C at 14–16 kbar and 450–520 °C at 14–18 kbar for the easternmost and westernmost parts of the studied area, respectively. A detailed study of metamorphic assemblages associated with individual deformation fabrics together with analysis of quartz deformation microstructures and textures allowed characterisation of the observed deformation structures in terms of their subduction-exhumation memory. An integration of the lithostratigraphic, metamorphic and structural data documents a subduction of distal and proximal parts of the Saxothuringian passive margin to high-pressure conditions and their subsequent exhumation during two distinct stages. The initial stage of exhumation has an adiabatic character interpreted as the buoyancy driven return of continental material from the subduction channel resulting in underplating and progressive nappe stacking at the base of the Teplá-Barrandian upper plate. With the transition from continental subduction to continental collision during later stages of the convergence, the underplated high-pressure rocks were further exhumed due to shortening in the accretionary wedge. This shortening is associated with the formation of large-scale recumbent forced folds extending across the entire studied area.

Plagioclase rims around metastable kyanite crystals appear during decompression of high-pressure felsic granulites from the high-grade internal zone of the Bohemian Massif (Variscan belt of Central Europe). The development of the plagioclase corona is a manifestation of diffusion-driven transfer of CaO and Na2O from the surrounding matrix and results in isolation of kyanite grains from the quartz- and K-feldspar-bearing matrix. This process establishes Si-undersaturated conditions along the plagioclase- kyanite interface, which allow crystallization of spinel during low-pressure metamorphism. The process of the plagioclase rim development is modeled thermody- namically assuming local equilibrium. The results combined with textural observations enable estimation of equilibration volume and diffusion length for Na and Ca that extends ~400-450 and ~450-550 lm, respectively, around each kyanite crystal. Low esti- mated bulk diffusion coefficients suggest that the diffusion rate of Ca and Na is controlled by low diffusivity of Al across the plagioclase rim.

In the central part of the Erzgebirge, the age of granulite-facies metamorphism corresponds to the age of formation of mafic eclogites exhumed at the base of the recently defined Lower Crystalline nappe. Granulite-facies conditions in acidic continental crust and eclogite-facies conditions in mafic eclogites developed at the same time of about 342 Ma, suggesting the existence of two distinct tectonic units with contrasting initial geothermal gradients. Existing petrological and geochronological data are used to present a new model for Carboniferous collision at the western margin of the Bohemian Massif. We propose that juxtaposition of rocks with contrasting thermal histories is the result of subduction of the Saxothuringian crust and its later collision with a thickened continental orogenic root existing to the SE. A short time span between the formation ages of high-pressure rocks and cooling ages of the host lithologies suggests extremely fast exhumation of a coupled subduction zone–orogenic root system during continuing Variscan collision.

