South Bohemian Batholith Research Articles

Ultrahigh-temperature granites and a curious thermal eye in the post-collisional South Bohemian batholith of the Variscan orogenic belt (Europe)

Abstract Comprehensive zircon thermometry that takes into account zircon saturation temperatures, Ti-in-zircon measurements, and zircon morphologies and microstructures can provide key information on the thermal evolution of a granite batholith. The Variscan South Bohemian batholith (Germany, Austria, and Czech Republic) comprises a series of granitoid units that intruded between ca. 330 and ca. 300 Ma. We categorize the granitic rocks according to their emplacement temperature into very low temperature (T) (VLT; <750 °C), low T (LT; 750–800 °C), medium T (MT; 800–850 °C), high T (HT; 850–900 °C), and ultrahigh T (UHT; >900 °C). The first stage of batholith formation (ca. 330–325 Ma) is characterized by LT to MT melting of mainly metasedimentary sources driven by their isothermal exhumation. In turn, ca. 322 Ma HT and UHT granites in the southern half of the batholith reveal an ephemeral thermal anomaly in the subbatholithic crust, which is presumably linked to a hidden mafic intrusion. The HT and UHT granites are weakly peraluminous, high-K, I-type rocks. Although sharing some features with A-type granites such as high Zr and rare earth element contents, they differ from classical A-type granites in being magnesian, not enriched in Ga over Al, and having high Ba and Sr contents. A ring structure of ca. 317 Ma MT and/or LT plutons is observed around the HT and/or UHT granite complex and interpreted as an aftermath of the hotspot event. This study is an example of how deep-crustal hotspots, presumably caused by mantle magmatism, can significantly enhance the effects of decompressional crustal melting in a post-collisional setting.

Polyphase deformation along the South Bohemian Batholith-Moldanubian nappes boundary – The Freyenstein Fault System (Bohemian Massif/Austria)

Abstract This work describes the Freyenstein Fault System, which extends over 45 km in the southeastern part of the Bohemian Massif (Lower Austria). It represents a ductile shear zone overprinted by a brittle fault located at the eastern edge of the South Bohemian Batholith towards the Moldanubian nappes. It affects Weinsberg- and a more “fine-grained” granite, interlayered aplitic granite and pegmatite dikes as well as paragneiss of the Ostrong Nappe System. The ductile shear zone is represented by approximately 500 m thick greenschist-facies mylonite dipping about 60° to the southeast. Shear-sense criteria like clast geometries, SCC`-type shear band fabrics as well as abundant microstructures show top to the south/ southsouthwest normal shearing with a dextral strike-slip component. Mineral assemblages in mylonitized granitoid consist of pre- to syntectonic muscovite- and biotite-porphyroclasts as well as dynamically recrystallized potassium feldspar, plagioclase and quartz. Dynamic recrystallization of potassium feldspar and the stability of biotite indicate upper green-schist-facies metamorphic conditions during the early phase of deformation. Fluid infiltration at lower greenschist-facies conditions led to local sericitization of feldspar and synmylonitic chloritisation of biotite during a later stage of ductile deformation. Finally, a brittle overprint by a north-south trending, subvertical, sinistral strike-slip fault that shows a normal component is observed. Ductile normal shearing along the Freyenstein Shear Zone is interpreted to have occurred between 320 Ma and c. 300 Ma. This time interval is indicated by literature data on the emplacement of the hostrock and cooling below c. 300°C inferred from two Rb-Sr biotite ages measured on undeformed granites close to the shear zone yielding 309.6 ± 3 Ma and 290.9 ± 2.9 Ma, respectively. Brittle sinistral strike-slip faulting at less than 300°C presumably took place not earlier than 300 Ma. Early ductile shearing along the Freyenstein Fault System may be genetically, but not kinematically linked to the Strudengau Shear Zone, as both acted in an extensional regime during late Variscan orogenic collapse. A relation to other major northeast-southwest trending faults of this part of the Bohemian Massif (e.g. the Vitis-Pribyslav Fault System) is indicated for the phase of brittle sinistral movement.

