The Central Iberian arc, an orocline centered in the Iberian Massif and some implications for the Variscan belt

An arcuate structure, comparable in size with the Ibero-Armorican arc, is delineated by Variscan folds and magnetic anomalies in the Central Iberian Zone of the Iberian Massif. Called the Central Iberian arc, its sense of curvature is opposite to that of the Ibero-Armorican arc, and its core is occupied by the Galicia-Trás-os-Montes Zone of NW Iberia, which includes the Rheic suture. Other zones of the Iberian Massif are bent by the arc, but the Ossa-Morena and South Portuguese zones are not involved. The arc formed during the Late Carboniferous, at final stages of thermal relaxation and collapse, and an origin related with right-lateral ductile transpression at the scale of the Variscan belt is proposed. The Central Iberian arc explains the width of the Central Iberian Zone, clarifies the position of the allochthonous terranes of NW Iberia, and opens new perspectives for correlations with the rest of the Variscan belt, in particular, with the Armorican Massif, whose central zone represents the continuation of the southwest branch of the arc detached by strike-slip tectonics.     

Early Variscan HP metamorphism in the western Iberian Allochthon—A 390 Ma U–Pb age for the Bragança eclogite (NW Portugal)

HP rocks (eclogites and granulites) occur in the upper unit of the western Iberian allochthonous complexes in Spain and Portugal. This HP metamorphism was considered as Early Variscan or Ordovician in Spain and Precambrian in Portugal. We have dated eclogites retrogressed into granulites in the Bragança massif in NE Portugal by U–Pb on rutile and zircon. The upper intercept of the discordia gives 390±4 Ma that we consider as the age of the HP/HT metamorphism.     

The metahyaloclastitic matrix of a unique metavolcanic block reveals subduction in the Somozas Mélange (Cabo Ortegal Complex,NW Iberia): tectonic implications for the assembly of Pangea

The allochthonous Cabo Ortegal Complex (NW Iberian Massif) contains a ~500 m thick serpentinite‐matrix mélange located in the lowest structural position, the Somozas Mélange. The mélange occurs at the leading edge of a thick nappe pile constituted by a variety of terranes transported to the East (present‐day coordinates; NW Iberian allochthonous complexes), with continental and oceanic affinities, and represents a Variscan suture. Among other types of metaigneous (calcalkaline suite dated at 527–499 Ma) and metasedimentary blocks, it contains close‐packed pillow‐lavas and broken pillow‐breccias with a metahyaloclastitic matrix formed by muscovite–paragonite–margarite–garnet–chlorite–kyanite–hematite–epidote–quartz–rutile. Pseudosection modelling in the MnCNTKFMASHO system indicates metamorphic peak conditions of ~17.5–18 kbar and ~550 °C followed by near‐isothermal decompression. This P–T evolution indicates subduction/accretion of an arc‐derived section of peri‐Gondwanan transitional crust. Subduction below the Variscan orogenic wedge evolved to continental collision with important dextral component. Closure of the remaining oceanic peri‐Gondwanan domain and associated release of fluid led to hydration of the overlying mantle wedge and the formation of a low‐viscosity subduction channel, where return flow formed the mélange. The submarine metavolcanic rocks were deformed and detached from the subducting transitional crust and eventually incorporated into the subduction channel, where they experienced fast exhumation. Due to the cryptic nature of the high‐P metamorphism preserved in its tectonic blocks, the significance of the Somozas Mélange had remained elusive, but it is made clear here for the first time as an important tectonic boundary within the Variscan Orogen formed during the late stages of the continental convergence leading to the assembly of Pangea.     

