Plutonism in the Variscan Odenwald (Germany): from subduction to collision

 Latest Devonian to early Carboniferous plutonic rocks from the Odenwald accretionary complex reflect the transition from a subduction to a collisional setting. For ∼362 Ma old gabbroic rocks from the northern tectonometamorphic unit I, initial isotopic compositions (ε_Nd=+3.4 to +3.8;⁸⁷Sr/⁸⁶Sr =0.7035–0.7053;δ¹⁸O=6.8–8.0‰) and chemical signatures (e.g., low Nb/Th, Nb/U, Ce/Pb, Th/U, Rb/Cs) indicate a subduction-related origin by partial melting of a shallow depleted mantle source metasomatized by water-rich, large ion lithophile element-loaded fluids. In the central (unit II) and southern (unit III) Odenwald, syncollisional mafic to felsic granitoids were emplaced in a transtensional setting at approximately 340–335 Ma B.P. Unit II comprises a mafic and a felsic suite that are genetically unrelated. Both suites are intermediate between the medium-K and high-K series and have similar initial Nd and Sr signatures (ε_Nd=0.0 to –2.5;⁸⁷Sr/⁸⁶Sr=0.7044–0.7056) but different oxygen isotopic compositions (δ¹⁸O=7.3–8.7‰ in mafic vs 9.3–9.5‰ in felsic rocks). These characteristics, in conjunction with the chemical signatures, suggest an enriched mantle source for the mafic magmas and a shallow metaluminous crustal source for the felsic magmas. Younger intrusives of unit II have higher Sr/Y, Zr/Y, and Tb/Yb ratios suggesting magma segregation at greater depths. Mafic high-K to shoshonitic intrusives of the southern unit III have initial isotopic compositions (ε_Nd=–1.1 to –1.8;⁸⁷Sr/⁸⁶Sr =0.7054–0.7062;δ¹⁸O=7.2–7.6‰) and chemical characteristics (e.g., high Sr/Y, Zr/Y, Tb/Yb) that are strongly indicative of a deep-seated enriched mantle source. Spatially associated felsic high-K to shoshonitic rocks of unit III may be derived by dehydration melting of garnet-rich metaluminous crustal source rocks or may represent hybrid magmas. Received: 7 December 1998 / Accepted: 27 April 1999     

Ore deposits of the French Massif Central: insight into the metallogenesis of the Variscan collision belt