The Cretaceous Yungmengshan core complex in northern China contains a large syntectonic granodiorite batholith that intrudes a slightly older diorite intrusion. A major gently dipping ductile decollement shear zone is developed along the contact of the diorite and granodiorite. The shear zone is invaded by a large volume of granitic and pegmatite veins associated with the main granodiorite batholith during activity of the shear zone under high-grade metamorphic conditions. Progressively older veins are more strongly deformed into tight cylindrical fold structures rotated into parallelism with the lineation and foliation in the shear zone. Parallelism of veins to the foliation is partly due to this rotation, but also to foliation-parallel injection of younger syntectonic pegmatite veins. Several small-scale structures have been recognized that allow distinction of solid-state deformation of veins. Granite veins do not extend much above the ductile shear zone that seems to act as a lid and an effective depository to intruding granite veins from the underlying batholith. There was considerable volume increase in the footwall and lower part of the shear zone by vein intrusion. Gently dipping ductile shear zones are a common feature of many continental metamorphic core complexes throughout the world, for example in the Basin and Range (Reynolds & Lister 1987; Fletcher & Bartley 1994; Foster & Fanning 1997; Foster et al. 2001), Algeria (Caby et al. 2001), New Guinea (Baldwin et al. 1993) and in the Aegean Sea (Lister et al. 1984; Walcott & White 1998; Pe-Piper et al. 2002). The shear zones are thought to accommodate most of the crustal-scale extension that is typical for the development of such core complexes. Core complexes are also commonly associated with the intrusion of granitoid plutons and associated rhyolitic volcanism (e.g. Caby et al. 2001; Foster et al. 2001; Pe-Piper et al. 2002). Many granitoid intrusions in core complexes can be shown to have intruded during active deformation of the ductile shear zones of the complex (Caby et al. 2001; Foster et al. 2001; Pe-Piper et al. 2002). In such cases, parts of a pluton may cut mylonitic rocks, while other parts are mylonitized with the same orientation of mylonitic structures as in the truncated older parts, and this is used as evidence of syntectonic intrusion. In the Yunmengshan of northern China, a Cretaceous core complex developed in Archaean basement and Proterozoic metasediments at the northern rim of the North China Craton (Fig. 1) (Davis et al. 1996). The core complex is centred on a granodiorite batholith, which is truncated at the northern, eastern and southern sides by brittle and ductile shear-zone segments. The segments dip gently away from the granodiorite batholith towards the north, east and south (Figs 1 and 2). Mapping by Davis et al. (1996) has shown that, although these shearzone segments are of different metamorphic grade and presumably of different age in different parts of the pluton, they share a common orientation of stretching lineations. They are therefore inferred to have formed as part of one tectonic shear-zone system that acted as a major decollement of the core complex. Its present variation in dip direction and dip is probably due to folding over the developing core complex, a feature also observed in many other complexes (e.g. Fletcher & Bartley 1994). Along the NE side of the core complex, the Yunmengshan granodiorite is bordered by metadioritic rocks and the contact is affected by a ductile shear zone, the Sihetang shear zone discussed in this paper (Fig. 2). The metadiorites are intrusive into Proterozoic metasediments (Fig. 2) (Miyun metadiorite; Davis et al. 1996), From: BRUHN, D. & BURLINI, L. (eds) 2005. High-Strain Zones: Structure and Physical Properties. Geological Society, London, Special Publications, 245, 65–80. 0305-8719/05/$15.00 # The Geological Society of London 2005. but the intrusive contact is rarely exposed and is marked by a Mesozoic brittle thrust fault in most places (Fig. 2). This fault post-dates intrusion of the diorite and granodiorite, and seems to postdate and cut the Sihetang ductile shear zone. The thrust and the Sihetang shear zone are cut in the SE by the brittle Hefangkou normal fault (Fig. 1). The Sihetang shear zone is an amphibolite facies ductile shear zone with a thickness of 20–50 m (Fig. 2) (Davis et al. 1996). The shear zone is centred on the contact between the metadiorite and the granodiorite, and has strongly mylonitized the igneous rocks. Despite the presence of the shear zone, it is clear that the contact between the granite and granodiorite of the Yunmengshan batholith and the Miyun metadiorite was intrusive: xenoliths of metadiorite occur in the granite, and granite veins intrude the metadiorite. The metadiorite therefore predates the granitic phases. Dating of both units has given concordant U–Pb zircon ages of 159 + 2 Ma for the Miyun metadiorite and ages between 151 and 127 Ma for the Yunmengshan granodiorite (Davis et al. 1996). Although the main body of the granodiorite is deformed by the Sihetang shear zone, there is a large volume of granite dykes that show different stages of deformation, and a decrease in deformation intensity with a decrease in relative age within the shear zone, based on cross-cutting relations. This suggests that the shear zone was active while intrusive material was added from lower levels of the granodiorite pluton. The Sihetang shear zone is not affected by late brittle deformation and forms a flat ‘roof’ to the batholith. It is only invaded by minor veins, although these represent a considerable volume of material. Some work has been published on synkinematic intrusion of granite veins into ductile shear zones (Zurbriggen et al. 1998; Pawley et al. 2002), but three-dimensional (3D) control on geometry is limited in these studies. The excellent outcrop condition and the sharp colour contrast between the metadiorite and the leucocratic veins in the Sihetang shear zone Proterozoic metasediments

Early structures in the central part of the Kaoko orogenic belt of NW Namibia suggest that the initial stage of collision was governed by underthrusting of the medium-grade Central Kaoko zone below the high-grade Western Kaoko zone, resulting in the development of an inverted metamorphic gradient. In the Western zone, early structures were overprinted by a second phase of deformation, which is associated with localization of the transcurrent Puros shear zone along the contact between the Western and Central zones. During this second phase, extensive partial melting and intrusion of ∼550 Ma granitic bodies occurred in the high-grade Western zone. In the Central zone, the second phase of deformation led to complete overprinting of the early foliation in the zone adjacent to the Puros shear zone, and to the development of kilometre-scale folds in the more distal parts. Strain partitioning into transcurrent deformation along the Puros shear zone and NE–SW oriented shortening in the Central zone is consistent with a sinistral transpressional regime during the second phase of deformation. Transcurrent deformation continued during cooling of the entire belt, giving rise to the localized low-temperature Village Mylonite Zone that separates a segment of elevated Mesoproterozoic basement from the rest of the Western zone in which only Pan-African ages have so far been observed. The data suggest that the boundary between the Western and Central Kaoko zones represents a modified thrust zone controlling the tectonic evolution of the Pan-African Kaoko belt.