Reconstruction of the prograde PT history of high-P migmatitic paragneisses via melt-reintegration approach and thermodynamic modelling (Allochthonous Complexes,NW Iberian Massif)

The Upper Units of the allochthonous complexes of the NW Iberian Massif constitute a terrane with continental affinity. They represent the vestiges of a Cambrian magmatic arc developed in the periphery of Gondwana (West African Craton) which was involved in the Devonian Variscan collision, undergoing high-P, high-T metamorphism. This includes ultramafic rocks, high-P mafic rocks (eclogites and granulites) and high-P migmatitic paragneisses. The latter rocks show an extensive migmatization with the leucosomes oriented parallel to the regional foliation. The migmatitic paragneisses are composed of garnet, kyanite, biotite, quartz, plagioclase, K-feldspar, rutile and Ti-hematite. Thermodynamic modelling using the measured bulk composition in the NCKFMASTHO system indicates metamorphic peak conditions of ~15 kbar and ~800 to 835°C, followed by a significant cooling. The prograde evolution is assessed by means of a melt-reintegration approach, using the composition of the garnet and its inclusions. An appropriate composition of liquid is added to the measured bulk composition to emulate the pre-melting bulk composition. Modelling of this melt-reintegrated composition allows to identify a colder high-P episode below ~500°C. Zircon crystals extracted from the leucosomes show overgrowths crystallized from the partial melt at c. 389 Ma (U–Pb system). The P–T–t path proposed reveals a subduction of the peri-Gondwanan arc-derived section down to mantle depths. An isobaric heating stage occurred as a result of residence at great depths and/or inception of a transient oceanic basin at c. 395 Ma. The ensuing near-isothermal exhumation occurred due to the extension related to the inception of the basin, reaching the thermal peak shortly before c. 389 Ma. Subsequent cooling is related to the underthrusting of colder oceanic and transitional crust below the HP-HT Upper Units.     

Provenance of the HP–HT subducted margin in the Variscan belt (Cabo Ortegal Complex,NW Iberian Massif)

The Variscan Upper Allochthon is a continental‐affinity terrane that recorded a Cambrian–Ediacaran magmatic arc generation, a subsequent transition to a passive margin, and a collision‐related high‐P metamorphism during the Devonian–Carboniferous amalgamation of Pangea. The objective of this article is to decipher which continental margin subducted in the Devonian high‐P–high‐T (HP–HT) event. To do so, a provenance study is presented using combined U–Pb (n = 613) and Lu–Hf (n = 463) isotopic LA–ICP–MS zircon analyses and Sm–Nd whole–rock (n = 5) determinations. These analyses have been performed on five samples of the Banded Gneisses (Cabo Ortegal Complex, NW Iberia), which forms a part of the HP–HT bottom member of the Upper Allochthon. Palaeozoic–Neoproterozoic zircon ages (34.7%) have a maximum abundance at 522–512 Ma, peaks at 575, 561, 545 Ma and minor abundance peaks between 780 and 590 Ma, and show from their Lu–Hf compositions a volcanic arc mixing pattern. This arc was probably related to the Cadomian arc system. The Mesoproterozoic population is scarce and scattered (2.8%), and due to its Lu–Hf pattern, it is proposed that this population is also West Africa Craton derived. The Paleoproterozoic population (39.6%) is concentrated at 2.07 Ga and it is linked to the Eburnean Orogeny, where depleted mantle derived magmas intruded an Archean craton margin. This craton is represented by the Archean population (22.8%), which is grouped at 3.0, 2.68‐2.61 and 2.52‐2.48 Ga, and shows long‐term reworking processes and at least two juvenile magma intrusions. These data show that the Variscan Upper Allochthon has a West African provenance and therefore, it strongly suggests that the NW Iberian allochthonous complexes and their correlative European terranes are also West Africa derived. These results allow us to finally clarify that the first high‐P event, recorded during the eo‐Variscan amalgamation of Pangea, was attained by the subduction of the margin of Gondwana under Laurussia.     

Re-interpreting the Devonian ophiolites involved in the Variscan suture: U–Pb and Lu–Hf zircon data of the Moeche Ophiolite (Cabo Ortegal Complex,NW Iberia)