The French Massif Central (FMC) represents the whole West European Variscan (WEV) belt, in terms of both the geodynamic evolution and the metallic content. Thus, a study of the metallogenic evolution of the FMC may elucidate the conditions that allow the mineralisation of a collision belt, since recent collision belts, e.g. the Himalayas or the Alps show that mineralisation does not necessarily result from the collision process. The Palaeozoic history of the FMC is divided into three geodynamic stages unevenly involved from the metallogenic view point. The Eo-Variscan stage (Cambrian to Silurian) was not important; the Meso-Variscan stage (Devonian-Early Carboniferous) was of limited importance; and most of the mineralisations formed during the Neo-Variscan stage (Late Carboniferous-Early Permian). In addition, some more mineralisation was produced during the Mesozoic because of the thermal reactivation linked with the Alpine orogenies. The Eo-Variscan stage (Cambrian-Silurian) corresponded to the pre-collision history, marked at the WEV belt scale by a fragmentation of the northern Gondwana (immature crust evolved from the Late Proterozoic Cadomian orogeny), up to the break-up of the crust and the formation of oceanic basins (Cambrian-Ordovician), followed by their resorption by subduction during the Silurian. In the FMC, no subduction-related magmatism is known (being rare at the WEV belt scale), and consequently subduction-related mineralisation, e.g. porphyry copper, is unknown in the WEV belt. Although some ophiolitic remnants are known, they never display Cyprus-type VMS deposits, nor massive podiform chromitites. Beside platformal sedimentary deposits on passive margins, the only deposits formed during the Eo-Variscan stage were of the SEDEX type, linked with the early rifting of the Gondwanian crust. The Meso-Variscan stage (Devonian-Early Carboniferous) corresponded to the collision proper, with the formation of crustal-scale nappe structures and the intrusion of collision-related peraluminous granites. Although these granites were enriched in rare metals they did not yield significant hydrothermal mineralisation, due to the great depth of their emplacement, as the similar granites in the Himalayas. However, they were a source of rare metals (in particular, uranium) for later mineralisation events. At the WEV belt scale Devonian distensive events are coeval with the collision. They were recorded by the formation of sedimentary basins of limited time and space extent, corresponding to the splitting of the continental crust (up to formation of oceanic domains in many cases), and were characterised by a bi-modal (“spilite-keratophyre”) volcanism. These basins formed in transtensional (or pull-apart) settings along major strike-slip faults, a peculiarity of the Variscan collision belt (which may conveniently be described as a “strike-slip orogen”). In such basins, many deposits linked with the volcanic thermal energy were formed: SEDEX deposits of the Meggen-type, iron deposits of the Lahn-Dill-type and VMS base metal deposits, the latter being the only ones known in the FMC (Brévenne deposits). The Neo-Variscan stage corresponded to the “hypercollision” and was characterised by a shift from compressional tectonics (late thickening of the crust during the Sudetian event and long-lasting dextral strike-slip tectonics along NW-SE to NE-SW fault zones) towards extensional tectonics (“basin and range” of the Late Stephanian-Early Permian), as well as by high heat flows, recorded by LP-HT metamorphism, extensive granitisation and granulitisation of the lower crust. These characteristics record the development of a lithospheric delamination process. In response to the energetic input released by this process, numerous hydrothermal deposits were formed in the FMC, as well as in the whole WEV belt, during the Neo-Variscan stage. These are mainly: (1) high-temperature granite-centered tungsten deposits, mainly associated with cordierite-bearing high level intrusions of Namurian-Westphalian age; (2) rare metal granites (and the associated hydrothermal tin mineralisations), resulting from fluid-induced low-degree partial melting of the middle crust in relation with the devolatilisation of the granulitised lower crust; (3) shear-zone hosted gold and antimony deposits, related to crustal-scale hydrothermal circulation, triggered by the transition to extensional tectonics at about 300 Ma; and (4) uranium deposition in extensional settings related to the Early Permian distension. The Post-Variscan mineralising events recorded the renewal of thermal flows in the lithosphere linked with early Alpine events (mainly the Trias-Lias distension in the Tethyan realm and the middle Cretaceous opening of the Bay of Biscay in the Pyrenean realm). They resulted in low-enthalpy geothermal systems, leading to a variety of deposits, mainly: (1) F-Ba districts, reworking F and Ba from Late Variscan granites and ignimbrites; (2) a major uranium deposit (Lodève), reworking uranium from the Permian Lodève basin; and (3) Zn-Pb districts of the MVT-type. Finally, the mineralisation of the Variscan collision belt is mainly the consequence of the Neo-Variscan lithospheric delamination process. By contrast, the absence of such a process in collision belts like the Himalayas or the Alps is the key of them being devoid of mineralisation. It appears that the mechanical energy released by the collision itself is not sufficient to mobilise and concentrate the trace elements involved in the metallogenic processes. Received: 1 September 1998 / Accepted: 3 February 1999     

Two-stage partial melting during the Variscan extensional tectonics (Montagne Noire,France)

International Journal of Earth Sciences - One of the striking features that characterise the late stages of the Variscan orogeny is the development of gneiss and migmatite domes, as well as...     

The East Variscan Shear Zone: new insights into its role in the Late Carboniferous collision in southern Europe

A comparison of the petro-tectonic features recorded in the Variscan Massifs scattered throughout the Alps, the Corsica-Sardinia-Maures-Tanneron Massif, the Calabria-Peloritani Arc, and the Northern Apennines, has allowed us to propose that they belonged to the same geodynamic realm until Late Carboniferous time. In the interval 330–300 Ma, the development of a regional dextral strike–slip shear zone, the East Variscan Shear Zone (EVSZ), affected all the massifs, leading to their spatial separation. The EVSZ developed, together with numerous regional shear zones, under a transpressional tectonic regime deriving from the Late Carboniferous collision between Gondwana, peri-Gondwana microcontinents (Armorica and Avalonia), and Laurussia plates. The EVSZ evidently played a key role in the evolution of the subsequent Alpine and Apenninic cycles, acting as a pre-existing tectonic barrier. Our proposed geodynamic reconstruction does not reflect the acquisition of new data, but is based on the analysis and review of the recent geological literature.     

