Morphology and growth trends of accessory zircons from various granitoids of the South-western Bohemian Massif (Moldanubicum,Austria)

As the main objective of the present study, the morphological development of accessory zircons from four granitoids (pearl gneiss, fine-grained granite, coarse-grained gneiss, and Weinsberg granite) of the South-western Bohemian Massif was described in detail. On the one hand, this was realized by the classical approach, including a statistical evaluation of external zircon morphologies with the typology scheme. On the other hand, direct insight into the growth of single crystals was established by the production of crystal sections parallel and perpendicular to the crystallographic c-axis and by their subsequent imaging with the electron microprobe. Regarding the second method, eventual morphological trends were represented as a function of the growth rates of single crystal forms. Except for the coarse-grained gneiss, zircon crystals of the investigated granitoids show similar morphological trends according to both methods, starting with a dominant {1 0 0} prism and equally sized pyramids. Final crystal habit, however, is marked by the predominance of {1 1 0} and {2 1 1}. Zircon crystals from the coarse-grained gneiss run through a completely different development with a change of the prism habit from {1 1 0} to {1 0 0} and a more or less static growth of {1 0 1} with only slight modifications in size. Comparison of the results with data from the literature underlines the role of magma chemistry, magma temperature, and cooling rate as the main factors responsible for growth trends of accessory zircon.     

The origin and evolution of the Mexican Cordillera as registered in modern detrital zircons

Continents are thought to be built at convergent margins by mantle-derived mafic inputs, but the processes that transform them into more felsic and long-lasting constituents are still unclear. Here we use U-Pb geochronology, trace elements and Hf isotopic compositions of modern detrital zircons from western Mexico to illustrate how a typical mafic island-arc built on an oceanic basin can be rapidly transformed into a mature intermediate to felsic continent by the combined effects of subduction erosion and rear-arc sediment underthrusting, prompted by the closure of a back-arc basin. In western Mexico, the most significant mass transfer from mantle to crust occurred during the Early Cretaceous in the context of an intra-oceanic island arc, locally dubbed Alisitos-Guerrero. Subsequently, the process of “continentalization” was achieved by the deep reprocessing of the newly formed lithologies once they were dynamically incorporated into the melting source during the development of the Mexican Orogen. The Mexican continental crust did not only grow by arc-related inputs, however, as there was at least one magmatic episode in the Jurassic where extensional-driven mantle upwellings triggered melting of upper-crustal sediments. In this view, mafic island arcs and back-arc basins are ephemeral geologic features that can be easily consumed into trenches, but the geochemical evidence of their existence, and possibly even a significant portion of their mass, can be preserved in younger and more felsic continental igneous rocks.     

兴凯地块伊曼群的形成时代与物源：碎屑锆石与岩浆锆石U-Pb年代学证据

本文对出露在兴凯地块南部伊曼群的变沉积岩和侵入其中的火成岩进行了系统的锆石LA-ICP-MS U-Pb定年和Hf同位素研究,旨在查明伊曼群的形成时代与物源,并讨论侵入岩的成因和构造背景。伊曼群二云母片岩中锆石呈自形-半自形,显示典型的岩浆生长环带,暗示其岩浆成因。定年结果表明:100个碎屑锆石分析点给出了555～1322Ma的年龄区间,主要峰值年龄为555Ma、612Ma、700Ma、739Ma、769Ma、839Ma、936Ma;其中约533Ma的锆石ε_(Hf)(t)值介于-4.1～+3之间,二阶段模式年龄t_(DM2)介于1305～1755Ma之间;700～839Ma的锆石ε_(Hf)(t)值介于-3.7～+7.8之间,二阶段模式年龄t_(DM2)介于1246～1907Ma之间。侵入伊曼群的白云母二长花岗岩的定年结果显示为443±3Ma,此年龄对应ε_(Hf)(t)值介于-17.6～-4.2之间,二阶段模式年龄t_(DM2)介于1688～2529Ma之间。结合二云母片岩中碎屑锆石的最小年龄以及穿切其中花岗岩的形成时代,可以判定伊曼群二云母片岩的形成时代介于443～555Ma之间,应为新元古代晚期至晚奥陶世之间。基于伊曼群中碎屑锆石的年龄众数及与区域岩浆事件的对比,伊曼群的沉积物源主要来自松嫩地块和佳木斯地块的新元古代火成岩。此外,白云母二长花岗岩显示出埃达克岩的地球化学属性,岩浆源于加厚下地壳的部分熔融,暗示晚奥陶世研究区可能发生了陆壳加厚事件。     