Variscan potassic dyke magmatism of durbachitic affinity at the southern end of the Bohemian Massif (Lower Austria)

Dykes in the Strudengau area (SW Moldanubian Zone, Austria) can be mineralogically divided into lamprophyres (spessartites and kersantites) and felsic dykes (granite porphyries, granitic dykes and pegmatoid dykes). Geochemical analyses of 11 lamprophyres and 7 felsic dykes show evidence of fractional crystallization. The lamprophyres are characterized by metaluminous compositions, intermediate SiO2 contents and high amounts of MgO and K2O; these rocks have high Ba (800–3000 ppm) and Sr (250–1000 ppm) contents as well as an enrichment of large-ion lithophile elements over high field strength elements, typical for enriched mantle sources with variable modifications due to fractionation and crustal contamination. This geochemical signature has been reported from durbachites (biotite- and K feldspar-rich mela-syenites particularly characteristic of the Variscan orogen in Central Europe). For most major elements, calculated fractionation trends from crystallization experiments of durbachites give an excellent match with the data from the Strudengau dykes. This suggests that the lamprophyres and felsic dykes were both products of fractional crystallization and subsequent magma mixing of durbachitic and leucogranitic melts. Rb–Sr geochronological data on biotite from five undeformed kersantites and a locally deformed granite porphyry gave cooling ages of c. 334–318 Ma, indicating synchronous intrusion of the dykes with the nearby outcropping Weinsberger granite (part of the South Bohemian Batholith, c. 330–310 Ma). Oriented matrix biotite separated from the locally deformed granite porphyry gave an Rb–Sr age of c. 318 Ma, interpreted as a deformation age during extensional tectonics. We propose a large-scale extensional regime at c. 320 Ma in the Strudengau area, accompanied by plutonism of fractionated magmas of syncollisional mantle-derived sources, mixed with crustal components. This geodynamic setting is comparable to other areas in the Variscan belt documenting an orogenic wide extension by the end of the Carboniferous.

Polyphase tectonics and late Variscan extension in Austria (Moldanubian Zone, Strudengau area)

New data suggest syn-convergent extrusion and polyphase tectonics followed by late Variscan extension in the Strudengau area of the southern Moldanubian zone in Austria. The tectonic history can be summarized as follows: (1) The oldest ductile event is observed in HT/LP metamorphic pelitic gneisses, which preserve E-dipping foliation planes (D1-fabric) with NW–SE-trending lineations. (2) The overlying gneisses record HT/HP conditions with decompression-induced anatexis in the central part of the domain. These gneisses exhibit N–S trending, horizontal lineations along steep-dipping foliation planes (D2-fabric) crosscutting the D1-fabric of the pelitic gneisses. Along the margin, these rocks have been strongly mylonitized under amphibolite facies conditions (D2). D2 is interpreted as a significant vertical shear zone, which juxtaposes the HT/LP rocks against the orogenic lower crust. (3) Lastly, the whole area is overprinted by localized shear zones (D3-fabric) with top-to-the-NW kinematics. This newly discovered Strudengau shearing event is associated with isoclinal folding that possesses axial planes parallel to the mylonitic foliation and fold axes parallel to the stretching lineations. Initial mylonitization occurred under greenschist facies, representing the latest ductile event of the Strudengau area. The new geochronological data presented here indicate a narrow time frame (c. 323–318 Ma) for the D3 deformation. Therefore, this event is contemporaneous with the intrusion of the granites of the South Bohemian Batholith (330–310 Ma). The nearby South Bohemian Batholith and generally steep dyke swarms in the Strudengau area and to the north trend in a NE–SW preferred orientation, interpreted to be D3-synkinematic magmatism. In a regional context, the NW–SE stretching during D3 together with the synkinematic intrusion of dykes is associated with late orogenic extension in the Austrian Moldanubian Zone. Kinematic data of brittle normal faults and tension gashes are consistent with NW–SE-oriented extension under cooler conditions.

Late Variscan stress-field rotation initiating escape tectonics in the south-western Bohemian Massif: a far field response to late-orogenic extension

The late Variscan tectonic evolution of the Muhlzone, located at the south-western margin of the Bohemian Massif, is characterized by subhorizontal magmatic shearing under low-differential stress conditions in intrusive and in situ granites of the South Bohemian Batholith. The magmatic shearing converted into ductile shearing and escape tectonics along conjugate strike-slip zones during the late stage of magma solidification (~325 Ma). Escape tectonics lasted for at least 30 Ma. The transition from subhorizontal to strike-slip shearing reflects the rotation of the palaeo-stress field by ~76° around an axis approximately parallel to σ 1 . Via a transient high-temperature stage of solid-state deformation the rotation caused σ 2 and σ 3 almost to swap positions during continuous contraction and cooling of the crust from >700 °C to >500 °C. The primary factor governing stress-field rotation in the Muhlzone were far-field effects of the regional tectonic evolution, specifically stress release by NE–SW crustal extension at the eastern margin of the Bohemian Massif. Extensional strain reduced the magnitude of the subhorizontal ENE–WSW oriented σ 2 to values below the magnitude of the steep σ 3 , which most likely remained constant. The σ 1 stress vector maintained a shallow SSE plunging orientation throughout the Late Variscan evolution. Stress-field rotation and the conversion from subhorizontal to steep shearing are favoured by deformation under low differential stress. High temperatures of metamorphism, or melt present in migmatites or plutons, provide the low shear strength that is required for deformation at low differential stress. Hence, in crust composed of high-grade metamorphic or partially molten rocks, small changes in the tectonic environment can be sufficient for stress-field rotation leading to significant changes in the tectonic style.