New U–Pb zircon data of a mylonitic greenschist from the Moeche Ophiolite, one of the mafic units involved in the Variscan suture in the Cabo Ortegal Complex (NW of the Iberian Massif), yielded an age of 400 ± 3 Ma. Consequently, this unit can be considered one of the Devonian ophiolites, the most extended group of oceanic units in the Variscan belt. The mafic rocks show transitional compositions between N-MORB and island-arc tholeiites, although Lu–Hf isotope signatures of its zircons clearly indicate contribution from an old continental source. εHf values in the analysed zircons are negative (generally below εHf = ?5), and thence, they are not compatible with their generation from a juvenile mantle source. Accordingly, the igneous protoliths were generated in a setting where juvenile mafic magmas interacted with an old continental crust. The Devonian ophiolites from the Variscan suture have been repeatedly interpreted as remnants of the Rheic Ocean. However, the presence of a continental source in the origin of the mafic rocks of the Moeche Ophiolite allows discarding an intraoceanic setting for their generation, at least for the NW Iberian counterparts. The tectonic setting for the Devonian ophiolites of NW Iberia is very likely represented by an ephemeral oceanic basin opened within a continental realm. Herein, the real Rheic Ocean suture could only be located west of the terrane represented by the upper units of the allochthonous complexes. Apparently that suture is not represented in NW Iberia.     

Constraints on the provenance of the uppermost allochthonous terrane of the NW Iberian Massif: inferences from detrital zircon U–Pb ages

ABSTRACT Insight into the origin and pre-orogenic palaeogeographical links of terranes involved in the assembly of collisional mountain belts is fundamental to the understanding of orogenic processes. Here we address the provenance and possible tectonic settings of the uppermost allochthonous terrane of the NW Iberian Variscan Belt through a 213-nm Laser Ablation ICP-MS study of U–Pb ages of detrital zircons. The age groups of zircons from greywackes in this terrane ( c . 480–610, 1900–2100, 2400–2500 Ma) and the lack of Mesoproterozoic zircons suggest an origin in a Neoproterozoic – Early Palaeozoic peri-Gondwanan realm along the periphery of the west African craton. It is further inferred that the greywackes were deposited in the periphery of a crustal unit that had been detached from the Gondwanan margin in relation to the opening of the Rheic ocean in Cambro-Ordovician times. This terrane was thrusted back upon the Gondwanan margin during the course of the Variscan collision and closure of the intervening ocean.     

Reconstruction of the mid-Devonian HP-HT metamorphic event in the Bohemian Massif (European Variscan belt)

Mid-Devonian high-pressure (HP) and high-temperature (HT) metamorphism represents an enigmatic early phase in the evolution of the Variscan Orogeny. Within the Bohemian Massif this metamorphism is recorded mostly in allochthonous complexes with uncertain relationship to the major tectonic units. In this regard, the Mariánské Lázně Complex (MLC) is unique in its position at the base of its original upper plate (Teplá-Barrandian Zone). The MLC is composed of diverse, but predominantly mafic, magmatic-metamorphic rocks with late Ediacaran to mid-Devonian protolith ages. Mid-Devonian HP eclogite-facies metamorphism was swiftly followed by a HT granulite-facies overprint contemporaneous with the emplacement of magmatic rocks with apparent supra-subduction affinity. New Hf in zircon isotopic measurements combined with a review of whole-rock isotopic and geochemical data reveals that the magmatic protoliths of the MLC, as well as in the upper plate Teplá-Barrandian Zone, developed above a relatively unaltered Neoproterozoic lithospheric mantle. They remained coupled with this lithospheric mantle throughout a geological timeframe that encompasses separate Ediacaran and Cambrian age arc magmatism, protracted early Paleozoic rifting, and the earliest phases of the Variscan Orogeny. These results are presented in the context of reconstructing the original architecture of the Variscan terranes up to and including the mid-Devonian HP-HT event.     

Mineralogy and pressure–temperature–time path of Cretaceous granulite gneisses, south-eastern San Gabriel Mountains, southern California

