Variscan Sm-Nd and Ar-Ar ages of eclogite facies rocks from the Erzgebirge, Bohemian Massif

Abstract The Erzgebirge Crystalline Complex (ECC) is a rare example where both‘crustal’eclogites and mantle-derived garnet-bearing ultramafic rocks (GBUs) occur in the same tectonic unit. Thus, the ECC represents a key complex for studying tectonic processes such as crustal thickening or incorporation of mantle-derived material into the continental crust. This study provides the first evidence that high-pressure metamorphism in the ECC is of Variscan age. Sm-Nd isochrons define ages of 333 ± 6 (Grt-WR), 337± 5 (Grt-WR), 360± 7 (Grt-Cpx-WR) (eclogites) and 353 ± 7 Ma (Grt-WR) (garnet-pyroxenite). ⁴⁰Ar/³⁹Ar spectra of phengite from two eclogite samples give plateau ages of 348 ± 2 and 355 ± 2 Ma. The overlap of ages from isotopic systems with blocking temperatures that differ by about 300 ° C indicates extremely fast tectonic uplift rates. Minimum cooling rates were about 50° C Myr-¹. As a consequence, the closure temperature of the specific isotopic system is of minor importance, and the ages correspond to the time of high-pressure metamorphism. Despite textural equilibrium and metamorphic temperatures in excess of 800° C, clinopyroxene, garnet and whole rock do not define a three-point isochron in three of four samples. The metamorphic clinopyroxenes seem to have inherited their isotopic signature from magmatic precursors. Rapid tectonic burial and uplift within only a few million years might be the reason for the observed Sm-Nd disequilibrium. The ε_Nd values of the eclogites (+4.4 to +6.9) suggest the protoliths were derived from a long-term depleted mantle, probably a MORB source, whereas the isotopically enriched garnet-pyroxenite (ε_Nd–2.9) might represent subcontinental mantle material, emplaced into the crust prior to or during collision. The similarity of ages of the two different rock types suggests a shared metamorphic history.     

Inverted metamorphic field gradient towards a Variscan suture zone (Champtoceaux Complex, Armorican Massif, France)

We describe, date and constrain the P–T conditions of a syntectonic inverted metamorphic sequence associated with continental collision and crustal‐scale thrusting in one of the key regions of the late Palaeozoic Variscan belt of Western Europe – the Champtoceaux Complex (Armorican Massif, France), interpreted as a trace of the Variscan suture zone between Laurussia and Gondwana. The Complex consists of several stacked units, some of them eclogite‐bearing, that are sandwiched between two main pieces of continental crust – the Parautochthon and the Upper Allochthon. Moderately to steeply dipping foliation parallels the main lithological boundaries. From the bottom to the top of the metamorphic rock pile, the following sequence testifies to the syntectonic temperature increase: chlorite–biotite‐bearing metagreywackes (Parautochthon); orthogneisses with eclogite lenses; micaschists with chloritoid–chlorite–garnet; orthogneisses; micaschists with staurolite–biotite–garnet with chloritoid inclusions (Lower Allochthon); and migmatites with boudins of eclogite and kyanite–biotite–garnet‐bearing metapelitic lenses (Upper Allochthon). Mylonitic amphibolites with lenses of serpentinized peridotite mark the boundary between the Lower Allochthon and the overlying Upper Allochthon, suggesting the presence of a major thrust. It is inferred that the latter is responsible for the development of the inverted metamorphic zoning. Multiequilibrium thermobarometry and pseudosections calculated with thermocalc indicate that equilibration temperatures of the syntectonic peak metamorphic assemblages increase upwards in the rock pile from <500 °C in the Parautochthon to >650 °C in the Upper Allochthon. All units equilibrated at similar pressures between 7 and 10 kbar. In the Upper Allochthon, chronological results on muscovite suggest initial cooling from c. 343 Ma (muscovite Rb–Sr) to c. 337 Ma (muscovite ⁴⁰Ar–³⁹Ar). A subsequent very rapid temperature decrease is suggested by the synchronous closure of the muscovite and biotite K–Ar and biotite Rb–Sr isotopic systems (c. 337–335 Ma). This cooling is also recorded in the Upper Micaschists of the Lower Allochthon and in the Parautochthon with muscovite ⁴⁰Ar–³⁹Ar ages of c. 336–334 and 332 Ma, respectively. Ages of c. 343 Ma inferred from disturbed muscovite spectra from the Parautochthon are possibly linked to a previous higher pressure metamorphic event in this unit. It is suggested that the development of the inverted metamorphic zoning in the Champtoceaux Complex is due to the emplacement of a hot nappe over colder units and is contemporaneous with major crustal thrusting and associated pervasive ductile deformation. The preservation of this inverted field gradient was possible because of fast cooling, tentatively associated with the syn‐compressional denudation of the tectonic pile, expressed by the detachment at the top of the nappe pile. The efficiency of cooling is best shown by the near‐coincidence of Rb–Sr and ⁴⁰Ar–³⁹Ar ages, obtained on both sides of the major thrust. Finally, we highlight similarities with other regions of the West‐European Variscan belt (Iberian massif, French Massif Central) and suggest that inverted metamorphic zoning is systematically associated with the contact between the Lower and Upper Allochthons.     

