NW Bohemian Massif Research Articles

Crustal vs. mantle contributions in the Erzgebirge/Krušné hory Mts. magmatism: Implications for generation of zoned, A-type silicic rocks in the late-Variscan Altenberg-Teplice Caldera, Central Europe

Late-Variscan (S-, A-type) granites and rhyolitic ignimbrites/lavas (~325 Ma) of the Altenberg-Teplice Caldera (Erzgebirge/Krušné hory Mts., NW Bohemian Massif) were investigated by using mineral and whole-rock chemistry and Nd–Pb isotopic systematics. Focus was given to the normally zoned Teplice Rhyolite (TR) volcanic successions and, to a lesser extent, the younger comagmatic rapakivi granite porphyry to assess the involvement of magma mixing and to identify differentiation processes in the petrogenesis of A-type rocks. Mineralogical, geochemical, and isotope data disprove A-type magma evolution by closed-system fractional crystallization and favor crystal-liquid separation under complex open-system conditions. Although no compelling mineralogical or chemical evidence for basic-acid magma mixing was found in the caldera, all rocks have constant isotopic fingerprints (εNd(i) = −1.9 to −3.7, 207Pb/204Pb(i) = 15.53–15.65) and two-stage Nd model ages (TDM2 = 1.2–1.3 Ga) that represent intermediate ranges between crustal and mantle components of the Cadomian and Lower Paleozoic basement. Thus, the caldera rocks were derived from a heterogeneous source region without basic magma input. The A-type magmas were generated chiefly by biotite dehydration melting of metasediments and non-restitic granodioritic-tonalitic rocks, likely prompted by heat advection from basement exhumation. Contrasting degrees of fertility, together with the distribution of heterogeneous protoliths relative to the heat source, played a critical role in the production of nearly coeval S- and A-type magmas in the Erzgebirge. The shift from S- to A-type magmatism was then controlled by a change in protolith compositions during the late-orogenic regime. Origin of the rapakivi granite porphyry (An27–43, similar to the TR range) can be explained by self-mixing in the lowest zone of the chamber owing to ponding/recharge of hotter, isotopically similar granitic magma. This study reinforces the idea that generation of A-type rocks and zoned silicic systems does not necessarily require basic magma input despite heterogeneous isotopic fingerprints and presence of rapakivi-textured rocks. The concept of S- and A-type magmas requiring significantly different pressures, H2O contents, and protoliths in the source region does not conform to the Erzgebirge/Krušné hory Mts. magmatism. Similar pressures and water-absent conditions at the melting site suggest that mixed protoliths and their melting temperatures were the decisive variables in magma production.

Cryptic metasomatic agent measured in situ in Variscan mantle rocks: Melt inclusions in garnet of eclogite, Granulitgebirge, Germany

AbstractGarnet of eclogite (formerly termed garnet clinopyroxenite) hosted in lenses of orogenic garnet peridotite from the Granulitgebirge, NW Bohemian Massif, contains unique inclusions of granitic melt, now either glassy or crystallized. Analysed glasses and re‐homogenized inclusions are hydrous, peraluminous, and enriched in highly incompatible elements characteristic of the continental crust such as Cs, Li, B, Pb, Rb, Th, and U. The original melt thus represents a pristine, chemically evolved metasomatic agent, which infiltrated the mantle via deep continental subduction during the Variscan orogeny. The bulk chemical composition of the studied eclogites is similar to that of Fe‐rich basalt and the enrichment in LILE and U suggest a subduction‐related component. All these geochemical features confirm metasomatism. In comparison with many other garnet+clinopyroxene‐bearing lenses in peridotites of the Bohemian Massif, the studied samples from Rubinberg and Klatschmühle are more akin to eclogite than pyroxenites, as reflected in high jadeite content in clinopyroxene, relatively low Mg, Cr, and Ni but relatively high Ti. However, trace elements of both bulk rock and individual mineral phases show also important differences making these samples rather unique. Metasomatism involving a melt requiring a trace element pattern very similar to the composition reported here has been suggested for the source region of rocks of the so‐called durbachite suite, that is, ultrapotassic melanosyenites, which are found throughout the high‐grade Variscan basement. Moreover, the Th, U, Pb, Nb, Ta, and Ti patterns of these newly studied melt inclusions (MI) strongly resemble those observed for peridotite and its enclosed pyroxenite from the T‐7 borehole (Staré, České Středhoři Mountains) in N Bohemia. This suggests that a similar kind of crustal‐derived melt also occurred here. This study of granitic MI in eclogites from peridotites has provided the first direct characterization of a preserved metasomatic melt, possibly responsible for the metasomatism of several parts of the mantle in the Variscides.