The Vilalba gold district,a new discovery in the Variscan terranes of the NW of Spain: A geologic,fluid inclusion and stable isotope study

This work defines the Vilalba gold district, a new gold region in the NW of Spain comprising three mineralized areas (Castro de Rei, Valiña–Azúmara and Arcos) and two different types of mineralization: 1) W–Au skarn in Castro de Rei, and 2) Au–As (Ag–Pb–Zn–Cu–Sb) vein/disseminated mineralization in Valiña–Azúmara and Arcos. Mineralization is mainly hosted by impure limestone and black slates (lower Cambrian) and spatially related to Variscan and Late-Variscan structures. Rhyolite dikes occur in Arcos (299 ± 6 Ma) and Castro de Rei, both consisting of peraluminous, high-potassium and calc-alkaline rocks. These dikes represent the fractionated expression of unexposed post-tectonic granitoids located at depth. The Castro de Rei area exhibits a contact aureole with biotite ± cordierite and calc-silicate hornfels. Prograde skarn consists of garnet, pyroxene, quartz, wollastonite and scheelite. The retrograde alteration took place in two stages and mainly consists of amphibole, epidote, zoisite, quartz, calcite, chlorite and sulfides. Gold mineralization in the skarn occurs as invisible gold in sulfoarsenides and as electrum related to Bi–Te–S minerals. The mineralization of Valiña–Azúmara and Arcos comprises two stages. The first consists of As-rich pyrite and arsenopyrite with invisible gold. The second stage mainly consists of base metal sulfides and Ag–Pb–Zn–Cu–Sb sulfosalts.Hydrothermal fluids involved in the skarn formation are low salinity (up to 6.6 wt.% NaCl eq.) aqueous carbonic fluids (H₂O + CO₂ ± CH₄ ± N₂) evolving into aqueous fluids during the last stage of gold mineralization. Stable isotope geochemistry indicates the importance of a magmatic fluid in the early stages (δ¹⁸O_SMOW from 7.5 to 11‰; δ³⁴S_CDT from 2.7 to 5.9‰) that evolved through interaction with host-rocks and mixing with metamorphic and/or meteoric fluids. The δ³⁴S_CDT values (7.8–10.9‰) of second retrograde stage sulfides suggest the incorporation of ³⁴S-enriched from host-rocks. In Valiña–Azúmara and Arcos, the fluid inclusion and isotope studies indicate similar hydrothermal fluids to those associated with the second retrograde skarn stage. Taking into account the fluid inclusion study and mineral geothermometry, an attempt was made to determine the P–T conditions of ore formation. The prograde skarn of Castro de Rei formed at temperatures and pressures ranging from 520 to 560 °C and from 1.7 to 2.5 kb. The mineralization of Valiña–Azúmara formed at minimum P–T conditions of 1.7 kb and 355 °C. The estimated temperatures for the Arcos mineralization are similar to those of Valiña–Azúmara (up to 383 °C) but at lower pressures.We propose the model of intrusion related gold systems (IRGSs) to explain the mineralizations of the Vilalba gold district. In this model, the Castro de Rei skarn and the Valiña–Azúmara mineralization represent a proximal and a distal deposit, respectively, relative to an unexposed granitoid situated at depth. We also consider Arcos as a distal deposit in relation to another unexposed granitoid located further south in the district.     

Provenance analysis of the Paleozoic sequences of the northern Gondwana margin in NW Iberia: Passive margin to Variscan collision and orocline development

The Cantabrian Zone of NW Iberia preserves a voluminous, almost continuous, sedimentary sequence that ranges from Neoproterozoic to Early Permian in age. Its tectonic setting is controversial and recent hypotheses include (i) passive margin deposition along the northern margin of Gondwana or (ii) an active continental margin or (iii) a drifting ribbon continent. In this paper we present detrital zircon U–Pb laser ablation age data from 13 samples taken in detrital rocks from the Cantabrian Zone sequence ranging from Early Silurian to Early Permian in depositional age. The obtained results, together with previously published detrital zircon ages from Ediacaran–Ordovician strata, allow a comprehensive analysis of changing provenance through time. Collectively, these data indicate that this portion of Iberia was part of the passive margin of Gondwana at least from Ordovician to Late Devonian times. Zircon populations in all samples show strong similarities with the Sahara Craton and with zircons found in Libya, suggesting that NW Iberia occupied a paleoposition close to those regions of present-day northern Africa during this time interval. Changes in provenance in the Late Devonian are attributed to the onset of the collision between Gondwana and Laurussia.Additionally, the Middle Carboniferous to Permian samples record populations consistent with the recycling of older sedimentary sequences and exhumation of the igneous rocks formed before and during the Variscan orogeny. Late-Devonian to Permian samples yield zircon populations that reflect topographic changes produced during the Variscan orogeny and development of the lithospheric scale oroclinal buckling.     