The Saxo-Danubian Granite Belt: magmatic response to post-collisional delamination of mantle lithosphere below the southwestern sector of the Bohemian Massif (Variscan orogen)

The Saxo-Danubian Granite Belt: magmatic response to post-collisional delamination of mantle lithosphere below the southwestern sector of the Bohemian Massif (Variscan orogen)On the basis of the synchronicity of geochronological data and the similarity of granite types, it is proposed that the mid-Carboniferous Fichtelgebirge/Erzgebirge Batholith in the Saxothuringian Zone of the central European Variscan Fold Belt and the South Bohemian Batholith in the Moldanubian Zone (including the intervening Oberpfalz and Bavarian Forest granite areas) belong to one coherent and cogenetic, ca. 400 km long plutonic megastructure. Unlike older (syn-collisional) plutonic structures in the Bohemian Massif, this Saxo-Danubian Granite Belt (nov. nom.) has developed discordant to the Devonian/Early Carboniferous collision-related tectonic architecture of the Bohemian Massif. It is argued that the Saxo-Danubian Granite Belt formed in response to a post-collisional detachment of lithospheric mantle below the south-western sector of the Bohemian Massif.

Biotite stability in peraluminous granitic melts: Compositional dependence and application to the generation of two-mica granites in the South Bohemian batholith (Bohemian Massif, Czech Republic)

Crystallization experiments have been conducted in the system Na 2O–K 2O–MgO–FeO–Al 2O 3–SiO 2–H 2O (with 4% normative corundum) in order to constrain the stability of biotite as a function of water activity and the Mg# of biotite [Mg/(Mg +Fe total)] in equilibrium with peraluminous granitic melts. The temperature at which biotite breakdown starts is strongly dependent on the Mg# of biotite. At 500 MPa, the temperature of biotite breakdown to form orthopyroxene increases from 750 °C to 830 °C, as the Mg# of biotite increases from 0.4 to 0.5. Considering that the system investigated is relevant for Ca-poor peraluminous biotite-bearing rocks (metapelites), the biotite dehydration curves obtained are used to discuss the melting reactions and the temperatures that lead to the formation of two distinct types of two-mica granites found in the South Bohemian batholith (specifically the Eisgarn and Deštná granites). The phase relationships were determined experimentally for the composition of these two granites in order to constrain the composition of the biotite in equilibrium with the melt in the protoliths. We demonstrate that Eisgarn granitic melts may have been generated at temperatures in the range 830–850 °C from melting reactions involving biotite with a Mg# up to 0.5 as a reactant. In contrast, Deštná granitic melts cannot have been generated from dehydration melting reactions involving biotite.

Pre-Variscan geological events in the Austrian part of the Bohemian Massif deduced from U–Pb zircon ages

In an attempt to elucidate the pre-Variscan evolution history of the various geological units in the Austrian part of the Bohemian Massif, we have analysed zircons from 12 rocks (mainly orthogneisses) by means of SHRIMP, conventional multi-grain and single-grain U–Pb isotope-dilution/mass-spectrometry. Two of the orthogneisses studied represent Cadomian metagranitoids that formed at ca. 610 Ma (Spitz gneiss) and ca. 580 Ma (Bittesch gneiss). A metagranite from the Thaya batholith also gave a Cadomian zircon age (567±5 Ma). Traces of Neoproterozoic zircon growth were also identified in several other samples, underlining the great importance of the Cadomian orogeny for the evolution of crust in the southern Bohemian Massif. However, important magmatic events also occurred in the Early Palaeozoic. A sample of the Gfohl gneiss was recognised as a 488±6 Ma-old granite. A tonalite gneiss from the realm of the South Bohemian batholith was dated at 456±3 Ma, and zircon cores in a Moldanubian metagranitic granulite gave similar ages of 440–450 Ma. This Ordovician phase of magmatism in the Moldanubian unit is tentatively interpreted as related to the rifting and drift of South Armorica from the African Gondwana margin. The oldest inherited zircons, in a migmatite from the South Bohemian batholith, yielded an age of ca. 2.6 Ga, and many zircon cores in both Moravian and Moldanubian meta-granitoid rocks gave ages around 2.0 Ga. However, rocks from the Moldanubian unit show a striking lack of zircon ages between 1.8 and 1.0 Ga, reflecting an ancestry from Armorica and the North African part of Gondwana, respectively, whereas the Moravian Bittesch gneiss contains many inherited zircons with Mesoproterozoic and Early Palaeoproterozoic ages of ca. 1.2, 1.5 and 1.65–1.8 Ga, indicating a derivation from the South American part of Gondwana.