The Kaoko Belt belongs to the Neoproterozoic mobile belt system of western Gondwana, whose geodynamic evolution is assumed to have resulted from collision between the Congo Craton (present Africa) and the Rio de la Plata Craton (present South America). Several magmatic intrusion periods can be distinguished in the coastal area of this belt, based on conventional U-Pb, SHRIMP and Pb-Pb evaporation analyses on zircons. The prevailing igneous rock types are of granitic to tonalitic composition. A Palaeoproterozoic terrain with U-Pb magmatic emplacement ages between ~2.03 and 1.96 Ga may be correlated with the Eburnian event (~1.8 to 2.0 Ga), which is widespread in Africa. Additionally, two distinct magmatic events appear to be significant in the southwestern Congo Craton in late Palaeoproterozoic and Mesoproterozoic times, indicated by magmatic ages around 1.77 Ga and between ~1.52 and ~1.45 Ga. These two events have so far not been reported from the Dom Feliciano Belt (southeastern Brazil), which is considered to represent the South American counterpart of the Kaoko Belt. Therefore, a possible link of these two belts prior to the Pan-African orogeny (~750 to ~450 Ma) can neither be confirmed nor rejected on the basis of our data. Emplacement ages for several Pan-African granitoids, obtained by U-Pb and SHRIMP analyses, range from ~730 to ~655 Ma. The youngest granitoids, representing the last major magmatic activity in that area, were emplaced at ~550 Ma during a transpressional regime at peak Pan-African temperatures.

In the central part of the Krusne hory (Erzgebirge) Mountains, the parautochthonous metasedimentary basement has been overthrust by a crustal nappe of fine- and coarse-grained orthogneisses. The thrust boundary is defined by the presence of mafic eclogites with preserved subduction-related fabric. Westward thrusting of an allochthonous unit is associated with the development of the main metamorphic foliation and lineation in non-eclogitic lithologies. Buttressing of the allochthonous body from the west is responsible for the development of late, large-scale folds with N–S trending hinges and vertical axial planes. Subsequent N–S compression leads to large-scale folding of both the parautochthonous and allochthonous units. This deformation produces km-scale antiforms with hinges plunging to the west and is associated with the development of the E–W stretching lineation as a result of complete reworking of earlier fabric in the limb zones. N–S shortening is also associated with the development of small-scale folds and brittle-ductile kink bands suggesting a decrease in temperature, and, thus, uplift of the whole studied area during this event. The last stage of deformation is characterised by the development of kink-band folds and a crenulation cleavage. These structures suggest a sub-vertical direction of principal compression, developed exclusively in those parts of the area in which the N–S compression produced steep planar fabric.

The metamorphism of low-alumina metapelites along high pressure gradients involves only a few continuous and discontinuous reactions that produce minor variations in modal mineralogy. This is the case with metamorphic rocks forming the (para-)autochthonous sequence in the vicinity of the village of Měděnec (Krusne hory Mts., Bohemian Massif). Metamorphic reactions taking place among micas, garnets and feldspars can be described using Tschermak exchange and NaK−1 exchange. Pressure-temperature conditions for some of these continuous reactions were calculated from mineral compositions. These calculations suggest that the Tschermak and NaK−1 exchanges are sensitive monitors of pressure and temperature conditions in these rocks, the mineral assemblages of which are unsuitable for experimentally calibrated thermobarometers. The results show metamorphic temperatures of 580–630 °C at a pressure of 13–15 kbar during metamorphism of Pre-Variscan basement beneath a nappe sequence.

Journal of Petrology, 51 (11), ISSN:0022-3530, ISSN:1460-2415

At the eastern margin of the Bohemian Massif (Variscan belt of Central Europe), large bodies of felsic granulite preserve mineral assemblages and structures developed during the early stages of exhumation of the orogenic lower continental crust within the Moldanubian orogenic root. The development of an early steep fabric is associated with east–west-oriented compression and vertical extrusion of the high-grade rocks into higher crustal levels. The high-pressure mineral assemblage Grt-Ky-Kfs-Pl-Qtz-Liq corresponds to metamorphic pressures of ∼18 kbar at ∼850 °C, which are minimum estimates, whereas crystallization of biotite occurred at 13 kbar and ∼790 °C during decompression with slight cooling. The late stages of the granulite exhumation were associated with lateral spreading of associated high-grade rocks over a middle crustal unit at ∼4 kbar and ∼700 °C, as estimated from accompanying cordierite-bearing gneisses. The internal structure of a contemporaneously intruded syenite is coherent with late structures developed in felsic granulites and surrounding gneisses, and the magma only locally explored the early subvertical fabric of the felsic granulite during emplacement. Consequently, the emplacement age of the syenite provides an independent constraint on the timing of the final stages of exhumation and allows calculation of exhumation and cooling rates, which for this part of the Variscan orogenic root are 2.9–3.5 mm yr−1 and 7–9.4 °C Myr−1, respectively. The final part of the temperature evolution shows very rapid cooling, which is interpreted as the result of juxtaposition of hot high-grade rocks with a cold upper-crustal lid.