The Palaeozoic evolution in the Alps: from Gondwana to Pangea

More than 50% of the Alps expose fragments of Palaeozoic basement which were assembled during the Alpine orogeny. Although the tectonic and metamorphic history of the basement units can be compared to that of the Variscan crust in the Alpine foreland, most of the basement pieces of the Alps do not represent the direct southern continuation of Variscan structural elements evident in the Massif Central, the Vosges–Black Forest or the Bohemian massif. The basement units of the Alps all originated at the Gondwana margin. They were derived from a Precambrian volcanic arc suture fringing the northern margin of Gondwana, from which they were rifted during the Cambrian–Ordovician and Silurian. A short-lived Ordovician orogenic event interrupted the general rifting tendency at the Gondwana active margin. After the Ordovician, the different blocks drifted from the Gondwana margin to their Pangea position, colliding either parallel to Armorica with Laurussia or with originally peri-Gondwanan blocks assembled presently in Armorica. From the Devonian onwards, many basement subunits underwent the complex evolution of apparently oblique collision and nappe stacking. Docking started in the External massifs, the Penninic and Lower and middle Austroalpine units in approximately Devonian/early Carboniferous times, followed by the Upper Austroalpine and the South Alpine domains, in the Visean and the Namurian times, respectively. Wrenching is probably the best mechanism to explain all syn and post-collisional phenomena since the Visean followed by post-orogenic collapse and extension. It explains the occurrence of strike-slip faults at different crustal levels, the formation of sedimentary troughs as well as the extrusion and intrusion of crustal and mantle-derived magmas, and allows for contemporaneous rapid uplift of lower crustal levels and their erosion. From the Stephanian onwards, all regions were deeply eroded by large river systems.     

Geochronological and petrological constraints from the evolution in the Saxon Granulite Massif,Germany, on the Variscan continental collision orogeny

Controversy over the plate tectonic affinity and evolution of the Saxon granulites in a two‐ or multi‐plate setting during inter‐ or intracontinental collision makes the Saxon Granulite Massif a key area for the understanding of the Palaeozoic Variscan orogeny. The massif is a large dome structure in which tectonic slivers of metapelite and metaophiolite units occur along a shear zone separating a diapir‐like body of high‐P granulite below from low‐P metasedimentary rocks above. Each of the upper structural units records a different metamorphic evolution until its assembly with the exhuming granulite body. New age and petrologic data suggest that the metaophiolites developed from early Cambrian protoliths during high‐P amphibolite facies metamorphism in the mid‐ to late‐Devonian and thermal overprinting by the exhuming hot granulite body in the early Carboniferous. A correlation of new Ar–Ar biotite ages with published P–T–t data for the granulites implies that exhumation and cooling of the granulite body occurred at average rates of ~8 mm/year and ~80°C/Ma, with a drop in exhumation rate from ~20 to ~2.5 mm/year and a slight rise in cooling rate between early and late stages of exhumation. A time lag of c. 2 Ma between cooling through the closure temperatures for argon diffusion in hornblende and biotite indicates a cooling rate of 90°C/Ma when all units had assembled into the massif. A two‐plate model of the Variscan orogeny in which the above evolution is related to a short‐lived intra‐Gondwana subduction zone conflicts with the oceanic affinity of the metaophiolites and the timescale of c. 50 Ma for the metamorphism. Alternative models focusing on the internal Variscan belt assume distinctly different material paths through the lower or upper crust for strikingly similar granulite massifs. An earlier proposed model of bilateral subduction below the internal Variscan belt may solve this problem.     

The Variscan evolution in the External massifs of the Alps and place in their Variscan framework

In the general discussion on the Variscan evolution of central Europe the pre-Mesozoic basement of the Alps is, in many cases, only included with hesitation. Relatively well-preserved from Alpine metamorphism, the Alpine External massifs can serve as an excellent example of evolution of the Variscan basement, including the earliest Gondwana-derived microcontinents with Cadomian relics. Testifying to the evolution at the Gondwana margin, at least since the Cambrian, such pieces took part in the birth of the Rheic Ocean. After the separation of Avalonia, the remaining Gondwana border was continuously transformed through crustal extension with contemporaneous separation of continental blocks composing future Pangea, but the opening of Palaeotethys had only a reduced significance since the Devonian. The Variscan evolution in the External domain is characterised by an early HP-evolution with subsequent granulitic decompression melts. During Visean crustal shortening, the areas of future formation of migmatites and intrusion of monzodioritic magmas in a general strike–slip regime, were probably in a lower plate situation, whereas the so called monometamorphic areas may have been in an upper plate position of the nappe pile. During the Latest Carboniferous, the emplacement of the youngest granites was associated with the strike–slip faulting and crustal extension at lower crustal levels, whereas, at the surface, detrital sediments accumulated in intramontaneous transtensional basins on a strongly eroded surface.     