Tracing the origin of Southwest Japan using the Hf isotopic composition of detrital zircons from the Akiyoshi Belt

The Permian Akiyoshi Belt, which is the oldest unmetamorphosed accretionary complex in Southwest Japan, plays an important role in understanding the origin of the Japanese Islands; however, its tectonic affinity is still not well constrained. Herein, we present the first Hf isotopic data for U–Pb dated detrital zircons of the Late Permian sandstones in the Akiyoshi Belt. Almost all the late Palaeozoic detrital zircons yielded highly positive ?_Hf (t) values compatible with a juvenile source, suggesting that the major provenance of the trench sediments was from the South China block and not from the Bureya‐Jiamusi‐Khanka massif. We propose that in the late Palaeozoic, a block of juvenile lower crust existed outboard of the South China block, and it was being melted during the westward subduction of the Palaeo‐Pacific oceanic plate to produce voluminous volcanic and plutonic rocks with positive ?_Hf (t) values.     

Zircon Hf isotope perspective on the origin of granitic rocks from eastern Bavaria, SW Bohemian Massif

The petrogenetic potential of in situ laser ablation Hf isotope data from melt precipitated zircons was explored through the analyses of about 700 individual crystals derived from about 20 different granitic intrusions covering the Variscan basement segment of eastern Bavaria, SE Germany. In combination with geochemical features, four major suites of granitic rocks can be distinguished: (1) NE Bavarian redwitzites (52–57 wt% SiO₂, intrusion ages around 323 Ma) have chondritic εHf(t) values (+0.8 to –0.4). The redwitzites are hybrid rocks and the Hf data are permissive of mixing of a mantle progenitor and crustal melts. (2) Various intermediate rock types (dioritic dyke, granodiorite, palite, 59–63 wt% SiO₂, 334–320 Ma) from the Bavarian Forest yield negative εHf(t) values between –3.4 and –5.1. These values which apparently contradict a mantle contribution fingerprint an enriched (metasomatized) mantle component that was mixed with crustal material. (3) Voluminous, major crust forming granites sensu stricto (67–75 wt% SiO₂, 328–298 Ma) are characterized by a range in εHf(t) values from –0.5 to –5.6. Different crustal sources and/or modification of crustal melts by various input of juvenile material can explain this variation. (4) Post-plutonic (c. 299 Ma) porphyritic dykes of dacitic composition (64–67 wt% SiO₂) from the southern Bavarian Forest have chondritic εHf(t) values (+0.6 to –1.1) and display large intergrain Hf isotope variation. The dykes form a separate petrogenetic group and the Hf data suggest that the zircons crystallized when a pristine mantle-derived parental melt was modified by infiltration of crustal material. The zircon Hf data form a largely coherent positive array with the whole-rock Nd data and both systems yield similar two-stage depleted mantle model ages (1.1–1.7 Ga).     

A review of the pre-Permian geology of the Variscan French Massif Central

The Massif Central, like the southern part of the Massif Armoricain, belongs to the north Gondwana margin. The Massif Central consists of a stack of nappes resulting from six main tectonic-metamorphic events. The first, D₀, is coeval with a Late Silurian (ca 415 Ma) high-pressure (HP) (or ultra high-pressure) metamorphism for which the associated structures are poorly documented. The Early Devonian D₁ event, responsible for top-to-the-southwest nappe displacement, is coeval with migmatization and the exhumation of HP rocks around 385–380 Ma. In the northern part of the Massif Central, metamorphic rocks with retrogressed eclogites are covered by Late Devonian undeformed sedimentary rocks. The Late Devonian-Early Carboniferous D₂ event involves top-to-the-northwest shearing, coeval with an intermediate pressure-temperature metamorphism dated around 360–350 Ma. The Visean D₃ event is a top-to-the-south ductile shearing, which is widespread in the southern Massif Central. Coevally, in the northern Massif Central, the D₃ event corresponds to the onset of synorogenic extension. The next two events, D₄ and D₅, of Early and Late Carboniferous age, correspond to the syn- and late orogenic extensional tectonic regimes, respectively. The former is controlled by NW–SE stretching whereas the latter is accommodated by NNE–SSW stretching. These structural and metamorphic events are reconsidered in a geodynamic evolution model. The possibilities of one or two cycles involving microcontinent drifting, rewelding and collision are discussed.     

