Variscan Research Articles

A complex accretionary assembly of Pangea developed in the range c. 400–340 Ma: the four successive events of high-P/ultra-high-P metamorphism of the Variscan Orogen

ABSTRACT In the most internal zone of the Variscan Orogen, a stack of ophiolites and (ultra)high-P belts hold evidence for c. 60 Myr (c. 400–340 Ma) of almost continuous metamorphism and accretion onto the continental margin of Gondwana. Subduction of the peri-Gondwanan lithosphere led to three successive (ultra)high-P metamorphic episodes prior to the continental collision between Gondwana and Laurussia that shaped Pangea. A first high-P, high-T metamorphic belt developed during subduction of a peri-Gondwanan magmatic arc in the Early Devonian. Related to this event is the ensuing opening of intra-Gondwana Middle Devonian oceanic basins, which rapidly closed leading to ophiolite accretion. The second high-P metamorphic episode is related to the formation of a serpentinitic mélange after the sequential subduction of oceanic and transitional crust during Middle Devonian time. Tectonic blocks of subducted crustal material recording high-P, low-T metamorphism were detached from the slab and incorporated into a low-viscosity channel developed between the downgoing slab and the mantle wedge. As convergence progressed, a Late Devonian high-P, low- to intermediate-T metamorphic belt developed due to subduction of the most external margin of Gondwana. Subsequently, the scenario switched to continental collision. In this context, Early Carboniferous intracontinental subduction of the Gondwanan foreland below the accretionary wedge triggered ultra-high-P metamorphism and ensuing exhumation via trans-lithospheric diapirism and doming up to the base of the accretionary wedge.

A composite terrain-and-provenance analysis tied to specific times of a basement-foreland drainage system and its gravel-sand-clay deposits (Bohemian Massif, SE Germany)

Provenance analysis and terrain analysis backed by radiometric age dating have been combined for the first time and conducted as a composite in a foreland-basement transition zone with mutual benefits for both disciplines. A meticulous provenance analysis enables a fine-tuning during studies of the landscapes and relief generations, while a detailed terrain analysis is essential for the disentanglement of lithological-geodynamic processes that often are telescoped into each other by sedimentological processes. The basic drivers for the denudation, transport and deposition processes of the various clast communities (gravel, sand, clay) are the climate change from a tropical (Neogene) to a glacial/temperate Earth and the brittle neotectonics along the margin of the study area (Rhine Graben Rift N-S, Eger/Ohře Graben Rift E-W). As a result of this composite approach the landscape formation in the study area is described as follows: (1) (Paleogene)-Neogene peneplanation in the basement, (2) Quaternary straight to low sinuosity drainage in the basement, (3) Late Pleistocene to Holocene high-sinuosity drainage in the proximal foreland and (4) Holocene (to Late Pleistocene) meandering fluvial rivers evolving in the distal foreland. In additions to these geomorphological results progress can be made in the geodynamic evolution of the study/source area. Five Proterozoic to Late Paleozoic lithological units are identified: (1) Para-metamorphic rocks, (2) greenstones, (3) basic volcano-sedimentary rocks, (4) sedimentary units abundant in chert and (5) felsic volcanics. By means of these sedimentological finds a more detailed picture of the Variscan orogeny can be presented.

Ordovician tectonics of the South European Variscan Realm: new insights from Sardinia