Variscan orogeny in Corsica: new structural and geochronological insights,and its place in the Variscan geodynamic framework

In Western Corsica, remnants of pre-batholitic lithological and metamorphic assemblages are preserved as km-scale septa enclosed within Lower Carboniferous to Early Permian plutons. Two groups of septa were recognized: (1) the Argentella and Agriates-Tenda fragments correspond to Neoproterozoic rocks deformed and metamorphosed during the Cadomian–Panafrican orogeny, and (2) the Zicavo, Porto-Vecchio, Solenzara–Fautea, Belgodère, Topiti, and Vignola fragments consist of Variscan metamorphic rocks. The lithological content and the main ductile deformation events for each septum are presented. In the Zicavo, Porto-Vecchio, and Topiti septa, a top-to-the-SW ductile shearing (D1 event) coeval with an amphibolite facies metamorphism is responsible for crustal thickening at ca 360 Ma. This main event was preceded by eclogite and granulite facies metamorphic events preserved as restites within migmatites dated at ca 345–330 Ma. A top-to-the-SE ductile shearing (D2 event) coeval with the crustal melting accommodated the exhumation of the D1 event. In contrast, the Belgodère segment is peculiar as it exhibits a top-to-the-E vergence, although retrogressed high-pressure rocks are also recognized. The pre-Permian fragments are arranged in four NW–SE-striking stripes that define a SW–NE zoning with (1) a Western domain in Topiti, Vignola, Zicavo, Porto-Vecchio, and Solenzara–Fautea; (2) a Neoproterozoic basement with its unconformable Early Paleozoic sedimentary cover in Argentella; (3) an Eastern metamorphic domain in Belgodère; (4) another Neoproterozoic basement with its Upper Paleozoic sedimentary cover in Agriates-Tenda. The Argentella basement is separated from the Western and Eastern domains by two sutures: S1 and S2. The Variscan Corsica represents the Eastern part of the Sardinia–Corsica–Maures segment. The comparison of this segment with other Variscan domains allows us to propose some possible correlations. We argue that the Western domain, Argentella, Belgodère, and Agriates-Tenda domains can be compared with the Southern Variscan belt exposed in French Massif Central–Southern Massif Armoricain, Armorica microblock, Léon block, respectively.     

Two-stage collision-related extrusion of the western Dabie HP–UHP metamorphic terranes, central China: Evidence from quartz c-axis fabrics and structures

The western Dabie orogen (also known as the Hong'an block) forms the western part of the Dabie–Sulu HP–UHP belt, central China. Rocks of this orogen have been subjected to pervasive ductile deformation, and include numerous quartz schists and felsic mylonites cropping out in ductile shear zones. Quartz textures in these mylonites contain important clues for understanding the movement sense of late-collisional extrusion and exhumation of high-pressure–ultrahigh-pressure (HP–UHP) rocks from the lower crustal level to the upper crustal level during Middle Triassic and Early Jurassic. The orientation and distribution of quartz crystallographic axes were used to confirm the regional shear sense across the orogen. The asymmetry of c-axis patterns consistently indicates top-to-the-southeast thrusting across the orogen in early structural stages. Later stages of deformation show different senses of movement in northern and southern parts of the orogen, with top-to-the-northwest sinistral shearing recorded in rocks north of the Xinxian HP–UHP eclogite-facies belt, and top-to-the-southeast dextral shearing south of the same unit.Based on our study on quartz c-axis fabrics and marco- to micro-scale structures, simultaneous southeastward shearing within a large part of the orogen and normal faulting north of the Xinxian HP–UHP unit is explained by upward extrusion in early stages of deformation. The extrusion process has been attributed to syn- and late-collisional processes, accounting for some earlier deformation in the western Dabie orogen such as metamorphic sequences around the core of the Xinxian HP–UHP eclogite-facies unit. Much higher pressure of deformation is also indicated in the aligned glaucophane and omphacite from blueschist and eclogite in the field. An orogen-parallel eastward extrusion of the Xinxian HP–UHP eclogite-facies unit, however, occurred diachronously in later stages of deformation. Therefore, a tectonic model combining an early upward extrusion with a later eastward extrusion is presented. Two different stages and types of extrusion for exhumation of HP–UHP rocks are suitable to all of east central China. Geochronological data shows that the first, upward extrusion occurred during Middle Triassic, the second, eastward extrusion occurred during Late Triassic to Early Jurassic. These two extrusions are correlative with two stages of rapid exhumation of the Dabie HP–UHP rocks, respectively. These two-stage late-collisional (Middle Triassic to Early Jurassic) extrusion events bridge the gap between syn-collisional (Early to Middle Triassic) vertical extrusion and post-collisional (Cretaceous) eastward-directed lateral escape and provide vital clues to understanding the more detailed processes of exhumation of HP–UHP rocks.     