Evolution of nappes in the eastern margin of the Bohemian Massif: a kinematic interpretation

The entire pile of nappes in the eastern margin of the Bohemian massif is characterized by two stages of Variscan nappe emplacement each exhibiting a different kinematic and metamorphic evolution.The older emplacement (D1) probably occurred around 350-340 Ma ago and was synmetamorphic. The nappes show a typical systematic superposition of higher grade metamorphic units over lower grade ones. Thus, the crystalline complexes showing a HT-MP Barrovian imprint (Svratka allochthonous unit and Moldanubicum) were thrust over an intermediate unit affected by MTMP recrystallization (Bíte orthogneiss and its country rock), and at the base of the D1 nappe pile the Inner Phyllite Nappe (Biý Potok Unit) is characterized by LT/LP metamorphism.The second stage of tectonic evolution (D2) is characterized by a thin-skinned northward-oriented nappe emplacement that occurred under LT-LP conditions dated at 320-310 Ma. The whole nappe sequence formed during the first tectonometamorphic period (D1) was transported northward over the autochthonous »Deblín polymetamorphic and granitic complex« of Upper Proterozoic age and its Devonian sedimentary cover with very low metamorphism. During this second tectonic event the Brno granite massif (580 Ma) was only marginally incorporated in the Variscan nappe tectonics which resulted in kilometer-scale cover and basement duplexes. The tectonic evolution of the nappe pile ended with stage D3, represented by large- to medium-scale east-vergent folds with limited displacement.

---

Zusammenfassung Der Deckenbau am Ostrand der Böhmischen Masse erfolgte in zwei aufeinanderfolgenden Stadien, die sich sowohl in ihrer Kinematik als auch in ihrer Metamorphoseentwicklung deutlich voneinander unterschieden.Die ältere Phase (D1 ca. 350-340 Ma) ist durch synmetamorphe Überschiebungen charakterisiert. Sie führt zu einer metamorphen Inversion der überschobenen Deckeneinheiten, so daß generell hohe metamorphe Einheiten schwach metamorphe tektonisch überlagern. Der Svratka Komplex und das Moldanubikum als hangendste Decken sind durch MP/HT Paragenesen vom Barrow-Typ gekennzeichnet. Beide Einheiten sind auf den MP/MT-metamorphen Bite-Gneis und seine Rahmengesteine überschoben. Die Bílý potok Einheit als liegende Decke zeigt nur noch eine LP/ LT Regionalmetamorphose.Das jüngere Stadium (D2 ca. 320-310 Ma) ist durch eine Thin-skinned Tektonik mit nordvergentem Deckentransport unter LP/LT Bedingungen charakterisiert. Der gesamte, invers metamorphe D1-Deckenstapel wird dabei nach N über den autochtonen Deblín Komplex bzw. seine devonische Sedimenthülle überschoben.Das Brno Granit Massiv (580 Ma) wird nur randlich in diesen variszischen Deckenbau einbezogen. Die tektonische Entwicklung endet mit einem mittel bis großräumigen E-vergenten Faltenbau (D3 phase).

---

Résumé L'empilement des nappes a la bordure orientale du Massif de Bohème est caractérisé par deux stades de mise en place présentant différentes évolutions cinématiques et métamorphiques.La tectonique majeure de mise en place des nappes crustales intervient lors d'un métamorphisme de type barrowien, calé autour de 350-340 Ma. L'empilement qui en résulte montre une superposition systématique d'unités à fort degré de métamorphisme sur des unités moins métamorphiques. Ainsi les complexes cristallins, montrant des reliques de métamorphisme de haute à moyenne pression-haute température (unités cristallines de Svratka et du Moldanubien), chevauchent une unité intermédiaire affectée par un métamorphisme de moyenne à basse pression-moyenne température (l'orthogneiss de Bíte et son encaissant). A la base de cette pile édifiée durant la tectonique D1, l'unité des phyllites internes (unité de Bílý potok) est caractérisée par un métamorphisme de basse témperature-basse pression.Le second stade D2 de l'évolution tectonique est caractérisé par une tectonique pelliculaire à vergence nord datée à 320-310 Ma. L'empilement résultant de D1 est ainsi transporté vers le nord, au dessus du complexe autochtone d'âge protérozoïque supérieur (groupe de Deblín) et sa couverture sédimentaire dévonienne très faiblement métamorphisée.Le massif granitique de Brno (580 Ma) n'est que marginalement incorporé à cette tectonique de nappe varisque. Ceci se traduit par des duplex socle-couverture d'échelle plurikilométrique. L'évolution tectonique s'achève lors d'une troisième phase, marquée par de grands plis à vergence est. Le déplacement associé est alors d'amplitude limitée.