Some late precambrian microfossils from the Bohemian Massif and their correlation

A brief account is given of the main biostratigraphical conclusions resulting from the micropalaeontological study of the Bohemian Upper Proterozoic. The relatively rich microfossil material presently known from the Barrandian area enables comparison to be made both with Middle and Upper Brioverian microfossils of the West European Brioverian complex and also with those from the Upper Riphean and Vendian platform sediments. A correlation between the relatively close geographical areas of Bohemia, Lusatia and Saxony on the basis of microorganisms is evident. Biocommunities from siliceous rocks display algal-mat assemblages. Their features are very similar to those of biogenic rocks reported from other regions, especially Australia and North America. In the Moldanubian Supergroup, the ?eský Krumlov Formation, in which graphitized phytoclasts with anatomical structures of primitive land plants have recently been found, was examined. The question of the age of this formation remains open and its study is still in progress. The correlation of the sedimentary complex of the East Sudeten (the presumably Proterozoic Záb?eh Formation) with the Palaeozoic assemblage is possible due to the finds of Chitinozoa. Remains of megascopic algae were recently found in this assemblage, along with chilinozoan chambers. This association is highly specialized and comprises new taxa of higher Thallophytes.     

Timing of Variscan HP-HT metamorphism in the Moldanubian Zone of the Bohemian Massif: U-Pb SHRIMP dating on multiply zoned zircons from a granulite from the Dunkelsteiner Wald Massif, Lower Austria

In an attempt to better constrain the timing of Variscan HP-HT metamorphism in the SE Bohemian Massif we have dated zoned zircons from a garnet-kyanite granulite of granitic composition from the Dunkelsteiner Wald Massif, Lower Austria, by means of sensitive high-resolution ion microprobe (SHRIMP) technique. In order to combine isotopic information with crystal growth textures, CL and BSE images were systematically taken from the dated zircons. A characteristic threefold concentric zoning was found in many zircons. This involves pre-Variscan protolithic cores followed by two distinct metamorphic/anatectic overgrowth shells of Variscan age. The inner overgrowth shell is characterized by a weak CL but bright BSE signal, and yields high contents of uranium (0.1 to 0.2 wt.%). A pooled U-Pb Concordia age for this zone is 342.0?±?3.0?Ma (n?=?11, MSWD?=?0.12). The second, outer, overgrowth shell is always bright in the CL image, dark in the BSE image, and has generally low uranium contents (mostly <500?ppm). A pooled U-Pb Concordia age for this zone is 337.1?±?2.7?Ma (n?=?11, MSWD?=?0.22). These results imply that the Variscan HT crystallisation history of the Moldanubian granulites took place over a period of a few million years and was not an extremely rapid subduction-exhumation process. SHRIMP measurements in the protolithic cores yield a cluster of (sub)concordant ages between ??390 and 460?Ma and a few outliers at higher ages mostly represented by cores in cores. Core domains, which are large, homogeneous and with undisturbed igneous oscillatory zoning, yielded preferentially ages between 430 and 460?Ma. We therefore consider that granitic protolith formation took place at that time. The still older inner cores are interpreted as inherited into the granitic melt.     

Hadean-archean detrital zircons from Jatulian quartzites and conglomerates of the Karelian craton

Using local procedures, the new results on the isotope ages and composition of mineral inclusions were obtained for detrital zircons from Paleoproterozoic Jatulian terrigenous quartzites and polymictic conglomerates in Central and Western Karelia. For Eastern Laurasia, signs of the existence of Hadean and ancient Eoarchean matter were found for the first time (zircon grains of 3871 ± 38.6 and 3837 ± 42.1 Ma concordant ages). The multimodal distribution of ages within 3.45−2.61 Ga was revealed. The discovery of the oldest zircon grains provides the conditions for valid global correlations of geological events that determined the deposition and formation of the continental crust of the North Atlantic supercraton.     