Mid-Cretaceous granulite gneisses crop out in a narrow belt in the Cucamonga region of the south-eastern foothills of the San Gabriel Mountains, southern California. Interlayered mafic granulites and pelitic, carbonate, calc-silicate and quartzofeldspathic metasediments record hornblende granulite subfacies metamorphism at approximately 8 kbar and 700–800°C. Regional deformation and formation of banded gneisses ceased by c. 108 Ma. although mafic-intermediate magmatism and high-grade metamorphism continued locally as late as c. 88 Ma. Garnet zoning in metapelitic gneisses suggests that peak metamorphism was followed locally by a period of near-isobaric cooling, but this interpretation requires diachronous cooling of the granulite belt which cannot be demonstrated without detailed thermo-chronological data. It is more likely that the entire terrane remained at granulite facies P–T conditions until 88 Ma, followed by rapid uplift associated with juxtaposition against adjacent middle and upper crustal arc terranes. Uplift occurred between c. 88 and 78 Ma at rates of approximately 1–2 km Ma^-1. The geotectonic evolution of the Cucamonga granulites is similar to mid-Cretaceous high- P granulites in the Sierra Nevada and Salinian block of central California. Late Cretaceous uplift common to these granulites may provide an important tectonic link between dismembered Mesozoic batholithic terranes in the California Cordillera.     

Zircon dating of North Bohemian granulites, Czech Republic: further evidence for the Lower Carboniferous high-pressure event in the Bohemian Massif

U-Pb zircon and rutile multigrain ages and ²⁰⁷Pb/²⁰⁶Pb zircon evaporation ages are reported from high-pressure felsic and metapelitic granulites from northern Bohemia, Czech Republic. The granulites, in contrast to those from other occurrences in the Bohemian Massif, do not show evidence of successive HT/MPLP overprints. Multigrain size fractions of nearly spherical, multifaceted, metamorphic zircons from three samples are slightly discordant and yield a U-Pb Concordia intercept age of 348 ± 10 Ma, whereas single zircon evaporation of two samples resulted in ²⁰⁷Pb/²⁰⁶Pb ages of 339 ± 1.5 and 339 ± 1.4 Ma, respectively. A rutile fraction from one sample has a U-Pb Concordia intercept age of 346 ± 14 Ma. All ages are identical, within error, and a mean age of 342 ± 5 Ma was adopted to reflect the peak of HP metamorphism. Because rutile has a lower closing temperature for the U-Pb isotopic system than zircon, the results and the P-T data imply rapid uplift and cooling after peak metamorphism. The above age is identical to ages for high-grade metamorphism reported from the southern Bohemian Massif and the Granulite Massif in Saxony. It can be speculated that all these granulites were part of the same lower crustal unit in early Carboniferous, being separated later due to crustal stacking and subsequent late Variscan orogenic collapse.     

Sm–Nd and U–Pb dating of high-pressure granulites from the Złote and Rychleby Mts (Bohemian Massif, Poland and Czech Republic)

In the Orlica‐?nie?nik complex at the NE margin of the Bohemian Massif, high‐pressure granulites occur as isolated lenses within partially migmatized orthogneisses. Sm–Nd (different grain‐size fractions of garnet, clinopyroxene and/or whole rock) and U–Pb [isotope dilution‐thermal ionization mass spectrometry (ID‐TIMS) single grain and sensitive high‐resolution ion microprobe (SHRIMP)] ages for granulites, collected in the surroundings of ?ervený D?l (Czech Republic) and at Stary Giera?tów (Poland), constrain the temporal evolution of these rocks during the Variscan orogeny. Most of the new ages cluster at c. 350–340 Ma and are consistent with results previously reported for similar occurrences throughout the Bohemian Massif. This interval is generally interpreted to constrain the time of high‐pressure metamorphism. A more complex evolution is recorded for a mafic granulite from Stary Giera?tów and concerns the unknown duration of metamorphism (single, short‐lived metamorphic cycle or different episodes that are significantly separated in time?). The central grain parts of zircon from this sample yielded a large spread in apparent ²⁰⁶Pb/²³⁸U SHRIMP ages (c. 462–322 Ma) with a distinct cluster at c. 365 Ma. This spread is interpreted to be indicative for variable Pb‐loss that affected magmatic protolith zircon during high‐grade metamorphism. The initiating mechanism and the time of Pb‐loss has yet to be resolved. A connection to high‐pressure metamorphism at c. 350–340 Ma is a reasonable explanation, but this relationship is far from straightforward. An alternative interpretation suggests that resetting is related to a high‐temperature event (not necessarily in the granulite facies and/or at high pressures) around 370–360 Ma, that has previously gone unnoticed. This study indicates that caution is warranted in interpreting U–Pb zircon data of HT rocks, because isotopic rejuvenation may lead to erroneous conclusions.     