---

, . , 350-340 . . , , - ( ), , - ( ). , D 1, (- ) - . D 2 , 320-310 ., D 1, , , ( ) . (580 . ) , »« -, . , .

     

Tectonometamorphic evolution of the Bohemian Massif: evidence from high pressure metamorphic rocks

The Variscan orogenic belt, of which the Bohemian Massif is a part, is typically recognized for its characteristic low pressure, high temperature metamorphism and a large volume of granites. However, there are also bodies of high pressure rocks (eclogites, garnet peridotites and high pressure granulites) which are small in size but widely distributed throughtout the Massif. Initially the high pressure rocks were considered to be relicts of a much older orogenic event, but the increasing data derived from isotopic and geochronological investigations show that many of these rocks have Palaeozoic protoliths. Metamorphic ages from the high pressure rocks define no single event. Instead, a number of discrete clusters of ages are found between about 430 Ma and the time of the dominant low pressure event at around 320–330 Ma.Most of the eclogite and granulite facies rocks are assigned to allochthonous nappes that arrived close to the end of the low pressure event, but before final granite intrusion. The nappes contain a mixture of different units and the relationship between rocks with high pressure relicts and host gneisses with no apparent signs of deep burial is still problematic. Some of the high pressure rocks retain evidence of multiple stages of partial re-equilibration during uplift. Moreover, it can be shown in certain instances that host gneisses also endured a multistage metamorphic development but with a peak event convergent with one of the breakdown stages in the enclosed rocks with high pressure relicts. It thus appears that the nappe units are composite bodies probably formed during episodic intracrustal thrusting. Fluids derived from prograde dehydration reactions in the newly under thrusting slab are taken to be the catalysts that drove the partial re-equilibrations.On the scale of the whole Massif it can be seen within the units with high pressure relicts that the temperature at the peak recorded pressure and that during the breakdown are variable in different locations. It is interpreted that regional metamorphic gradients are preserved for given stages in the history and thus the present day dismembered nappe relicts are not too far removed from their original spatial distribution in an original coherent unit. From the temperature information alone it is highly probable that the refrigerating underthrusting slab was situated in the north-west. However, this north-west to south-east underthrusting probably represents the major 380–370 Ma event and is no guide to the final thrusting that emplaced the much thinned nappe pile with high pressure relicts.Granite genesis is attributed to the late stage stacking, during the final Himalayan-type collision stage, of thinned crust covered by young, water-rich, sediments — erosion products of the earlier orogenic stages. Regional metamorphism at shallow depths above the voluminous granites was followed by final nappe emplacement which rejuvenated the granite ascent in places. Correspondence to: P. J. O'Brien     

Assimilation and diffusion during xenolith-magma interaction: a case study of the Variscan Karkonosze Granite, Bohemian Massif

Magmatic enclaves from the Rudolfov quarry near Liberec (Czech Republic) are interpreted to represent remnants of lamprophyric melt that intruded the Karkonosze granite at a stage at which the granite was not fully solidified. Based on the observation that larger enclaves are generally more circular than the smaller ones, we conclude that bigger blobs of mafic magma became more spherical during flow in the gravity field (sink or float). This flow is also interpreted to be responsible for the incorporation of mineral grains into the enclaves and may have facilitated the assimilation of granitic melt. Linear mixing trends on Harker diagrams for most network-forming and mainly slow-diffusing or fluid-immobile elements indicate such an assimilation process between granite and lamprophyre. In contrast, all fast-diffusing or fluid-mobile elements display scattered element distributions, implying that chemical diffusion also played a role. Pressure and temperature for this late magmatic stage are estimated at around 1 kbar and 500°C. These results suggest that two processes modified the composition of the enclaves in the Karkonosze granite: (1) assimilation (mechanical mixing) of granitic melt during the injection of the lamprophyric melt and the subsequent flow of the forming enclaves in the gravity field (responsible for the linear mixing trends) and (2) diffusion-controlled redistribution of elements between both solidifying rock types at the magmatic stage and/or due to late-stage magmatic fluids (responsible for the scattering and deviation from the linear mixing trends).     