Triassic breakup of Pangea in southern Mexico: Thermochronological evidence from the Tianguistengo formation

Southern Mexico is a key area for unraveling the tectonic evolution of North America because it contains the stratigraphic and structural record of the major tectonic events that shaped this continental mass, such as the breakup of Pangea and the growth of the North America Cordilleran Orogeny. However, multiple reactivations of faults and erosion of the stratigraphic record do not permit to adequately assess the timing of these tectonic events. Although most authors suggested that lithospheric extension and exhumation of continental blocks during Pangea breakup started in Mexico by Early Jurassic time, works published in the last decade provide an increasing number of thermo-tectonic evidence of an earlier phase of continental thinning. In this work, we present detrital apatite thermochronological and geochemical data (trace elements including rare earth elements) from fluvial deposits of the Tianguistengo formation, which is the oldest stratigraphic unit of the Otlaltepec Basin, a major basin in southern Mexico that has been linked to Pangea breakup. Our data show that at least a part of the Tianguistengo formation was derived from the adjacent Pennsylvanian–Cisuralian Totoltepec pluton. Apatite fission-track-based time–temperature modelling for unreset apatite populations suggests that the main exhumation of the Totoltepec pluton, which prompted the deposition of a part of the Tianguistengo formation, took place during Late Triassic time. Thus, our results suggest that Pangea breakup in southern Mexico started at least by Middle–Late Triassic time (240–230 Ma), as it is recorded in the Otlaltepec Basin.     

The Andes as a peripheral orogen of the breaking-up Pangea

Formation conditions of the peripheral orogen are expressed most fully in the Central Andes, a mountain system almost not yielding in height to the Himalayan-Tibetan system but formed at the margin of ocean without any relations to intercontinental collision. The marine transgression and rejuvenation of subduction in the Early Jurassic during the origination of foldbelt at the margin of Pangea marked the transition to a new supercontinental cycle, and the overall further evolution began and continues now in the frame of the first half of this cycle. The marginal position of this belt above the subduction zone, the rate and orientation of convergence of the lithospheric plates, the age of “absolute” movement of the continental plate, variation in slab velocity, and subduction of heterogeneities of the oceanic crust were the crucial factors that controlled the evolution of the marginal foldbelt. At the stage of initial subsidence (Jurassic-Mid-Cretaceous), during extension of the crust having a moderate thickness (30–35 km), the Andean continental margin comprises the full structural elements of an ensialic island arc that resembled the present-day Sunda system. These conditions changed with the separation and onset of the western drift of the South American continent. Being anchored in the mantle and relatively young, the slab of the Andean subduction zone served as a stop that brought about compression that controlled the subsequent evolution. Due to the contribution of deep magma sources along with marine sediments and products of tectonic erosion removed to a depth, the growth of crust above the subduction zone was favorable for heating of the crust. By the middle Eocene, when compression enhanced owing to the acceleration of subduction, the thermal evolution of the crust had already prepared the transition to the orogenic stage of evolution, i.e., to the progressive viscoplastic shortening and swelling of the mechanically weakened lower crust and the concomitant reverse faulting and thrusting of the upper crust. The general compression of the Central Andes by more than 250 km relative to the stop created by the oceanic slab accommodated up to 40% of the western continental drift over this time and increased the thickness of the crust up to 65–75 km. It is suggested that the onset of fast uplift and growth of the mountain edifice 12–10 Ma ago was predetermined by the approach of the submarine Juan Fernandez Ridge in the south and the Nazca Ridge in the north toward the continental margin and their involvement in subduction. As a result, the Central Andes were transformed into a region of advanced shortening and vigorous orogeny.     