Monazite in a Variscan mylonitic paragneiss from the Münchberg Metamorphic Complex (NE Bavaria) records Cadomian protolith ages

Small oval‐shaped, unshielded monazite grains found in a Variscan garnet–muscovite‐bearing mylonitic paragneiss from the Liegendserie unit of the Münchberg Metamorphic Complex in the northwestern Bohemian Massif, central Europe, yield only pre‐Variscan ages. These ages, determined with the electron microprobe, have maxima at c. 545, 520 and 495 Ma and two side‐maxima at 455 and 575 Ma, and are comparable with previously determined ages of detrital zircon reported from paragneisses elsewhere in the NW Bohemian Massif. The pressure (P)–temperature (T) history of this mylonitic paragneiss, determined from contoured P–T pseudosections, involved an initial stage at 6 kbar/600 °C, reaching peak P–T conditions of 12.5 kbar/670 °C with partial melting, followed by mylonitization and retrogression to 9 kbar/610 °C. The monazite, representing detrital grains derived from igneous rocks of a Cadomian provenance between 575 and 455 Ma, has survived these Variscan metamorphic/deformational events unchanged because this mineral has probably never been outside its P–T stability field during metamorphism.     

U–Pb evidence for a polyorogenic evolution of the HP–HT units of the NW Iberian Massif

. A isotope dilution thermal ionisation mass spectrometry U-Pb geochronological study was carried out on the high-pressure and high-temperature units (HP-HT units) overlying the oceanic suture in the Allochthonous Complexes of the NW Iberian Variscan Belt. The rocks investigated are seven granulite- to eclogite-facies paragneisses and one leucosome within mafic high-pressure granulites in the Ordenes and Cabo Ortegal Complexes of NW Spain. U-Pb dating of zircon, monazite, titanite and rutile reveal the presence of a pervasive Early Ordovician metamorphic event at ca. 500-480 Ma and a later Early Devonian event at ca. 400-380 Ma. The U-Pb ages, in conjunction with petrological and structural data, indicate that the high-pressure event recorded by these rocks is Early Ordovician in age. Monazite ages in the paragneisses suggest that peak metamorphic conditions were reached at ca. 500-485 Ma. Subsequently, the rock ensemble underwent exhumation accompanied by partial melting and zircon growth at ca. 485-470 Ma. Melting of mafic granulites was coeval with this latter episode as indicated by zircon crystallisation age in the leucosomes dated at ca. 486 Ma. Based on these data and on the general features of magmatism and metamorphic evolution, it is proposed that this process took place at a convergent plate boundary within a peri-Gondwanan oceanic domain. Monazite, titanite and rutile data in some of the samples studied show evidence of a second metamorphic episode that took place between ca. 400 and 380 Ma (with a peak at ca. 390-385 Ma). This Early Devonian event, at variance with the previous one, was not pervasive, but, rather, was localised in areas of intense Variscan tectonothermal reworking. It is claimed that this later metamorphic event was recorded by the U-Pb system in areas where monazite and titanite growth was enhanced by fluid circulation in highly strained rocks (Variscan shear zones). According to previous structural studies and Ar-Ar dating of fabrics, this Early Devonian episode took place as the HP-HT units were deformed and thrusted upon the ophiolitic units in the early stages of the Variscan collision.     

Exhumation of a Variscan orogenic complex: insights into the composite granulitic–amphibolitic metamorphic basement of south-east Corsica (France)