Garnet pyroxenite and eclogite in the Bohemian Massif: geochemical evidence for Variscan recycling of subducted lithosphere

High-temperature, high-pressure eclogite and garnet pyroxenite occur as lenses in garnet peridotite bodies of the Gföhl nappe in the Bohemian Massif. The high-pressure assemblages formed in the mantle and are important for allowing investigations of mantle compositions and processes. Eclogite is distinguished from garnet pyroxenite on the basis of elemental composition, with mg number <80, Na₂O > 0.75 wt.%, Cr₂O₃ < 0.15 wt.% and Ni < 400 ppm. Considerable scatter in two-element variation diagrams and the common modal layering of some eclogite bodies indicate the importance of crystal accumulation in eclogite and garnet pyroxenite petrogenesis. A wide range in isotopic composition of clinopyroxene separates [_Nd, +5.4 to –6.0; (⁸⁷Sr/⁸⁶Sr)_i, 0.70314–0.71445; ¹⁸O_SMOW, 3.8–5.8%o] requires that subducted oceanic crust is a component in some melts from which eclogite and garnet pyroxenite crystallized. Variscan Sm-Nd ages were obtained for garnet-clinopyroxene pairs from Dobeovice eclogite (338 Ma), Úhrov eclogite (344 Ma) and Nové Dvory garnet pyroxenite (343 Ma). Gföhl eclogite and garnet pyroxenite formed by high-pressure crystal accumulation (±trapped melt) from transient melts in the lithosphere, and the source of such melts was subducted, hydrothermally altered oceanic crust, including subducted sediments. Much of the chemical variation in the eclogites can be explained by simple fractional crystallization, whereas variation in the pyroxenites indicates fractional crystallization accompanied by some assimilation of the peridotite host.     

Hydrothermal alteration, fluid flow and volume change in shear zones: the layered mafic–ultramafic Kettara intrusion (Jebilet Massif, Variscan belt, Morocco)

During emplacement and cooling, the layered mafic–ultramafic Kettara intrusion (Jebilet, Morocco) underwent coeval effects of deformation and pervasive fluid infiltration at the scale of the intrusion. In the zones not affected by deformation, primary minerals (olivine, plagioclase, clinopyroxene) were partially or totally altered into Ca‐amphibole, Mg‐chlorite and CaAl‐silicates. In the zones of active deformation (centimetre‐scale shear zones), focused fluid flow transformed the metacumulates (peridotites and leucogabbros) into ultramylonites where insoluble primary minerals (ilmenite, spinel and apatite) persist in a Ca‐amphibole‐rich matrix. Mass‐balance calculations indicate that shearing was accompanied by up to 200% volume gain; the ultramylonites being enriched in Si, Ca, Mg, and Fe, and depleted in Na and K. The gains in Ca and Mg and losses in Na and K are consistent with fluid flow in the direction of increasing temperature. When the intrusion had cooled to temperatures prevailing in the country rock (lower greenschist facies), deformation was still active along the shear zones. Intense intragranular fracturing in the shear zone walls and subsequent fluid infiltration allowed shear zones to thicken to metre‐scale shear zones with time. The inner parts of the shear zones were transformed into chlorite‐rich ultramylonites. In the shear zone walls, muscovite crystallized at the expense of Ca–Al silicates, while calcite and quartz were deposited in ‘en echelon’ veins. Mass‐balance calculations indicate that formation of the chlorite‐rich shear zones was accompanied by up to 60% volume loss near the centre of the shear zones; the ultramylonites being enriched in Fe and depleted in Si, Ca, Mg, Na and K while the shear zones walls are enriched in K and depleted in Ca and Si. The alteration observed in, and adjacent to the chlorite shear zones is consistent with an upward migrating regional fluid which flows laterally into the shear zone walls. Isotopic (Sr, O) signatures inferred for the fluid indicate it was deeply equilibrated with host lithologies.     

Partially retrograded eclogites of the Münchberg Massif, Germany: records of a multi-stage Variscan uplift history in the Bohemian Massif

Abstract Eclogites with a wide range in bulk composition are present in the Münchberg Massif, part of the Variscan basement of the Bohemian Massif in north-east Bavaria. New analyses of the primary phases garnet, omphacite, phengite and amphibole, as well as the secondary phases clinopyroxene II, various amphiboles, biotite/phlogopite, plagioclase, margarite, paragonite, prehnite and pumpellyite, reveal a complex uplift history. New discoveries were made of samples with very jadeite-rich primary omphacite as well as a secondary omphacite in a symplectite with albite. Various geothermobarometric techniques, together with thermodynamic databases (incorporating separately determined activity–composition values) and experimental data have clustered the minimum conditions for the primary assemblages to the P–T range 650 ± 60° C, 14.3 ± 1 kbar. However, jadeite (in omphacite)–kyanite–paragonite (in phengite) and zoisite–grossular (in garnet)–kyanite–quartz relationships suggests pressures of 25–28 kbar at the same temperatures. The fact that the secondary omphacite–plagioclase assemblage yields pressures within a few hundred bars of the minimum pressures for the plagioclase-free assemblages strongly suggests that the minimum values are serious underestimates.
Zoning, inclusion suites and breakdown reactions of primary phases, in addition to new minerals formed during uplift, define a polyphase metamorphic evolution which, from geochronological evidence, occurred solely within the Variscan cycle. The complex breakdown in other Bohemian Massif eclogites and the distinct variation in their temperatures during uplift suggest a multi-stage thrusting model for the regional evolution of the eclogites. Such an evolution has significance with respect to incorporation of mantle slices into crustal sequences and fluid derivation from successively subducted units, possibly driving the breakdown reactions.     