Although much is known about the Ordovician tectonics of the South European Variscides, aspects of their geodynamic evolution and palaeogeographic reconstruction remain uncertain. In Sardinia, Variscan tectonic units include significant vestiges of Ordovician evolution, such as a fold system that affected only the Cambrian–Lower Ordovician successions, and are cut by a regional angular unconformity. A comparison of the stratigraphy and tectonic structures of the successions below and above the Lower Ordovician unconformity and a reinterpretation of biostratigraphic data allow us to identify significant differences between the stacked tectonic units. The unconformity is sealed as follows: (i) in the Sulcis–Iglesiente Unit (Variscan External Zone, SW Sardinia) by Middle–Upper Ordovician continental and tidal deposits; and (ii) in the Sarrabus and Gerrei units (part of the Variscan Nappe Zone, SE Sardinia) by Middle–Upper Ordovician calc–alkaline volcanic rocks. Therefore, at the same time, one tectonic unit was situated close to a rifting setting and the others were involved in a convergent margin. Of note are the different durations associated with the unconformities in the tectonic units (17 Myr in the Sulcis–Iglesiente Unit, 6 Myr in the Sarrabus and Gerrei units) and the occurrence (or absence) of glacio-marine deposits indicating that the units were located at different palaeo-latitudes during the Ordovician. These results suggest that the SW and SE Sardinia blocks did not share the same geodynamic setting during the Ordovician, implying that they were situated in different palaeogeographic positions at this time and subsequently amalgamated during the Variscan Orogeny. Furthermore, stratigraphic and tectonic correlations with neighbouring areas, such as the eastern Pyrenees, imply alternative palaeogeographic reconstructions to those proposed previously for some peri-Mediterranean Variscan terranes.

Hot metamorphic complex in the Foreland Zone of the Variscan chain: insights from the Monte Filau orthogneiss (SW Sardinia), Italy

The Mt. Filau Orthogneiss belongs to the low-pressure (LP) - high-temperature (HT) Sulcis Metamorphic Complex located in the foreland zone of the Variscan belt (southern Sardinia). It consists of a core of orthogneiss and high metamorphic grade host rock tectonically juxtaposed below weakly- to non-metamorphic tectonic units, by the Tuerredda ductile shear zone. Our new petrographic and microstructural data on the fine–grained leucocratic facies of the Mt. Filau Orthogneiss are consistent with metamorphic peak under conditions of incipient partial melting in the sillimanite stability field at 680–700°C and 0.32–0.46 GPa. The retrograde P–T path occurred in the andalusite stability field, according to the growth of andalusite after sillimanite in leucocratic facies. The P-T conditions of metamorphic peak and incipient melting are both consistent with development, in the foreland zone, of a thermal anomaly accompanied by the upward rise of deep hot crust in the footwall of the Tueredda ductile shear zone. This transtensive shear zone acted as preferential channel for upward advection of deep hot melts during tectonically-driven isothermal decompression of middle crust. Therefore, we interpret the Sulcis Metamorphic Complex as the result of a tectonically controlled thermal perturbation, and underline the role of this late Variscan crustal scale tectonic structure in the development of hot metamorphic complex in an anomalous position within the foreland zone of the Variscan collisional belt.

The Lanthanide “Tetrad Effect” as an Exploration Tool for Granite-Related Rare Metal Ore Systems: Examples from the Iberian Variscan Belt

Highly fractionated granites and related magmatic-hydrothermal ore-forming processes can be traced by elemental ratios such as Nb/Ta, K/Rb, Y/Ho, Sr/Eu, Eu/Eu*, Zr/Hf, and Rb/Sr. The lanthanide “tetrad effect” parameter (TE1,3) can also be a useful geochemical fingerprint of highly fractionated granites. This work assesses its application as an exploration vector for granite-related mineralization in the Central Iberian Zone by examining TE1,3 variations with different elemental ratios and with the concentrations of rare metals and fluxing elements (such as F, P, and B). The multi-elemental whole-rock characterization of the main Cambrian–Ordovician and Carboniferous–Permian granite plutons and late aplite–pegmatite dykes exposed across the Segura–Panasqueira Sn-W-Li belt show that the increase in TE1,3 values co-vary with magmatic differentiation and metal-enrichment, being the Carboniferous–Permian granite rocks the most differentiated, and metal specialized. The Argemela Li-Sn-bearing rare metal granite and the Segura Li-phosphate-bearing aplite–pegmatite dykes deviate from this geochemical trend, displaying TE1,3 < 1.1, but high P2O5 contents. The results suggest that mineralized rocks related to peraluminous-high-phosphorus Li-Sn granite systems are typified by TE1,3 < 1.1, whereas those associated with peraluminous-high-phosphorus Sn-W-Li (lepidolite) and peraluminous-low-phosphorus Sn-Ta-Nb granite systems display TE1,3 > 1.1, reaching values as high as 1.4 and 2.1, respectively.