Zircon typology, geochronology and whole rock Sr?Nd isotope systematics of the Mecsek Mountain granitoids in the Tisia Terrane (Hungary)

Zircon typology and ages together with petrological data suggest similarities in the origin of the K–Mg-rich granitoids of the Mecsek Mountains (Moragy Unit, Tisia Terrane, S Hungary) and the durbachitic to K–Mg-rich rocks of the South and Central Bohemian Batholiths. Our investigations evidence a characteristic bimodal distribution of zircon types and ages. Zircons of “normal” magmatic habit and a typology around S24 show an age of 354±5 Ma. Zircons of tabular habit of subtype S4 indicate a possibly younger age of 339±10 Ma. The ages and typology of zircons from plutonic rocks in the Mecsek Mountains strongly resemble those from the Rastenberg granodiorite in the South Bohemian Batholith (Austria). This, as well as Rb/Sr and Sm/Nd isotope systematics, support the assumption of simultaneous formation and close spatial relationship between the Mecsek Mountain granitoids and the Rastenberg granodiorite. Thus, the Mecsek Mountains granitoids likely formed in a Late Palaeozoic palaeo-position S or SSW of the Rastenberg granodiorite. Having experienced a similar metamorphic and magmatic evolution during the Carboniferous, at least the northernmost part of the Tisia Terrane, the Moragy Unit, is therefore considered to represent a former part of the Helvetic-Moldanubian zone.

Electron-microprobe dating of monazites from Western Carpathian basement granitoids: plutonic evidence for an important Permian rifting event subsequent to Variscan crustal anatexis

Accessory monazites from 35 granitoid samples from the Western Carpathian basement have been analysed with the electron microprobe in an attempt to broadly constrain their formation ages, on the basis of their Th, U and Pb contents. The sample set includes representative granite types from the Tatric, Veporic and Gemeric tectonic units. In most cases Lower Carboniferous (Variscan) ages have been obtained. However, a much younger mid-Permian age has been recorded for the specialised S-type granites of the Gemeric Unit, and several small A- and S-type granite bodies in the Veporic Unit and the southern Tatric Unit. This distinct Permian plutonic activity in the southern part of the Western Carpathians is an important, although previously little considered geological feature. It appears to be not related to the Variscan orogeny and is interpreted here to reflect the onset of the Alpine orogenic cycle, with magma generation in response to continental rifting. The voluminous Carboniferous granitoid bodies in the Tatric and Veporic units comprise S- and I-type variants which document crustal anatexis accompanying the collapse of a compressional Variscan orogen sector. The Variscan magmas were most likely produced through the remelting of a subducted Precambrian volcanic arc-type crust which included both igneous and sedimentary reworked volcanic-arc material. Although the 2σ errors of the applied dating method are quite large and typically ±10–20 Ma for single samples, it would appear from the data that the Variscan S-type granitoids (333–367 Ma) are systematically older than the Variscan I-type granitoids (308–345 Ma). This feature is interpreted in terms of a prograde temperature evolution in the deeper parts of the post-collisional Variscan crust. In accordance with recently published zircon ages, this study shows that the Western Carpathian basement must be viewed as a distinct "eastern" tectonomagmatic province in the Variscan collision zone, where the post-collisional crustal melting processes occurred ~20 Ma earlier than in the central sector (South Bohemian Batholith, Hohe Tauern Batholith).