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.     

Permo-Carboniferous volcanism in late Variscan continental basins of the Bohemian Massif (Czech Republic): geochemical characteristic

Extensive Permo-Carboniferous volcanism has been documented from the Bohemian Massif. The late Carboniferous volcanic episode started at the Duckmantian–Bolsovian boundary and continued intermittently until Westphalian D to Stephanian B producing mainly felsic and more rarely mafic volcanics in the Central Bohemian and the Sudetic basins. During the early Permian volcanic episode, after the intra-Stephanian hiatus, additional large volumes of felsic and mafic volcanics were extruded in the Sudetic basins. The volcanics of both episodes range from entirely subalkaline (calc-alkaline to tholeiitic) of convergent plate margin-like type to transitional and alkaline of within-plate character. A possible common magma could not be identified among the Carboniferous and Permian primitive magmas, but a common geochemical signature (enrichment in Th, U, REE and depletion in Nb, Sr, P, Ti) in the volcanic series of both episodes was recognized. On the other hand, volcanics of both episodes differ in intensities of Nb, Sr and P depletion and also, in part, in their isotope signatures. High ⁸⁷Sr/⁸⁶Sr (0.707–0.710) and low ε_Nd (−6.0 to −6.1) are characteristic of the Carboniferous mafic volcanics, whereas low ⁸⁷Sr/⁸⁶Sr (0.705–0.708) and higher ε_Nd ranging from −2.7 to −3.4 are typical of the Permian volcanics. Felsic volcanics of both episodes vary substantially in ⁸⁷Sr/⁸⁶Sr (0.705–0.762) and ε_Nd (−0.9 to −5.1). Different depths of magma source or heterogeneity of the Carboniferous and Permian mantle can be inferred from variation in some characteristic elements of the geochemical signature for volcanics in some basins. The Sr–Nd isotopic data with negative ε_Nd values confirm a significant crustal component in the volcanic rocks that may have been inherited from the upper mantle source and/or from assimilation of older crust during magmatic underplating and ascending of primary basic magma. Two different types of primary magma development and formation of a bimodal volcanic series have been recognized: (i) creation of a unique magma by assimilation fractional crystallization processes within shallow-level reservoirs (type Intra-Sudetic Basin) and (ii) generation and mixing of independent mafic and felsic magmas, the latter by partial melting of upper crustal material in a high-level chamber (type Krkonoše Piedmont Basin). A similar origin for the Permo-Carboniferous volcanics of the Bohemian Massif is obvious, however, their geochemical peculiarities in individual basins indicate evolution in separate crustal magma chambers.     

从地壳上地幔构造看大陆碰撞带岩石圈的克拉通化

本篇讨论超级大陆汇聚后逐渐变为克拉通或扩大克拉通的作用过程,即指经及大陆碰撞地体汇聚后新形成的大陆块逐渐转变为刚性克拉通的作用过程。增生大陆岩石圈的克拉通化的作用后果,包括大陆地壳密度的增加,岩石圈地幔的增厚和大地热流值的下降,使大陆岩石圈逐渐刚性强化。大陆碰撞后形成的大陆块必须经过克拉通化的过程,才能逐渐成为刚性克拉通。作用过程主要包括:①上地壳沉积碎屑岩石结晶岩化和中地壳岩石角闪岩化;②下地壳岩石基性化;③大陆碰撞带下凹莫霍面的磨平;④岩石圈地幔底侵加厚形成陆根。从大陆碰撞带转变为克拉通的过程也是岩石圈地幔不断增厚而地壳缓慢变硬变冷的过程。这个过程包含以下作用:区域变质作用,交代作用和岩石圈幔源岩浆的底侵。这个过程时间尺度比碰撞造山作用大一个级次。长期的底侵作用使地壳岩石密度和强度不断加大,改变岩层的矿物成分和局部结构。当大陆岩石圈克拉通化到一定程度之后,由于下方软流圈的热能供应逐渐减缓,使岩石圈地温梯度缓慢下降,最终结果会形成大陆根。由于显生宙大陆碰撞带岩石圈强度弱,大陆碰撞时更容易造成岩石圈变形,因此大陆碰撞的板内效应主要发生在大陆内的显生宙碰撞带。显生宙大陆碰撞带如果再次受到大陆碰撞板内效应的作用,其克拉通化的过程必然会推迟。     