Constraints on the origin and the genesis of graphite-bearing rocks from the variegated sequence of the Bohemian Massif (Austria)

Summary Transitions between graphitic gneisses, graphite-bearing calcsilicate rocks and marbles of the Variegated Sequence (Bunte Serie) in the Moldanubian zone of the Bohemian massif were examined for their primary sedimentary signatures. The bulk chemistry of the graphitic gneisses is similar to those of common black shales. Chondrite-normalized rare earth element (REE) distribution patterns of graphite-bearing rocks resemble those of average pelitic sediments. Graphite ¹³C-values around –22%o (PDB) show primary organic isotopic characteristics in the silicate rocks and confirm their organogenic origin. Traces of metasomatic mobilizations were found for a number of elements such as Ba, Rb, Cs, the REEs and Mn.Carbon isotope thermometry using the calibration ofScheele andHoefs (1992) suggests a peak-temperature of 640–780°C for the graphite-bearing calcsilicates and marbles. Small scale isotopic studies at the interface between gneisses and marbles argue against a pervasive streaming CO₂-rich fluid derived from an external source and imply that the fluid was controlled more locally by lithology. Ore petrographic investigations showed iron-rich alabandite as an abundant phase within the calcsilicate rocks and marbles. The occurrence and metamorphic origin of alabandite within these rocks indicates high S- and low CO₂-, O₂-fugacities in local layers of the Variegated Sequence.

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Herkunft und Genese von graphitführenden Gesteinen der Bunten Serie aus der Böhmischen Masse (Österreich)

Zusammenfassung Übergänge zwischen Graphitgneisen, Kalksilikatgesteinen und Marmoren der Bunten Serie im Moldanubikum der Böhmischen Masse wurden auf primäre sedimentäre Merkmale hin untersucht. Die Gesamtzusammensetzung der Graphitgneise gleicht der von durchschnittlichen Schwarzschiefern. Chondrit-normierte Verteilungsmuster der Seltenen Erdelemente (SEE) der graphitführenden Gesteine spiegeln pelitische Sedimente wider. Kohlenstoffisotopenzusammensetzungen der Graphite mit ¹³C-Werten um –22 (PDB) aus den Graphitgneisen lassen eindeutig auf einen organogenen Ursprung der Graphite schließen. Hinweise auf metasomatische Mobilisierungen finden sich bei den Elementen Ba, Rb, Cs, den SEE und Mn.Graphit-Kalzit Isotopenthermometrie nachScheele undHoefs (1992) ergibt eine Peak-Temperatur zwischen 640–780°C für das prägende Metamorphoseereignis. Isotopengeochelnische Kleinbereichsuntersuchungen erlauben es, ein während der prägenden Metamorphose lithologische Grenzen pervasiv durchströmendes CO₂-reiches Fluid, das von einer externen Quelle abgeleitet wird, auszuschließen und weisen auf eine eher lokale, von der Lithologie abhängige, Prägung des Fluids hin. Im Zuge der erzpetrographischen Untersuchung wurde Alabandin in den Kalksilikatgesteinen und Marmoren gefunden. Die Gegenwart von Alabandin, der in diesen Gesteinen als metamorphe Bildung angesehen wird, kann als Anzeiger für bereichsweise höhere S- und niedere CO₂- bzw. O₂-Fugazitäten in lokalen Bereichen der Bunten Serie gewertet werden.

     

The Precambrian tectogenesis in the Bohemian Massif

The Cadomian and Variscan tectogeneses are two distinctive and easily distinguished cycles of a long-term geological process leading to the almost complete crustal consolidation of the Bohemian Massif. The internal parts of the massif are those of the main development of the Upper Proterozoic geosyncline and the intensive Cadomian metamorphism and plutonism.The Precambrian of the Bohemian Massif is divided into two principal regional chronostratigraphic units: the Moldanubian and the Brioverian; the age of the boundary between them is estimated at about 1000 Ma B. P. The Brioverian is subdivided into three units: the Lower, Middle and Upper. The Moldanubian is provisionally subdivided into two units: the Lower and the Upper.The maximum of the metamorphic activity of the Cadomian cycle falls approximately within the sedimentation interval of the Middle Brioverian, and that of the Variscan cycle in the Devonian. Compared with the Cadomian regional metamorphism which attained mostly amphibolite to granulite facies, the Variscan metamorphism generally did not exceed greenschist facies.The origin of the granitoid rocks of the massif is closely associated with the metamorphic processes of the two above-mentioned cycles. Cadomian granitoids are represented mostly by rocks of granodiorite-tonalite and durbachite types whereas the Variscan intrusives are dominated by rocks of granitic composition.