The paleotectonic evolution of the western Mediterranean: provenance insights from the internal Betics, southern Spain

Since the early Cenozoic, the closure of the Alpine Tethys in the Western Mediterranean has been accomplished by protracted subduction, followed by collision and orogenic collapse. The internal zones of the Betic Cordillera (southern Spain) and Rif (northern Morocco) experienced subduction metamorphism and subsequent exhumation due to the westward migration of the orogenic system. The detrital provenance of these meta-sedimentary units contains crucial insights into their pre-subduction stratigraphic arrangement, which is essential to constrain the pre-Cenozoic paleogeography and tectonic evolution of the Western Mediterranean. This study focuses on the Nevado-Filábride Complex (NFC) and the Eastern Alpujárride Complex in the Internal Betic Cordillera. New depth-profile zircon U-Pb LA-ICP-MS data from the NFC (N = 72) and Alpujárride (N = 21) and in situ apatite U-Pb data from a metabasite within the NFC allow us to establish the pre-subduction stratigraphy, sedimentary provenance, and paleotectonic configuration of the Iberian rifted margin during the Paleozoic–Early Mesozoic. Our dataset demonstrates that the NFC represents an intact Devonian to Early Jurassic stratigraphic sequence that records the evolution of the Western Mediterranean from the Variscan orogeny to rifting and opening of the Alpine Tethys. Detrital zircon U-Pb age modes of the NFC remain remarkably similar for over 200 Myr with only the differences being the progressive addition of new zircon modes related to depositional ages. Additionally, a comparison between the provenance record of the NFC and Alpujárride reveals notably similar zircon signatures of Carboniferous and Permian strata. This study reports the first data from the Triassic–Jurassic strata that record the opening of the Alpine Tethys. Additional evidence for rifting is present in the metabasites from the Veleta unit yielding in situ apatite U-Pb ages of 197 Ma, recording CAMP magmatism and Early Jurassic rifting in the Alpine Tethys. The data demonstrate a palinspastic connection between the NFC and the Alpujárride. Based on these extensive new provenance data, we propose a pre-subduction tectonic configuration in which the NFC and Alpujárride both represent adjacent attenuated continental fragments that are separated from the southern Iberian rifted margin by a narrow oceanic domain of the Alpine Tethys in the early Mesozoic.

Maturity Matters in Provenance Analysis: Mineralogical Differences Explained by Sediment Transport from Fennoscandian and Variscan Sources

The significance of mineralogical maturity as a provenance indicator has long been debated and we use this study to demonstrate that it can indeed be a powerful tool to track the distribution of sandstone reservoirs. We investigate the cause of the pronounced geographic and stratigraphic differences in mineralogical composition that are found in the Upper Triassic–Lower Jurassic Gassum Formation across the Norwegian–Danish Basin and surrounding areas. Zircon U-Pb dating of 46 sandstone samples including analysis of 4816 detrital grains are combined with quantifications of the detrital mineralogical composition and placed in a sequence stratigraphic framework. The results show that the Gassum Formation can be divided into a southeastern region with high mineralogical maturity and a less mature region to the northwest with more feldspars, rock fragments, micas, and heavy minerals. Both the mineralogical assemblage and the provenance signature have been thoroughly homogenized in the SE region where sediment supplies from the Fennoscandian Shield and the Variscan Orogen are evident. In the NW region, sediment was initially supplied from Fennoscandia only, but the provenance abruptly changed from the Telemarkia Terrane to comprising also the more distant Caledonian Orogen resulting in a different mineralogical assemblage. The change occurred during a basinwide regression and may be caused by tectonic movements in the hinterland that permanently changed the composition of the sediment supplied to the basin.