Contrasting modes of supercontinent formation and the conundrum of Pangea

Repeated cycles of supercontinent amalgamation and dispersal have occurred since the Late Archean and have had a profound influence on the evolution of the Earth's crust, atmosphere, hydrosphere, and life. When a supercontinent breaks up, two geodynamically distinct tracts of oceanic lithosphere exist: relatively young interior ocean floor that develops between the dispersing continents, and relatively old exterior ocean floor, which surrounded the supercontinent before breakup. The geologic and Sm/Nd isotopic record suggests that supercontinents may form by two end-member mechanisms: introversion (e.g. Pangea), in which interior ocean floor is preferentially subducted, and extroversion (e.g. Pannotia), in which exterior ocean floor is preferentially subducted.The mechanisms responsible remain elusive. Top–down geodynamic models predict that supercontinents form by extroversion, explaining the formation of Pannotia in the latest Neoproterozoic, but not the formation of Pangea. Preliminary analysis indicates that the onset of subduction in the interior (Rheic) ocean in the early Paleozoic, which culminated in the amalgamation of Pangea, was coeval with a major change in the tectonic regime in the exterior (paleo-Pacific) ocean, suggesting a geodynamic linkage between these events. Sea level fall from the Late Ordovician to the Carboniferous suggests that the average elevation of the oceanic crust decreased in this time interval, implying that the average age of the oceanic lithosphere increased as the Rheic Ocean was contracting, and that subduction of relatively new Rheic Ocean lithosphere was favoured over the subduction of relatively old, paleo-Pacific lithosphere. A coeval increase in the rate of sea floor spreading is suggested by the relatively low initial ⁸⁷Sr/⁸⁶Sr in late Paleozoic ocean waters. We speculate that superplumes, perhaps driven by slab avalanche events, can occasionally overwhelm top–down geodynamics, imposing a geoid high over a pre-existing geoid low and causing the dispersing continents to reverse their directions to produce an introverted supercontinent.     

Variscan plate dynamics in the circum-carpathian area

Abstract

Magmatic and metamorphic events, imprinted in the crystalline rocks of the so-called core mountains inside the Alpine structure of the Inner Carpathians, allow the re-construction of the history of the Rheic Ocean opening, its development and its final closure. Intra-Carpathian core-mountains are the remnants of the continents that drifted away from Gondwana and docked, initially, with Baltica as part of Avalonia and later on as parts of the Gondwana-derived Armorica Terrane Group or as a separate micro-continent.

All magmatic suites, mafic and felsic, present in the Carpathians core mountains, show similarities to those found in the European Variscan Belt. All described- and dated metamorphic and magmatic events also have equivalents in the evolution of the Caledonian-Variscan Belts of Europe. The most pronounced feature of all Carpathian core mountains is the syn-collisional, multistage I/S granitoid magmatism (370-340 Ma) related to subduction, mafic-magma influx, extensional decompression and slab melting. That episode marked the Laurussia - Gondwana collision and closure of the Rheic Ocean, as in the whole of Central and Western Europe.

The Carpathian core-mountains, currently dispersed inside the Alpine mountain chain, can be considered the broken fragments of the eastern prolongation of the Variscan orogenic belts – possibly part of the Moldanubian Unit.     

Variscan orogeny in the Black Sea region

International Journal of Earth Sciences - Two Gondwana-derived Paleozoic belts rim the Archean/Paleoproterozoic nucleus of the East European Platform in the Black Sea region. In the north is a belt...     

Two-stage prograde and retrograde Variscan metamorphism of glaucophane-eclogites, blueschists and greenschists from Ile de Groix (Brittany, France)