Cadomian Lower-crustal Contributions to Variscan Granite Petrogenesis (South Bohemian Batholith, Austria): a Comment

of opx, through partial melting of heterogeneous, highPREAMBLE K, biotite-bearing, lower-crustal source rocks. We inKlotzli et al. (2001) present a genetic model for a particular terpreted the orthopyroxenes in the SQMD rocks as granitic rock unit within the Variscan (Carboniferous) having crystallized early, and probably at somewhat South Bohemian Batholith. We studied and published higher P than the final emplacement of the magma. In on these same rocks some years previously (Finger & the course of protracted cooling and magmatic crysClemens, 1995). The interpretations of Klotzli et al. (2001) tallization, in a crustal section already heated by regional contrast so significantly with our own that we consider metamorphism, the orthopyroxenes were affected by it necessary to comment briefly on their paper. various exsolution and recrystallization processes. They The rock unit in question is the so-called Sarleinsbach quartz monzodiorite (SQMD), first described by Frasl & reacted with the residual melts and became marginally Finger (1988). This unit is an opx-bearing facies of the overgrown and replaced by clinopyroxene, amphibole Weinsberg granite—a very coarse-grained biotite granite and biotite–quartz symplectites (Haunschmid & Finger, and the most prominent rock type in the batholith. As 1994). Such magmatic overgrowths are to be expected with all granitic units assigned to the Weinsberg type, because aH2O and aKAlSi3O8 both increase in the the SQMD contains large K-feldspar phenocrysts. On residual liquids as crystallization proceeds toward the chemical variation diagrams, the data for the SQMD solidus. join smoothly with the other Weinsberg points, at the In contrast with this, Klotzli et al. (2001) hypothesize more mafic end of the compositional spectrum. Although that the pyroxenes are xenocrystal in origin, derived from such a variation is by no means definitive, this and the a Cadomian magmatic rock that crystallized some 200 field and textural evidence suggest that the SQMD is Myr earlier, and accidentally incorporated into the part of the Weinsberg granite. This has long been held Weinsberg magma. Klotzli et al. arrive at this conclusion to be the case. mainly through a geochronological study of the zircons of the SQMD. This work reveals the existence of a population of zircons with Variscan ages (interpreted as dating the formation and intrusion of the granite magma), ORIGIN OF PYROXENES IN THE and morphologically distinct (so-called J4-type) zircons SQMD with Cadomian ages (>520 Ma), clearly pre-dating the Variscan magmatic cycle. They observed the relationship We (Finger & Clemens, 1995) considered that the that pyroxene-rich variants of SQMD have a sysWeinsberg magma (as a whole) originated under granulite-facies conditions, i.e. within the P–T stability field tematically greater proportion of J4-type zircons than

Post‐collisional granite generation and HT–LP metamorphism by radiogenic heating: the Variscan South Bohemian Batholith

The Palaeozoic Variscan Orogen of Europe is a well‐documented example of a collision zone characterized by widespread late‐orogenic high‐temperature metamorphism and associated crustal magmatism. However, the heat source is still under debate. Based on the Bohemian Massif in the internal zone of the Variscides as case study, we present geological, geochemical, petrological and geochronological data arguing against a substantial mantle involvement in metamorphism and magma genesis in the area of the South Bohemian Batholith. In order to provide an alternative explanation consistent with heat transfer mechanism, we apply a two‐dimensional thermal–kinematic modelling approach. The model calculates the transient lithospheric temperature field during crustal thickening and subsequent thinning by erosiorf from material parameters and boundary conditions specific to the study area. Model results show that the increased contribution of radiogenic heat in the thickened crust can indeed cause a substantial temperature increase in the middle and lower crust. Model predictions are in good agreement with observations, e.g. the P–T–t evolution of the country rocks, the formation of syn‐kinematic migmatites, the large volumes of peraluminous granites derived from dehydration melting of metasediments and the small volumes of lamprophyric melts from the mantle lithosphere. The results of this study emphasize the importance of radiogenic heat as the source for high‐temperature metamorphism and granite petrogenesis in the Bohemian Massif and potentially in other areas of the Variscan Orogen.