Cenozoic tectono-sedimentary evolution of the Upper Moulouya continental basins (Moroccan Atlas system): Insights from lithostratigraphy, structural analysis and paleostress reconstruction

The Upper Moulouya massif (UMM) is located at the junction between the Middle and the High Atlas mountain ranges where outcrop a segment of the Moroccan Variscan belt and its unconformably overlying Meso-Cenozoic cover. In this paper, we propose the Cenozoic tectono-sedimentary evolution of the continental series filling the intramontane basins of the UMM using a multidisciplinary approach with multiscale geological-structural and paleostress analyses. The lithostratigraphic study shows that these series correspond to Oligo-Neogene fluvio-lacustrine sequences, alluvial fans deposits, and alluvial outwash plains, associated with volcanic rocks. Structural and paleostress analyses provide a complex tectono-sedimentary evolution marked by the superposition of four tectonic phases with changes in paleostress orientations. These tectonic phases are consistent with strike-slip tectonic regimes associated with compressional, transpressional, transtensional and extensional episodes: (i) the first tectonic phase is characterized by NW-SE trending σ1 axis, and NE-SW trending σ3 axis, with NNW-SSE to N–S sinistral strike-slip faults, ENE-WSW to E-W dextral strike-slip faults, and NE-SW reverse faults cross cutting the Mesozoic and Oligo-Miocene series; (ii) the second tectonic phase is characterized by ENE-WSW to E-W σ1 axis, and NNW-SSE to N–S σ3 axis where the subsidence of Pliocene lake basins is probably controlled by ENE-WSW to E-W extensional faults and NE-SW transtensional faults; (iii) the third phase is marked by NNE-SSW to NE-SW trending σ1 axis, and WNW-ESE to NW-SE trending σ3 axis, with submeridian dextral strike-slip faults, subequatorial sinistral strike-slip faults, NE-SW normal faults, and NW-SE reverse faults; (iv) the last tectonic phase is characterized by NNW-SSE to N–S σ1 axis, and ENE-WSW to E-W σ3 axis responsible for subequatorial reverse faults, submeridian normal faults, NE-SW sinistral strike-slip faults, and NW-SE dextral strike-slip faults.

Geodiversity Assessment Through the Évora–Montemor-o-Novo Region: on the Scope of Valorising the Mining Heritage of the Ossa-Morena Zone (SW Iberia, Portugal)

The SW of Iberia registers diverse examples of mining remnants from activities that ceased during the twentieth century, namely in the Ossa-Morena Zone. Such activities exposed outcrops that are part of the mining heritage of the Alentejo region (Portugal), and examples of that are found throughout the Évora–Montemor-o-Novo region at the ancient Montemor-o-Novo iron mines. GeSologically, the area is comprised within the Évora Massif and, besides displaying an important mining and quarrying heritage, also exhibits important geomorphological, structural and lithological features that are a key to understand the geodynamic evolution of the Variscan Orogeny at the SW of Iberia. This work intends to evaluate the geodiversity throughout the region and propose routes that integrate geodiversity features along with the mining heritage of selected ancient mines from the Montemor-o-Novo iron district. Furthermore, the region is characterised by immense cultural, historical and archaeological assets that, together with the geodiversity of this region, have been successfully used for scientific, formal and informal educational purposes. Mining heritage can contribute to the understanding of the role of mineral resources on the past, present and future of society as we know it. The proposal herein disclosed, although intending to promote geoconservation strategies regarding the mineral resources of the Ossa-Morena Zone, does not aim to make exploration and mining unfeasible in these locations. On the contrary, this work intends to promote strategies in which industry and geoheritage can work together on the knowledge transfer to society, contributing to the increase of geological literacy.