Alpine tectono‐metamorphic history of the continental units from Vardar zone: the Kopaonik Metamorphic Complex (Dinaric‐Hellenic belt,Serbia)

The easternmost zone of the Dinaric‐Hellenic belt is represented by the Vardar Zone, in which the Kopaonik Metamorphic Complex (KMC) is regarded as the lowermost unit. This complex is topped by the unmetamorphosed Brzece unit and is intruded by the Oligocene Kopaonik Intrusive complex. The KMC is characterized by a stratigraphy that includes metapelites and meta‐carbonates of Late Triassic age, associated with metabasites. It is characterized by a complex deformation history that comprises four phases: D1 to D4. The D1 phase structures occur only as relict structures, whereas the D2 phase structures are represented by isoclinal F2 folds, associated with a well‐developed S2 foliation. The estimated P‐T conditions for the D1 and D2 metamorphism are consistent with the upper greenschist facies. The D3 phase is characterized by west‐verging thrusts associated with upright folds. In contrast, the D4 phase is characterized by open folds (F4) associated with low‐angle normal faults. The D1 and D2 deformation phases developed during the shortening related to continental collision, whereas the subsequent D3 and D4 phases can be related to the progressive exhumation of the KMC. The D4 phase probably developed during extensional tectonics during and after emplacement of the Kopaonik Intrusive Complex. The data show that the continental units belonging to the Vardar zone had a long‐lived deformation history that was more complex that previously thought. Copyright © 2009 John Wiley & Sons, Ltd.     

Dehydration and anatexis of UHP metagranite during continental collision in the Sulu orogen

Dehydration and anatexis of ultrahigh‐pressure (UHP) metamorphic rocks during continental collision are two key processes that have great bearing on the physicochemical properties of deeply subducted continental crust at mantle depths. Determining the time and P–T conditions at which such events take place is needed to understand subduction‐zone tectonism. A combined petrological and zirconological study of UHP metagranite from the Sulu orogen reveals differential behaviours of dehydration and anatexis between two samples from the same UHP slice. The zircon mantle domains in one sample record eclogite facies dehydration metamorphism at 236 ± 5 Ma during subduction, exhibiting low REE contents, steep MREE–HREE patterns without negative Eu anomalies, low Th, Nb and Ta contents, low temperatures of 651–750 °C and inclusions of quartz, apatite and jadeite. A second mantle domain records high‐T anatexis at 223 ± 3 Ma during exhumation, showing high REE contents, steeper MREE–HREE patterns with marked negative Eu anomalies, high Hf, Nb, Ta, Th and U contents, high temperatures of 698–879 °C and multiphase solid inclusions of albite + muscovite + quartz. In contrast, in a second sample, one zircon mantle domain records limited hydration anatexis at 237 ± 3 Ma during subduction, exhibiting high REE contents, steep MREE–HREE patterns with marked negative Eu anomalies, high Hf, Nb, Ta, Th and U contents, medium temperatures of 601–717 °C and multiphase solid inclusions of albite + muscovite + hydrohalite. A second mantle domain in this sample records a low‐T dehydration metamorphism throughout the whole continental collision in the Triassic, showing low REE contents, steep MREE–HREE patterns with weakly negative Eu anomalies, low Th, Nb and Ta contents, low temperatures of 524–669 °C and anhydrite + gas inclusions. Garnet, phengite and allanite/epidote in these two samples also exhibit different variations in texture and major‐trace element compositions, in accordance with the zircon records. The distinct P–T–t paths for these two samples suggest separate processes of dehydration and anatexis, which are ascribed to the different geothermal gradients at different positions inside the same crustal slice during continental subduction‐zone metamorphism. Therefore, the subducting continental crust underwent variable extents of dehydration and anatexis in response to the change in subduction‐zone P–T conditions.     