The geometric evolution of structures in granite during continuous deformation from magmatic to solid-state conditions; an example from the central European Variscan Belt

Intrusive and in-situ granites of the late-Variscan South Bohemian batholith (Austria) were deformed continuously during crystallization and after solidification by horizontal crustal shortening. When the proportion of melt was relatively high, the granitic magmas accomodated deformation in (low-angle) thrust zones. With decreasing melt content shearing became more localized and the dip angle of shear planes increased during the formation of submagmatic reverse shear zones. During this submagmatic stage the orientation of shear planes was variable. They probably rotated from flat in the magmatic stage to a steeper orientation perpendicular to the shortening direction and/or parallel to post-magmatic conjugated strike-slip zones, that became active at high-T, solid state conditions. Thermobarometry and the compositions of plagioclase phenocrysts, subgrains, and recrystallized grains indicate that the magmatic, submagmatic, and high-T, solid state strike-slip deformation occurred at similar P-T conditions. Conjugate strike-slip zones are well known structures of solid-state crustal shortening. In syn-tectonic plutons, they may have magmatic or submagmatic precursors, and because they preferably crosscut the plutons rather than the country rock, this could be the result of earlier magmatic or submagmatic deformation, that evolves to solid state shear zones. Hot (migmatitic) crust during magma emplacement, coupled with low cooling rates and a constant direction of crustal shortening probably aids the generation and preservation of related magmatic, submagmatic, and postmagmatic structures.

Geochemical characterization and origin of granitoids from the South Bohemian Batholith in Lower Austria

Major and 31 minor elements have been determined in 39 large samples of Variscan granitoids from 6 plutons or intrusions from the South Bohemian Batholith (Rastenberg, Weinsberg, Mauthausen, Schrems, Eisgarn and Gebharts). The granitoids are mainly granites but also diorites, tonalites, trondhjemites, granodiorites. Average concentrations of Ba, Th, U, La, Ce, Pb, Nd, Sr and K in the Weinsberg, Mauthausen and Schrems granites exceed those in average felsic I- and S-type granites by factors ranging between 2.1 and 1.3. The granites melts formed at waterundersaturated conditions and intruded at 10 to 15 km depth during late-tectonic and post-tectonic phases of the Variscan orogeny (about 330 to 300 Ma ago). Hydrothermal or low temperature alteration is excluded for the majority of samples from a study of oxygen isotopes. The thickness of the plutons is estimated at about 6 km from heat balance constraints. By analogy with experimental partial melting, three different sources of the granitoids can be identified and chemically characterized: (1) The trondhjemites, tonalites and diorites in the early Rastenberg pluton are products of 15 to 40% melting respectively of a mafic (partly amphibolitic) lower crust. Redwitzites from the West Bohemian Massif which are comparable in age partly resemble the Rastenberg rocks. The mafic sources of the Rastenberg granitoids and redwitzites are crustally contaminated as reflected in their Sr-Nd isotopes. (2) The very large syn-tectonic Weinsberg pluton was formed from about 30% partial melting of a tonalitic lower crust at 800 to 850°C. Its low proportion of ca. 10% restite has a ferrodioritic composition. The post-tectonic fine-grained Mauthausen and Schrems granites which tend to a granodioritic mode, are very low in restite and are also products of melting of a tonalitic source. (3) The youngest (leuco-)granite, the Eisgarn pluton (high in Si, P, Li, Rb, Cs, U,87Sr/86Sr and low in Ca, Sr, Ba) reflects a pelitic source. The change from mafic to tonalitic to pelitic source composition for the granitoid sequence may indicate that the depth of melt formation decreased with time. The concentration of heavy rare earth elements decreased from Weinsberg to Eisgarn granites which indicates an increasing proportion of garnet in the source. The orogenic heat conformable with a heat flow of about 100 mWm-2 was provided by mafic intrusions. An alternative would be a drastic increase of the crustal thickness which cannot be recognized by barometry of the associated metamorphic rocks. Exposed metamorphic country rocks occur in higher amphibolite facies indicating about 5 kbar pressure. Mafic intrusions contain gabbros (Kleinzwettl) or have formed (quartz-)diorites (Gebharts), the latter being contaminated by granitic melts from partial melting of the wall rocks (MASH process). Largescale contamination by crustal materials can be observed in δ18O and in Sr-Nd isotopes. The major mafic activity was probably caused by depression of solidus temperatures in the mantle wedge above a subduction zone where water was available from dehydration of subducted ocean crust. This water initiated partial melting of ultramafic rocks and metasomatism in the uppermost mantle above the level of melting. The water also mobilized highly incompatible elements (Ba, Th, U, La, Ce, Pb, Nd, Sr and K) from the uppermost mantle and transported them into the lower crust. Indicators of a nearby subduction or collision zone of Late Variscan age in addition to the specific association of granitoidal rocks are abundant upper mantle tectonites. An alternate or additional source of metasomatic fluids may have been dehydration of lower crustal rocks during Variscan high-grade metamorphism.