U–Pb age constraints on the protolith, cooling and exhumation of a Variscan middle crust migmatite complex from the Central Iberian Zone: insights into the Variscan metamorphic evolution and Ediacaran palaeogeographic implications

The Figueira de Castelo Rodrigo–Lumbrales Anatectic Complex (FCR-LAC) located in the Central Iberian Zone of the European Variscan Belt is delimited by two shear zones, which have been interpreted as constraining its exhumation. New U–Pb zircon and apatite ages for the migmatites allowed us to obtain a peak metamorphic age of the anatectic complex (316.8 ± 2.0 Ma), and to estimate fast cooling (7–54°C Ma –1 ) and fast exhumation (0.2–1.3 mm a −1 ) after a vertical displacement of c. 6–9 km. Our results show a similar zircon age distribution between migmatites and the Excomungada Formation from the host Ediacaran–Cambrian Douro–Beiras Supergroup. Although the existence of an inherited zircon population lasting c. 500 Ma, absent in the metasedimentary units, suggests the contribution of Ollo de Sapo magmatic rocks during the Lower–Middle Pennsylvanian melting events. Detrital zircon ages of the Pinhão Formation (Douro Group) have contrasting sources to units from the Beiras Group, which may indicate the presence of a barrier that prevented older detrital components from reaching the Douro basin. However, at odds with what has been postulated before, such a barrier must have been placed farther south of the FRC-LAC, and thus not coincident with the Juzbado–Penalva do Castelo shear zone. Supplementary material: A full description of the stratigraphy and lithologies (Supplementary 1 and 2, respectively); the analytical conditons of the LA-ICP-MS analyses (Supplementary Material 3); the U–Pb zircon and apatite data of the FCR-LAC migmatites and the U–Pb detrital zircon data of the DBSG (Supplementary Material 4) are available at https://doi.org/10.6084/m9.figshare.c.6002006

Diversity of origin and geodynamic evolution of the mantle beneath the Variscan Orogen indicating rapid exhumation within subduction-related mélange (Moldanubian Zone, Bohemian Massif)

A variable assemblage of ultrabasic rocks along with minor eclogites investigated on a small area of a few km2 (Gföhl unit, Moldanubian Zone of the Bohemian Massif) reflects incorporation of contrasting mantle domains within a subduction-related tectonic mélange during the Variscan orogeny. Based on mineral composition, whole-rock chemistry, isotopic signatures, and pressure-temperature (P–T) estimates, four principal lithological types have been distinguished (1) spinel harzburgite (2) garnet lherzolite (3) spinel websterite and (4) eclogite. Spinel harzburgite, exclusively associated with HT migmatized gneisses, corresponds to the refractory oceanic lithosphere, as demonstrated by whole-rock composition (low Al2O3, CaO), chemistry of spinel (Cr# <0.4) and Al-rich pyroxene (Al2O3 up to 9 wt%). By contrast, garnet lherzolite, predominantly enclosed in HP felsic granulite, represents a fragment of the subcontinental mantle wedge, considering the strong fertile character (high Al2O3, TiO2, Yb) and spinel chemistry (Cr# >0.5). Websterites likely represent products of decompression partial melting of the asthenospheric mantle with a variable input of crustal component, whereas eclogites correspond to HP crystal cumulates from partial melts migrating through the Variscan mantle wedge. Both peridotite protoliths experienced various degrees of secondary refertilization, recorded as a cryptic metasomatic overprint, due to interaction with subduction-related silicate melts, from which numerous websterite and rare eclogite layers crystallized. The secondary mantle refertilization via melt-peridotite reaction is well-documented by decreasing bulk Mg# along with MgO/SiO2, elevated Al2O3/SiO2, TiO2 and FeOtot contents, and isotopic composition (87Sr/86Sr338 ~ 0.7051). Moreover, the positive correlation between highly fluid-immobile incompatible trace elements (e.g. Ti, Sc, V, Zr, Yb) and distribution of REE and Li is consistent with the melt refertilization trend.

Variscan Inheritance Induces Alpine Upper Crustal Delamination in East Spanish‐Portuguese Central System