A structural, petrological and geochronological (U‐Th‐Pb of zircon and monazite) study reveals that the lower crust sequences of the Variscan high‐grade basement cropping out between Solenzara and Porto Vecchio, south‐east Corsica (France) have been tectonically juxtaposed along with middle crustal rocks during the extrusion of the orogenic root of the Variscan chain. We propose that a system of high‐temperature, orogen‐parallel shear zones that developed under a transpressive dextral tectonic regime caused the exhumation of the entire sequence. This tectonic complex is thus made up of rocks having undergone different P–T conditions (eclogite‐?, high‐pressure granulite facies and amphibolite facies) at different times, reflecting the progressive foreland migration of the orogenic front. The Solenzara granulites were derived from burial of continental crust to high‐pressure (1.8–1.4 GPa) and high‐ to ultrahigh‐temperature conditions (900–1000 °C) during the Variscan convergence: U–Pb ELA‐ICPMS zircon dating constrained the timing of this metamorphism at c. 360 Ma. The gneisses cropping out at Porto Vecchio are middle crustal‐level rocks that reached their peak temperature conditions (700–750 °C at <1.0 GPa) at c. 340 Ma. The diachronism of the metamorphic events, the foliation patterns and their geometry suggest that the granulites were exhumed to middle crustal levels through channel flow tectonics under continuous compression. The amphibolite facies gneisses of Porto Vecchio and the granulites of Solenzara were accreted through the development of a major dextral mylonitic zone forming under amphibolite facies conditions: in situ monazite isotope dating (ELA‐ICPMS) revealed that this deformation occurred at c. 320 Ma and was accompanied by the emplacement of syntectonic high‐K melts. A final HTLP static overprint, constrained at 312–308 Ma by monazite U‐Th‐Pb isotope dating, is related to the emplacement of the igneous products of the Sardinia‐Corsica batholith and marks the transition from the Variscan orogenic event to the Permian extension.     

Clasts of Variscan high-grade rocks within Upper Viséan conglomerates – constraints on exhumation history from petrology and U-Pb chronology

In this paper, U‐Pb zircon, monazite and rutile data for crystalline rocks deposited as clasts in the Upper Viséan conglomerates at the eastern margin of the Bohemian Massif are reported. U‐Pb data of spherical zircon from three different granulite clasts yielded a mean age of 339.0 ± 0.7 Ma (±2σ), while oval and spherical grains of another granulite pebble define a slightly younger date of 337.1 ± 1.1 Ma. These ages are interpreted as dating granulite facies metamorphism. Thermochronology and the derived pressure–temperature (P–T) path of the granulite pebbles reflect two‐stage exhumation of the granulites. Near‐to‐isothermal decompression from at least 44 km to mid‐crustal depths of around 22 km was followed by a near‐isobaric cooling stage based on reaction textures and geothermobarometry. Minimum average exhumation rate corresponds to 2.8–4.3 mm year^?1. The extensive medium‐pressure/high‐temperature overprint on granulite assemblages is dated by U‐Pb in monazite at c. 333 Ma. This thermal event probably has a close link to generation and emplacement of voluminous Moldanubian granites, including the cordierite granite present in clasts. This granite was emplaced at mid‐crustal levels at 331 ± 3 Ma (U‐Pb monazite), whereas the U‐Pb zircon ages record only a previous magmatic event at c. 378 Ma. Eclogites and garnet peridotites normally associated with high‐pressure granulites are absent in the clasts but exotic subvolcanic and volcanic members of the ultrapotassic igneous rock series (durbachites) of the Bohemian Massif have been found in the clasts. It is therefore assumed that the clasts deposited in the Upper Viséan conglomerates sampled a structurally higher tectonic unit than the one that corresponds to the present denudation level of the Moldanubicum of the Bohemian Massif. The strong medium‐temperature overprint on granulites dated at c. 333 Ma is attributed to the relatively small size of the entirely eroded bodies compared with the presently exposed granulites.     

The Mejillonia suspect terrane (Northern Chile): Late Triassic fast burial and metamorphism of sediments in a magmatic arc environment extending into the Early Jurassic