LPHT metamorphism in a late orogenic transpressional setting, Albera Massif, NE Iberia: implications for the geodynamic evolution of the Variscan Pyrenees

During the Late Palaeozoic Variscan Orogeny, Cambro‐Ordovician and/or Neoproterozoic metasedimentary rocks of the Albera Massif (Eastern Pyrenees) were subject to low‐pressure/high‐temperature (LPHT) regional metamorphism, with the development of a sequence of prograde metamorphic zones (chlorite‐muscovite, biotite, andalusite‐cordierite, sillimanite and migmatite). LPHT metamorphism and magmatism occurred in a broadly compressional tectonic regime, which started with a phase of southward thrusting (D1) and ended with a wrench‐dominated dextral transpressional event (D2). D1 occurred under prograde metamorphic conditions. D2 started before the P–T metamorphic climax and continued during and after the metamorphic peak, and was associated with igneous activity. P–T estimates show that rocks from the biotite‐in isograd reached peak‐metamorphic conditions of 2.5 kbar, 400 °C; rocks in the low‐grade part of the andalusite‐cordierite zone reached peak metamorphic conditions of 2.8 kbar, 535 °C; rocks located at the transition between andalusite‐cordierite zone and the sillimanite zone reached peak metamorphic conditions of 3.3 kbar, 625 °C; rocks located at the beginning of the anatectic domain reached peak metamorphic conditions of 3.5 kbar, 655 °C; and rocks located at the bottom of the metamorphic series of the massif reached peak metamorphic conditions of 4.5 kbar, 730 °C. A clockwise P–T trajectory is inferred using a combination of reaction microstructures with appropriate P–T pseudosections. It is proposed that heat from asthenospheric material that rose to shallow mantle levels provided the ultimate heat source for the LPHT metamorphism and extensive lower crustal melting, generating various types of granitoid magmas. This thermal pulse occurred during an episode of transpression, and is interpreted to reflect breakoff of the underlying, downwarped mantle lithosphere during the final stages of oblique continental collision.     

Partial melting of deeply subducted continental crust during exhumation: insights from felsic veins and host UHP metamorphic rocks in North Qaidam,northern Tibet

A combined petrological, geochronological and geochemical study was carried out on felsic veins and their host rocks from the North Qaidam ultrahigh‐pressure (UHP) metamorphic terrane in northern Tibet. The results provide insights into partial melting of deeply subducted continental crust during exhumation. Partial melting is petrograpically recognized in metagranite, metapelite and metabasite. Migmatized gneisses, including metagranite and metapelite, contain microstructures such as granitic aggregates with varying outlines, small dihedral angles at mineral junctions and feldspar with magmatic habits, indicating the former presence of felsic melts. Partial melts were also present in metabasite that occurs as retrograde eclogite. Felsic veins in both the eclogites and gneisses exhibit typical melt crystalline textures such as large euhedral feldspar grains with straight crystal faces, indicating vein crystallization from anatectic melts. The Sr–Nd isotope compositions of felsic veins inside gneisses suggest melt derivation from anatexis of host gneisses themselves, but those inside metabasites suggest melt derivation from hybrid sources. Felsic veins inside gneisses exhibit lithochemical compositions similar to experimental melts on the An–Ab–Or diagram. In trace element distribution diagrams, they exhibit parallel patterns to their host rocks, but with lower element contents and slightly positive Eu and Sr anomalies. The geochemistry of these felsic veins is controlled by minerals that would decompose and survive, respectively, during anatexis. Felsic veins inside metabasites are rich either in quartz or in plagioclase with low normative orthoclase. In either case, they have low trace element contents, with significantly positive Eu and Sr anomalies in plagioclase‐rich veins. Combined with cumulate structures in some veins, these felsic veins are interpreted to crystallize from anatectic melts of different origins with the effect of crystal fractionation. Nevertheless, felsic veins in different lithologies exhibit roughly consistent patterns of trace element distribution, with variable enrichment of LILE and LREE but depletion of HFSE and HREE. There are also higher contents of trace elements in veins hosted by gneisses than veins hosted by metabasites. Anatectic zircon domains from felsic veins and migmatized gneisses exhibit consistent U–Pb ages of c. 420 Ma, significantly younger than the peak UHP eclogite facies metamorphic event at c. 450–435 Ma. Combining the petrological observations with local P–T paths and experimentally constrained melting curves, it is inferred that anatexis of UHP gneisses was caused by muscovite breakdown while anatexis of UHP metabasites was caused by fluid influx. These UHP metagranite, metapelite and metabasite underwent simultaneous anatexis during the exhumation, giving rise to anatectic melts with different compositions in various elements but similar patterns in trace element distribution.     

Magmatic stoping as an important emplacement mechanism of Variscan plutons: evidence from roof pendants in the Central Bohemian Plutonic Complex (Bohemian Massif)

The presence of numerous roof pendants, stoped blocks and discordant intrusive contacts suggests that magmatic stoping was a widespread, large-scale process during the final construction of the Central Bohemian Plutonic Complex, Bohemian Massif. The measured total length of the discordant contacts that cut off the regional cleavage and were presumably formed by stoping corresponds to about half of all contacts with the upper-crustal host rocks. In addition, at least some of the straight, cleavage-parallel intrusive contacts may also have recorded complex intrusive histories ending with piecemeal stoping of thin cleavage-bounded host rock blocks into the magma chamber. Based on the above, we argue that the fast strain rates required for emplacement of large plutons of the Central Bohemian Plutonic Complex into brittle upper crustal host rocks over relatively short-time span could not have been accommodated entirely by slow ductile flow or slip along faults. Instead, the emplacement was largely accommodated by much faster thermal cracking and extensive stoping independent of regional tectonic deformation. Finally, we emphasize that magmatic stoping may significantly modify the preserved structural patterns around plutons, may operate as an important mechanism of final construction of upper-crustal plutons and thus may contribute to vertical recycling and downward transport of crustal material within the magma plumbing systems in the crust.     