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Zusammenfassung Die assyntische und variszische Orogenese sind die wesentlichen strukturprägenden Ereignisse im Böhmischen Massiv. Die internen Teile des Massives sind aus der jung-proterozoischen Geosynklinale hervorgegangen und während der assyntischen Gebirgsbildung metamorphisiert und von Magmatiten durchdrungen worden. Das Präkambrium des Böhmischen Massives ist in zwei chronostratigraphische Einheiten zu gliedern: das Moldanubium und das Brioverian. Die Grenze liegt etwa bei 1000 Mill. Jahren. Das Brioverian läßt sich in drei Einheiten gliedern und das Moldanubium in zwei Einheiten.Das Maximum der metamorphen Überprägung während der assyntischen Orogenese fällt in die Sedimentationslücke im mittleren Brioverian; im variszischen Zyklus erfolgt die Metamorphose im Devon. Die assyntische Metamorphose erreicht Amphibolit- bis Granulitfazies, die variszische geht nicht über eine Grünschieferfazies hinaus.Die magmatischen Gesteine sind eng mit den beiden Orogenesen verbunden. Die assnytischen Magmatite sind vorwiegend Granodiorit-Tonalite und Durbachite, während die variszischen Magmatite aus Graniten bestehen.

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Résumé Les tectogenèses cadomienne et varisque sont deux cycles différents, bien discernables, d'un processus géologique de longue durée, qui a conduit à la cosolidation presque complète du Massif de Bohème. La partie interne du Massif est celle du développement principal du géosynclinal protérozoïque supérieur et du métamorphisme et du plutonisme cadomiens.Le Précambrien du Massif de Bohème se divise en deux unités chronostratigraphiques régionales: le Moldanubien et le Briovérien. On estime l'âge de la limite entre elles à 1000 millions d'années environ. Le Briovérien est subdivisé en trois unités: inférieure, moyenne et supérieure. Le Moldanubien est divisé provisoirement en deux unités: inférieure et supérieure.La culmination de l'activité métamorphique du cycle cadomien tombe à peu près dans l'intervalle de sédimentation du Briovérien moyen, celle du cycle varisque dans le Dévonien. Comparé avec le métamorphisme régional cadomien qui le plus souvent a atteint le faciès amphibolite, le métamorphisme varisque, en général, n'a pas dépassé le faciès schistes verts.L'origine des roches granitoïdes du Massif et associée avec les processus métamorphiques des deux sus-dits cycles. Les granitoïdes cadomiens sont représentés pour la plupart par des roches du type des granodiorites-tonalites et durbachites, tandis que parmi les roches intrusives varisques prédominent des roches de composition granitique.

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, , . - , , , . : Moldanubium Brioverian. 1 . Brioverian , a Moldanubium . Brioverian; . ; . . - , — qg.

     

New constraints on the origin and evolution of the Thomson Orogen and links with central Australia from isotopic studies of detrital zircons

Interpretation of the Thomson Orogen and its context within the Tasmanides of eastern Australia is hampered by vast areas of deep sedimentary cover which also mask potential relationships between central and eastern Australia. Within covered areas, basement drill cores offer the only direct geological information. This study presents new detrital zircon isotopic data from these drill cores and poorly understood outcropping units to provide new age and provenance information for sedimentary rocks from the Thomson Orogen. Two distinct detrital zircon signatures are revealed. One is dominated by Grenvillian-aged (1300–900 Ma) zircons with a significant peak at ~ 1180 Ma and lesser peak at ~ 1070 Ma. These age peaks, along with Lu–Hf isotopic compositions (median εHf(t) = + 1.5), dominantly mantle-like δ¹⁸O values (median = 5.53‰) and model ages of ~ 1.89 Ga, support a Musgrave Province (central Australia) source. The dominance of Grenvillian-aged material additionally points to deposition during the Petermann Orogeny (570–530 Ma) when the Musgrave Province was uplifted, shedding abundant material to the Centralian Superbasin. Comparable age spectra suggest that parts of the Thomson Orogen were connected to the Centralian Superbasin during this period. We use the term ‘Syn-Petermann’ to describe this signature which is observed in two drill cores adjacent to the North Australian Craton and scattered units in the outcropping Thomson Orogen. The second signature marks a significant provenance shift and is remarkably consistent throughout the Thomson Orogen. Age spectra exhibit dominant peaks at 600–560 Ma, lesser 1300–900 Ma populations and maximum depositional ages of ~ 495 Ma. This pattern is termed the ‘Pacific Gondwana’ detrital zircon signature and is recognised throughout eastern Australia, Antarctica and central Australia. Lu–Hf isotope data for Thomson Orogen rocks with this signature are highly variable with εHf(t) values between ‐ 49 and + 10 and dominantly supracrustal δ¹⁸O values suggesting input from different and more diverse source regions relative to those exhibiting the Syn-Petermann signature.     