The Mejillonia terrane, named after the Mejillones Peninsula (northern Chile), has been traditionally considered an early Paleozoic block of metamorphic and igneous rocks displaced along the northern Andean margin in the Mesozoic. However, U–Pb SHRIMP zircon dating of metasedimentary and igneous rocks shows that the sedimentary protoliths were Triassic, and that metamorphism and magmatism took place in the Late Triassic (Norian). Field evidence combined with zircon dating (detrital and metamorphic) further suggests that the sedimentary protoliths were buried, deformed (foliated and folded) and metamorphosed very rapidly, probably within few million years, at ca. 210 Ma. The metasedimentary wedge was then uplifted and intruded by a late arc-related tonalite body (Morro Mejillones) at 208 ± 2 Ma, only a short time after the peak of metamorphism. The Mejillones metamorphic and igneous basement represents an accretionary wedge or marginal basin that underwent contractional deformation and metamorphism at the end of a Late Permian to Late Triassic anorogenic episode that is well known in Chile and Argentina. Renewal of subduction along the pre-Andean continental margin in the Late Triassic and the development of new subduction-related magmatism are probably represented by the Early Jurassic Bólfin–Punta Tetas magmatic arc in the southern part of the peninsula, for which an age of 184 ± 1 Ma was determined. We suggest retaining the classification of Mejillonia as a tectonostratigraphic terrane, albeit in this new context.     

Cambro-Silurian magmatisms at the northern Gondwana margin (Penninic basement of the Ligurian Alps)

The Early Paleozoic evolution of the northern margin of Gondwana is characterized by several episodes of bimodal magmatism intruded or outpoured within thick sedimentary basins. These processes are well recorded in the Variscan blocks incorporated in the Ligurian Alps because they experienced low temperature Alpine metamorphism. During the Paleozoic, these blocks, together with the other Alpine basements, were placed between the Corsica-Sardinia and the Bohemian Massif along the northern margin of Gondwana. In this framework, they host several a variegated lithostratigraphy forming two main complexes(Complexs I and II) that can be distinguished by both the protoliths and their crosscutting relationships, which indicate that the acidic and mafic intrusives of Complex II cut an already folded sequence made of sediments, basalts and granitoids of Complex I. Both complexes were involved in the Variscan orogenic phases as highlighted by the pervasive eclogite-amphibolite facies schistosity(foliation II). However, rare relicts of a metamorphic foliation at amphibolite facies conditions(foliation I)is locally preserved only in the rocks of Complex I. It is debatable if this schistosity was produced during the early folding event e occurred between the emplacement of Complex I and II e rather than during an early stage of the Variscan metamorphic cycle.New SHRIMP and LA ICP-MS Ue Pb zircon dating integrated with literature data, provide emplacement ages of the several volcanic or intrusive bodies of both complexes. The igneous activity of Complex I is dated between 507 ± 15 Ma and 494 ± 5 Ma, while Complex II between 467 ± 12 Ma and 445.5 ± 12 Ma.The folding event recorded only by the Complex I should therefore have occurred between 494 ± 5 Ma and 467 ± 12 Ma. The Variscan eclogite-amphibolite facies metamorphism is instead constrained between ~420 Ma and ~300 Ma. These ages and the geochemical signature of these rocks allow constraining the Early Paleozoic tectono-magmatic evolution of the Ligurian blocks, from a middleeupper Cambrian rifting stage, through the formation of an Early Ordovician volcanic arc during the Rheic Ocean subduction, until a Late Ordovician extension related to the arc collapse and subsequent rifting of the PaleoThetys. Furthermore, the ~420-350 Ma ages from zircon rims testify to thermal perturbations that may be associated with the Silurian rifting-related magmatism, followed by the subduction-collisional phases of the Variscan orogeny.     

U–Pb detrital zircon data of the Rio Fuerte Formation (NW Mexico): Its peri-Gondwanan provenance and exotic nature in relation to southwestern North America

U–Pb detrital zircon studies in the Rio Fuerte Group, NW Mexico, establish its depositional tectonic setting and its exotic nature in relation to the North American craton. Two metasedimentary samples of the Rio Fuerte Formation yield major age clusters at 453–508 Ma, 547–579 Ma, 726–606 Ma, and sparse quantities of older zircons. The cumulative age plots are quite different from those arising from lower Paleozoic miogeoclinal rocks of southwestern North America and of Cordilleran Paleozoic exotic terranes such as Golconda and Robert Mountains. The relative age-probability plots are similar to some reported from the Mixteco terrane in southern Mexico and from some lower Paleozoic Gondwanan sequences, but they differ from those in the Gondwanan-affinity Oaxaca terrane. Major zircon age clusters indicate deposition in an intraoceanic basin located between a Late Ordovician magmatic arc and either a peri-Gondwanan terrane or northern Gondwanaland. The U–Pb magmatic ages of 151 ± 3 Ma from a granitic pluton and 155 ± 4 Ma from a granitic sill permit a revision of the stratigraphic and tectonic evolution of the Rio Fuerte Group. A regional metamorphism event predating the Late Jurassic magmatism is preliminarily ascribed to the Late Permian amalgamation of Laurentia and Gondwana. The Late Jurassic magmatism, deformation, and regional metamorphism are related to the Nevadan Orogeny.     