AbstractThe Spanish‐Portuguese Central System (SPCS) is an Alpine Mountain range with crystalline basement characterized by a two‐layer rheological structure. This structure formed after primary (protolith) and secondary (tectonometamorphic) processes during the extensional collapse of the Variscan Orogen. The SPCS structure is usually controlled by foreland‐directed thrusts and strike‐slip faults. However, the eastern SPCS is dominated by NW‐directed, imbricate backthrusts and lacks the main thrust directed to the foreland basin located southeast of the mountain range (Madrid Cenozoic Basin). The SPCS exhibits a crustal root (&gt;40 km depth) supporting SE‐directed crustal‐scale thrusting. Alpine backthrusts sole into an SE‐dipping décollement within the Variscan basement. Variscan extension‐related structures parallel the SE‐dipping geometry of Alpine backthrusts, so they provided favorably oriented rheological weaknesses to accommodate Alpine shortening. Backthrusts geometry, their hanging wall position within the fault that raised the SPCS and gravity modeling support an Alpine crustal delamination process. Tectonic wedging and delamination of the more competent basement occurred in the footwall of Variscan extensional faults (Daurius domain), which enforced the shearing off of a rheologically weaker upper layer of the crust, located in the hanging wall of the Variscan extensional faults (Arriaca domain) by inverting Variscan extensional faults. This led to NW‐directed incipient continental subduction of the weaker crust. Intraplate subduction and crustal delamination can be independent from lithosphere‐scale inheritance and be conditioned by structural inheritance in the overlying crust. Alpine shortening for the Cretaceous cover is around 17.7 km (10.5% shortening), and 11 km (7%) for the upper‐lower crust limit.

Upper crustal structure at the KTB drilling site from ambient noise tomography

Summary In this study, we show results from ambient noise tomography around the KTB (Kontinentales Tiefbohrprogramm der Bundesrepublik Deutschland), a continental deep drilling site located at the western edge of the Bohemian Massif, within the Variscan belt of Europe. At the KTB site, crustal rocks have been drilled down to 9 km depth. Before the drilling activity started, several active seismic surveys had been performed to explore its surroundings during the 1980s and early 1990s, in the frame of an extensive exploration of the area aimed at unravelling the characteristics of the continental lower crust that is exposed at surface in this location. Despite the exploration campaigns held at and around the KTB drilling site, there are important targets that are worth further investigation; these are related in particular to the obduction of lower crustal units to the surface, and to the mechanism of orogenic processes in general. Here we present a new 3-D shear wave velocity model of the area from cross-correlations of ambient seismic noise. The model is obtained by a unique data set composed of 2 yr of continuous data recorded at nine 3-component temporary stations (installed from July 2012 to July 2014) located on top and around the drilling site, and together with the data from 19 permanent stations throughout the region. This paper is focusing on the upper crustal layers, and we show velocity variations at short scales that correlate well with known geological structures in the region of the KTB site, at the surface and at depth. These are used to discuss features that are less well-resolved at present.

Reassessing the intrusive tempo and magma genesis of the late Variscan Aar batholith: U–Pb geochronology, trace element and initial Hf isotope composition of zircon

The Variscan orogeny was responsible for the formation of a significant volume of igneous basement throughout present-day Europe. Detailed understanding of these rocks has, however, been obfuscated by significant overprinting during younger geologic events. In order to better understand the formation of this basement, we present U–Pb dates, trace element concentrations and Hf isotope compositions of zircon from 17 intrusions of the Variscan Aar batholith, located in the Aar Massif, Central Alps, Switzerland. Laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) was used to generate a large set of U–Pb dates, trace element and Hf isotope compositions on untreated zircon, as well as zircon pretreated by chemical abrasion. Furthermore, a subset of samples was also analyzed for high-precision U–Pb geochronology using chemical abrasion, isotope dilution, thermal ionization mass spectrometry (CA-ID-TIMS). The U–Pb dates of both dating techniques are significantly dispersed, indicating that they are influenced by multiple forms of complexity, including inheritance, domains of secondary alteration likely related to Alpine overprint or growth, decay damage related Pb-loss, and potentially protracted magmatic growth. Decay-damage related Pb-loss is likely a subordinate source of age scatter within the data, therefore chemical abrasion pretreatment is not capable of completely mitigating the observed analytical scatter. After rejection of outliers, the remaining data still exhibit excess scatter of several percent among 206Pb/238U dates in individual samples, however it is possible to interpret reasonable geologic ages from these data. These new U–Pb zircon age interpretations indicate the Aar batholith grew incrementally through four major magmatic pulses, which occurred at approximately 348, 333, 309 and 298 Ma. Based on the trace element and Hf isotope geochemistry, the melt source(s) of the Aar batholith evolved throughout the duration of batholith formation and growth. The transitioning from (i) melting of depleted mantle at 348 Ma during a stage of active continental arc magmatism (εHf = + 12 to + 10), (ii) melting of metasomatically enriched lithospheric mantle, possibly contaminated by crust during the 333 Ma pulse (εHf = − 10 to − 3), followed by (iii) an increasing incorporation of a juvenile mantle components during the 309 and 298 Ma pulses (εHf = − 3 to + 6). Finally, these new U–Pb ages yield a more detailed understanding of the Variscan Aar batholith by integrating the new detailed mapping of Aar Massif for the Geological Atlas of Switzerland, allowing for more accurate characterization and categorization of variably deformed heterogeneous intrusive bodies.