Climate change in Central Europe and the sensitivity of the hard rock aquifer in the Bohemian Massif to decline of recharge: case study from the Bohemian Massif

Climate change in Central Europe is manifested by periods of drought during the summer months of years when there is a reduction in atmospheric precipitation and temporary accumulation of water in the form of snow during winter period. The shallow sub-surface aquifer in crystalline rocks is very vulnerable to this decline in rainfall. Monitoring and subsequent model simulations revealed that the groundwater level in recharge areas had been lowered by almost 10 m, but the study also showed that levels in discharge areas are significantly more stable. The transmissivity of hard rock aquifers evidently shows changes in both space and time. The temperature rise results in increasing values of evapotranspiration that is manifested by a gentle but long-lasting groundwater-level decline.     

U-Pb systematics of detrital zircons from the Serebryanka Group of the Central Urals

Based on the LA-ICP-MS data, detrital zircons from the tillite-type conglomerates of the Tanin Formation (Serebryanka Group) on the western slope of the Central Urals include approximately equal proportions of crystals with Neoarchean and Paleoproterozoic U-Pb ages. Therefore, we can assume that crystalline rocks of the basement beneath the eastern part of the East European Craton served as a provenance for aluminosilicate clastics in the initial Serebryanka period. Detrital zircons from sandstones of the Kernos Formation have the Meso-Neoarchean (∼15%), Paleoproterozoic (∼60%), and Mesoproterozoic (∼26%) age. Comparison of the obtained data with the results of the study of detrital zircons from Riphean and Vendian sandstones of the Southern Urals shows that the Riphean and Lower Vendian rocks are mainly represented by erosional products of Middle and Upper Paleoproterozoic crystalline rocks that constitute the basement of the East European Craton. In addition, a notable role belonged to older (Lower Proterozoic, Neoarchean and Mesoarchean) rock associations during the formation of the Serebryanka Group. The terminal Serebryanka time (Kernos Age) differed from its initial stage (Tanin Age) by the appearance of Mesoproterozoic complexes in provenances. According to available data, these complexes played an insignificant role in the formation of Riphean-Vendian rocks in the neighboring South Uralian segment. This implies a spatiotemporal diversity of clastic material sources for Upper Precambrian rocks in the western megazone of the Southern and Central Urals.     

Early metamorphic evolution and exhumation of felsic high-pressure granulites from the north-western Bohemian Massif

Summary Several granulite terrains are exposed in the Bohemian Massif of Central Europe. These were metamorphosed at pressures close to 12 kbar and temperatures of more than 800 °C c. 340 Ma ago. The corresponding penetrative deformation almost totally erased the record of the preceding metamorphic evolution. Nevertheless, rare relics such as mineral inclusions in large garnet grains are witness of this earlier evolution, which was previously related to significantly higher pressures and, thus, to a subduction-related event. The exemplary investigation of such mineral relics in a felsic granulite from the Granulitgebirge rather points to pressures of 13–14 kbar only at relatively low temperatures of 620 °C and, thus, to considerable, nearly isobaric heating before the exhumation of the granulites started at 800 °C or somewhat higher temperature. The inferred P–T evolution is compatible with a geodynamic model of lithospheric delamination, with crustal material having been involved. The delamination at c. 340 Ma ago followed long-lasting, continuous collision of Gondwana and Laurussia forming the Variscan orogen. Within the thickened continental crust, the delamination concerned mainly the dense basic material in the lower crust. This event also caused upwelling of the mantle asthenosphere. Both processes resulted in heating of the more felsic lower portion of the continental crust, thinner than before delamination. Heating by 200 °C or more caused prograde mineral reactions and created buoyancy forces, as the overlying crust became denser than the underlying hot and felsic granulites. As a consequence, considerable volumes of felsic granulite could have reached shallow crustal levels (corresponding to 3 to 4 kbar), conditions documented in granulite bodies in the north-western Bohemian Massif.     

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     

An Andean type Palaeozoic convergence in the Bohemian Massif

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