U-Pb SHRIMP and Nd isotopic data from the western Bohemian Massif (Bayerischer Wald,Germany): Implications for Upper Vendian and Lower Ordovician magmatism

U-Pb SHRIMP dating of zircons of metamagmatites from the Bayerischer Wald (Germany) reveals a complex evolution of this section of the Moldanubian Zone exposed in the western Bohemian Massif of the central European Variscan belt. In the south-western part of the Bayerischer Wald Upper Vendian magmatism is constrained by pooled ²⁰⁶Pb/²³⁸U mean ages of 555±12, 549±7 and 549±6 Ma from metarhyolites and a metabasite. Inherited zircon cores were not observed. Zircon overgrowths, yielding pooled ²⁰⁶Pb/²³⁸U ages of 316±10 and 319±5 Ma, provide evidence for Variscan metamorphic zircon growth; cathodoluminescence imaging reveals a two-stage metamorphic overprint.In contrast, Lower Ordovician magmatism and anatexis are documented in the north-eastern parts of the Bayerischer Wald by metagranitoids (480±6, 486±7 Ma), an eclogitic metabasite (481±8 Ma) and a leucosome (491 to 457 Ma). Inherited zircon cores are found in Lower Ordovician metagranitoids and the leucosome, indicating a Palaeoproterozoic-Archaean (ca. 2.7, 2.0 Ga) source region, presumably of Gondwana affinity (West African craton), and documenting Cadomian magmatism (ca. 640 Ma). Post-Cadomian metamorphism is inferred from concordant ages of 433±4 and 431±7 Ma.Upper Vendian magmatism is assumed at an active continental margin with ensialic back-arc development (ε_Nd(t) –3.01 to +1.22); the lack of inherited zircon is due to either derivation from juvenile (?volcanic arc) material or complete isotopic resetting of pre-existing zircon. An active continental margin setting, possibly with some lateral variation (accretion/collision) is envisaged for the Lower Ordovician, producing granitoids, rhyolites and leucosomes (ε_Nd(t) -0.5 to -6.27); MORB-type metabasites may be related to ZEV or Mariánské Lázně Complex metabasites. A tentative palaeogeographic reconstruction puts the “Bayerischer Wald” in close relationship with the Habach terrane (proto-Alps), as the “eastern” extension of terranes of the northern Gondwana margin.     

Dating granulite-facies structures and the exhumation of lower crust in the Moldanubian Zone of the Bohemian Massif

Deformation of granulite-facies rocks in the Moldanubian Zone of the southern Bohemian Massif is expressed in two intersecting planar fabrics - steeply disposed (S₁) and flat-laying (S₂) - which correspond to two deformation stages (D₁) and (D₂). The existing Sm-Nd garnet ages from banded granulite gneisses, new U-Pb zircon data from deformed granite intrusions within the granulite gneisses, and the P-T and field structural relations constrain the ages and P-T conditions of the two deformation phases. The early deformation (D₁) was associated with a HP-HT metamorphic stage with a minimum age of ca. 354 Ma which was followed by a near-isothermal decompression. A concordant U-Pb zircon age of 318ǃ Ma dates the emplacement of intrusions of deformed granite into the granulite gneisses and constrains deformation phase (D₂). This phase was associated with an LP-HT metamorphism dated in the region at ca. 340-330 Ma. The available structural and isotopic data indicate that granulites in the southern Bohemian Massif were exhumed from lower to middle crust during compression. The structural relations and P-T-t data for the studied granulites are consistent with their exhumation by near-vertical extrusion of the softened orogenic root.