Variscan lamprophyres of the South Armorican Domain and comparison with lamprophyres of the Western European Variscan belt

Variscan lamprophyres of the South Armorican Domain and comparison with lamprophyres of the Western European Variscan belt

Zircon Reveals Diverse Trends of Magma Crystallization from Two Types of Early Post-Collisional Diorites (Variscan Orogen, NE Bohemian Massif)

Abstract Amphibole- and clinopyroxene-bearing monzodiorites were emplaced at 340 Ma (CA-ID-TIMS zircon age), suggesting the formation of hydrous and dry magmas closely related in space and time in the NE Bohemian Massif. Hafnium and oxygen isotopes of zircon in less evolved rocks (&amp;lt;55 wt% SiO2) are similar between Amp and Cpx monzodiorites (εHf = −3.3 ± 0.5 and − 3.5 ± 0.8; δ18O = 6.4 ± 1.0 and 6.8 ± 0.7, respectively), consistent with a common source—a contaminated mafic magma derived from an enriched mantle. At the same time, the conditions of crystallization are distinct and zircon appears to be an excellent tool for distinguishing between hydrous and anhydrous crystallization conditions, a process that may be more ambiguously recorded by whole rock and major mineral chemistry. In particular, elements fractionated by either amphibole or plagioclase crystallization, such as Hf, Dy, and Eu, differ in zircon from amphibole- and clinopyroxene-bearing rocks, and Zr/Hf, Yb/Dy, and Eu/Dy are therefore useful indices of crystallization conditions. We show that the composition of zircon from hydrous dioritic magmas is not comparable with that of typical zircon from dioritic-granitic suites worldwide, suggesting a specific process involved in their formation. Here, we propose that fluid-present remelting of a mafic underplate is necessary to explain the rock textures as well as the composition of the whole rock, zircon, and other minerals of amphibole-bearing monzodiorites and that a similar process may control the formation of amphibole-rich dioritic rocks worldwide, including appinitic suites. Overall, we show that dioritic rocks represent snapshots of differentiation processes that occur in the early stages of magma evolution before the magma is homogenized into large-scale batholiths.

A field guide to the excursion in the Maures Massif (southern France): a complete transect of the Southern European Variscan belt

In this guide key outcrops of the Maures Massif, the southernmost part of the Variscan belt in France, are described. Here the features acquired during the fundamental steps of the evolution of the southern European Variscan belt can be recognised. The field trip develops along a W to E transect, perpendicular to the main architecture of the belt. It is organised in eight stops that can be easily reached by car or by walking. The excursion can be completed in one day, but to better enjoy these spectacular outcrops, two days are suggested. The stops show significative outcrops of the low-grade rocks of the External Zone up to the high-grade migmatite of the Internal Zone. Particular focus is given on the tectonic contact between the Internal and External zones of the belt marked by a km-scale transpressional shear zone, known as the Cavalaire Fault, whose structural, lithological, metamorphic and geochronological aspects are described in detail. This structure is a branch of a crustal-scale system of late Carboniferous shear zones known at the regional-scale as the East Variscan Shear Zone. The several branches were interconnected and all recorded a common evolution characterised by transpressional deformation starting from ~330 My.