Southern Granulite Terrain, South of the Palghat-Cauvery Shear Zone: Implications for India-Madagascar Connection

The present paper correlates the southern Madgascar terrain, south of the Ranotsara shear with the granulite terrain of southern India, occurring south of the Palghat-Cauvery (P-C) shear zone. Both the terrains have witnessed high temperature to ultra high temperature granulite metamorphism at 550 Ma and are traversed by shear zones and deep crustal faults. The 550 Ma old granulite terrains of Madagascar and southern India have similar lithologies, in particular, sapphirine bearing pelitic assemblages. Graphite deposits and gem occurrences are common to both these terrains. The 550 Ma old southern granulite terrain of southern India comprises of different blocks, the Madurai and the Kerala Khondalite belt, but all the blocks have similar lithologies with pelite—calc silicate rocks inter-banded with two pyroxene granulite bodies. These lithologies occur amidst an essentially charnockitic terrain. The protolith ages of the southern granulite terrain, south of the P-C shear zone ranges between 2400–2100 Ma. The terrain as a whole has witnessed the 550 Ma old granulite event. The granulite metamorphism took place under temperatures of 800–1000°C and at pressures of 9.5 to 5 Kbar.The source of heat for the high temperature granulite event of the southern Madagascar terrain has been linked to advective heat transfer along mantle deep faults. The source for the high temperature granulite metamorphism for the southern granulite terrain may be attributed to high temperature carbonatite and alkaline intrusives in an extensional setting which followed an initial crustal thickening.Many workers have linked Madagascar to southern India by connecting the Ranotsara shear either to the P-C shear zone or to the Achankovil shear zone, further south. The important factor is the lithologies of the Madagascar terrain, south of Ranotsara shear zone and the 550 Ma. old southern Indian granulite terrain are similar in many aspects. It will be more appropriate to link the Ranotsara shear to the curvilinear lineament bounding the Anaimalai-Kodaikanal ranges and which merges with the southern margin of the P-C shear zone.However, north of the Ranotsara shear/fault, the northern Madagascar terrain comprises of a dominant Itremo sequence (< 1850 Ma) and 780 Ma old calc-alkaline intrusives. The latter have similarities with that of Aravallis and the Sirohi, Malani sequences occurring further north east. The Rajasthan terrain has witnessed igneous intrusive activity at 1000–800 Ma. If we can broaden the area of investigations and include the above areas, the Madagascar-India connection can be better understood.     

Brittle deformation events at the western border of the Bohemian Massif (Germany)

Investigations of brittle deformation structures, present within the crystalline rocks of the Bavarian Oberpfalz, reveal a complex late to post-Variscan crustal evolution. Upper Carboniferous (mainly Westphalian) granites were emplaced into semibrittle to brittle rocks of the ZEV (zone of Erbendorf-Vohenstrauß) and the EGZ (Erbendorf greenschist unit), respectively. From both the alignment of the granites and the direction of granite-related tension gashes a north-east-south-west extension must be assumed for the period of magmatic activities. Apart from the granite intrusions, rapid crustal uplift (about 1.5 km/my) led to an increase in the geothermal gradient from < 30 °C/km (late Variscan pre-granitic) to > 40 °C/km (late Variscan post-granitic). The increased geothermal gradient persisted during the succeeding reverse faulting which results from late Carboniferous (probably Stephanian) east-west and northeast-south-west compression. Although not evidenced directly in the area considered, strike-slip faults seem to have played an important part during the late Variscan crustal evolution, particularly in the Early Permian. The strike-slip events indicate further crustal shortening and indentation under north-south compression.A similar indentation was present in Cretaceous time. After a weak phase of Early Cretaceous reverse faulting, which results from north-south compression, strike-slip faults formed under north-west-south-east and north-south compression. All these faults, in particular the strike-slip faults, seem to be related to the Cretaceous and lowermost Tertiary convergence of the Alpine/Carpathian orogeny.A late stage of crustal extension, characterized by a radial stress tensor (₂ = ₃), is indicated through high angle normal faults which probably formed during the subsidence of the adjacent Neogene Eger Graben.