On Ile de Groix, Variscan metamorphic former tholeiitic and alkaline basalts occur as glaucophane-eclogites, blueschists and greenschists in isolated lenses and layers within metapelites. Whole-rock '¹⁸O_SMOW values of the metabasites show limited variations (10.4-12.0‰) and no systematic differences among rock types and metamorphic grades. This provides no argument for large-scale blueschist-to-greenschist transformation driven by infiltration of externally derived fluids. Metamorphic mineralogical changes should have been triggered by internal fluids. Element variations in interlayered blue- and greenschists can be attributed to magmatic fractionation. Assemblages with garnet, clinopyroxene and glaucophane of a high-pressure/low-temperature (HP-LT) metamorphism M1, and NaCa-amphiboles (barroisite, magnesiohornblende, actinolite) of a medium-pressure/medium-temperature metamorphism M2 crystallized during deformation D1. Detailed core-rim zonation profiles display increasing and then decreasing Al^IV in glaucophane of M1. NaCa-amphiboles of M2, mantling glaucophane and crystallized in porphyroblasts, show first increasing, then decreasing, Al^IV and Al^VI. Empirically calibrated thermobarometers allowed P-T path reconstructions. In glaucophane-eclogites of a metamorphic zone I, a prograde evolution to M1 peak conditions at 400-500°C/10-12 kbar was followed by a retrograde P-T path within the glaucophane stability field. The subsequent M2 evolution was again prograde up to >600°C at 8 kbar and then retrograde. Similarly, in metamorphic zones II and III, prograde and retrograde paths of M1 and M2 at lower maximal temperatures and pressures exist. The almost complete metamorphic cycle during M2 signalizes that the HP-LT rocks escaped from an early erosion by a moderate second burial event and explains the long-lasting slow uplift with low average cooling rates.     

The Archaean terranes of central eastern Brazil: a review

This paper compiles and describes several lithostratigraphic sequences which have characteristics typical of granite-greenstone and granite-gneiss terranes. The Archaean high-grade metamorphic assemblages are also described and are considered to form part of the mobile belt areas. Three major Archaean domains appear to be present in central eastern Brazil, i.e., the São Francisco Craton and the Jequié and Goiás Mobile Belts. In addition, several other small, but no less important, ancient nuclei occur within the Tocantins Province, especially in central Goiás.The São Francisco Craton is mostly covered by thick platform sequences of Proterozoic age, and as a result it can be examined only around the margins where granite-greenstone terranes are partially exposed. There appears to be sufficient evidence to recognise a stable Archaean basement domain, except in the northeast where the craton has been largely affected by the Transamazonian cycle (2.2-1.8 Ga) and, therefore, may represent a separate province. The available data for the Jequié and Goiás belts suggest that these domains have been subjected to high-grade regional metamorphism, chiefly granulite facies, during the Jequié tectono-thermal event of ～2.7 Ga, equivalent to the Liberia-Limpopo events in Africa. The differentiated mafic to ultramafic complexes of central Goiás, of uncertain age, may also be related to the evolution of the Goiás Mobile Belt.In general, the Archaean terranes described here appear to display a rather fragmentary pattern with poorly defined boundaries due to substantial reworking, which makes them partially different from better preserved, major ancient domains in other continents.     

Tectonics of the Beja-Acebuches Ophiolite: a major suture in the Iberian Variscan Foldbelt

The Beja-Acebuches Ophiolitic Complex (BAOC) (south Portugal/Spain) corresponds to a high grade metamorphic belt along the boundary between Ossa-Morena and the South Portuguese Zones and comprises a lithostratigraphic sequence including (from top to bottom) metabasalts, (metamorphosed) multiple dyke intrusions in gabbro, flasergabbros and metaserpentinites. It is affected by three deformation phases. D₁ affects the ophiolite lower stratigraphic units and is represented by a mylonitic cleavage with a stretching lineation where shear criteria indicates the sense of shear to be towards the north-north-east; this deformation event can be related to the ophiolite emplacement above the crystalline footwall of the Serpa antiform, affecting Precambrian basement and Cambrian cover rocks. The obduction polarity ist north-eastwards, similar to the subduction polarity that generates the Beja Gabbroic Complex (BGC), implying a flake geometry. The second deformation phase, D₂, is represented by an intense WNW-ESE sinistral shear event which is responsible for the shattered appearance of the suture; D₂ is reactivated later by a more brittle D₃ event involving thrusting to the south-west, again with a sinistral component. ⁴⁰Ar/³⁹Ar isotopic ages were obtained for (metamorphosed) multiple dyke intrusions in the BAOC's gabbro (342.6 ± 1.4 Ma), for metagabbroic cumulates (340.7 ± 1.9 Ma), and for the undeformed/unmetamorphosed BGC (341.1 ± 1.3 Ma) occurring to the north of the ophiolitic suture. These ages reflect a last regional cooling event in the area which post-dates the ophiolite emplacement and the intrusion of the BGC through this oceanic sequence. Correspondence to: P. Fonseca