Stratigraphic-Tectonic Evolution and Characterization of the Carboniferous in the Karamay–Baikouquan Fault Zone in the Northwestern Margin of the Junggar Basin, Northwest China

The northwestern margin of the Junggar Basin has a complex tectonic history, which is challenging for hydrocarbon exploration. This study highlights the structural patterns, tectonic evolution, and controlling factors of the Karamay–Baikouquan Fault Zone (KBFZ) in the northwestern margin of the Junggar Basin based on well-log and 3D seismic data. The main findings of this study include the following: spatially, we have divided the structural patterns of KBFZ into three zones on the plane, namely, northern, central, and southern zones; temporarily, the tectonic evolution has been divided into five stages from Late Carboniferous to Cretaceous, namely, Early-Middle Carboniferous, Late Carboniferous-Early Permian, Middle-Late Permian, Triassic-Middle Jurassic, and Late Jurassic-Cretaceous. During the Hercynian and Indochina orogeny, the compressional stress from the movement of the Dalabute strike-slip fault and the Genghis–Junggar strike-slip fault produced various degrees of deformation in different locations in the KBFZ. The general structural style is a mixture of strike-slip and thrust-nappe. The main controlling factors include stress period, stress direction, and stratigraphic-lithologic associations. This study has implications for hydrocarbon exploration and development in the northwestern margin of the Junggar Basin.

Geology of the contact area between the Internal and External Nappe Zone of the Sardinian Variscan Belt (Italy): new insights on the complex polyphase deformation occurring in the hinterland-foreland transition zone of collisional belts

ABSTRACT This study aims to decipher the structural evolution of a sector of the Nappe Zone in the Sardinian Variscan Belt. We investigated the Barbagia Thrust between the Meana Sardo Unit (belonging to the External Nappe Zone) in the footwall and the Barbagia Unit (belonging to the Internal Nappe Zone) in the hanging wall. Combining the geological survey with meso- and microstructural analysis, we realized a 1:10.000 scale geological map highlighting the polyphasic evolution developed under low-grade metamorphic conditions during the Variscan orogeny. Both units preserve (i) an early phase generally observed far from the tectonic contact and mainly in the Meana Sardo Unit (D1), (ii) a syn-nappe ductile deformation linked to the Barbagia Thrust activity and a top-to-the S-SW sense of shear (D2) and (iii) a large-scale nappe refolding (D3). A late extensional stage (D4), with the development of collapse folds, marks the end of the orogenic cycle.

Tectonics and geothermal gradients from subduction to collision in the NW Variscan Iberian Massif

ABSTRACT The earliest tectonometamorphic record of tectonic slices incorporated to the base of an orogen holds the key to understand how an orogen is built. The tectonic pile of the NW Iberian section of the Variscan Orogen includes tectonic slices separated by crustal-scale thrusts. The earliest tectonometamorphic record in the uppermost parautochthon is calculated at 11–14 kbar and 450–500°C (P-T gradient about 13°C/km), suggesting a subduction-related metamorphic recrystallization at lower pressure than the overlying Lower Allochthon. Early conditions calculated in the autochthon (9–10 kbar and 425–450°C; 16°C/km) point to a relatively ‘cold’ collisional setting. Higher thermal gradients obtained from some sections of the autochthon (11–12 kbar and 700–725°C; 21°C/km) and the Lower Parautochthon (7.5 kbar and 550–700°C; 24–31°C/km), correspond to more advanced and ‘hot’ stages of collision. New U–Pb monazite geochronology indicates a 318–311 Ma age for the final formation of HT domes in the region. We propose the rapid decrease in P-T gradient (from <10 to 16°C/km) documents a fail to sustain further burial along a regular subduction zone. We consider the subsequent increase in the geothermal gradient (from 16 to 31°C/km) as the culmination of previous crustal accretion and the onset of crustal underthrusting and later processes in a collisional stage. We propose these switches in the early tectonometamorphic record of individual tectonic slices as potential markers to track the transition from subduction to collision in collisional orogens.