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

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

Depleted subcontinental lithospheric mantle and its tholeiitic melt metasomatism beneath NE termination of the Eger Rift (Europe): the case study of the Steinberg (Upper Lusatia, SE Germany) xenoliths

The ca. 30 Ma Steinberg basanite occurs at the NE termination of the Eger (Ohře) Rift in the NW Bohemian Massif, Central Europe, and belongs to the Cenozoic alkaline Central European Volcanic Province. The basanite hosts a suite of mantle xenoliths, most of which are harzburgites containing relatively magnesian olivine (Fo 90.5–91.6) and Al-poor (0.04–0.13 a pfu) orthopyroxene (mg# 0.90–0.92). Some of these harzburgites also contain volumetrically minor clinopyroxene (mg# 0.92–0.95, Al 0.03–0.13 a pfu) and have U-shaped LREE-enriched REE patterns. The Steinberg harzburgites are typical for the Lower Silesian - Upper Lusatian domain of the European subcontinental lithospheric mantle. They represent residual mantle that has undergone extensive partial melting and was subsequently affected by mantle metasomatism by mixed carbonatite-silicate melts. The Steinberg xenolith suite comprises also dunitic xenoliths affected by metasomatism by melt similar to the host basanite, which lowered the Fo content in olivine to 87.6 %. This metasomatism happened shortly before xenolith entrainment in the erupting lava. One of the xenoliths is a wehrlite (olivine Fo 73 %, clinopyroxene mg# 0.83–0.85, subordinate orthopyroxene mg# 0.76–0.77). Its clinopyroxene REE pattern is flat and slightly LREE-depleted. This wehrlite is considered to be a tholeiitic cumulate. One of the studied harzburgites contains clinopyroxene with similar trace element contents to those in wehrlite. This type of clinopyroxene records percolation of tholeiitic melt through harzburgite. The tholeiitic melt might be similar to Cenozoic continental tholeiites occurring in the Central European Volcanic Province (e.g., Vogelsberg, Germany).

Zircon (re)crystallization during short-lived, high-P granulite facies metamorphism (Eger Complex, NW Bohemian Massif)

The Eger Complex in the northwestern Bohemian Massif consists mainly of amphibolite facies granitic gneisses containing a subordinate volume of felsic granulites. Microstructural changes and modelling of metamorphic conditions for both rock types suggest a short-lived static heating from ~760 to ~850 °C at a constant pressure of ~16 kbar, which led to the partial granulitization of the granitoid rocks. Detailed study of the protolith zircon modifications and modelling of the Zr re-distribution during the transition from amphibolite to granulite facies suggests that the development of c. 340 Ma old zircon rims in the granulite facies sample is the result of recrystallization of older (c. 475 Ma) protolith zircon. This study suggests that the partial granulitization is a result of a short exposure of the Eger Complex metagranitoids to a temperature of ~850 °C at the base of an arc/fore-arc domain and their subsequent rapid exhumation during the Lower Carboniferous collision along the western margin of the Bohemian Massif.

Erratum to: “Petrology and geochemistry of the Tertiary alkaline intrusive rocks at Doupov, Doupovské hory Volcanic Complex (NW Bohemian Massif)” Journal of Geosciences 55: 251–278

Rock type Melteigite Urtite Essexite Sodalite monzosyenite Metasom. r. “Pseudolamprophyre” Hauyne-phyric phonolite 87Sr/86Sr (measured) 0.703823 0.703608 0.704384 0.705060 0.704241 0.703884 1 sigma 0.000066 0.000078 0.000068 0.000063 0.000082 0.000037 2 S(M) 0.000010 0.000015 0.000013 0.000009 0.000016 0.000010 143Nd/144Nd (measured) 0.512793 0.512789 0.512729 0.512694 0.512811 0.512722 1 sigma 0.000045 0.000061 0.000040 0.000065 0.000052 0.000039 2 S(M) 0.000011 0.000011 0.000009 0.000019 0.000013 0.000008 (Sr/Sr)30 0.703766 0.703577 0.704283 0.704866 0.704029 0.703787 (Nd/Nd)30 0.512770 0.512771 0.512708 0.512676 0.512788 0.512713

Petrology and geochemistry of the Tertiary alkaline intrusive rocks at Doupov, Doupovské hory Volcanic Complex (NW Bohemian Massif)

*Corresponding author Field observations, petrography, geochemical data and the results of K–Ar dating are presented for Oligocene (c. 30–29 Ma) alkaline intrusions at the former township of Doupov (Duppau) in the central part of the Doupovske hory Volcanic Complex (DHVC). The Doupov Intrusive Complex (DIC) is a very limited in area (< 2 km 2 ) but is petrographically highly variable. Several phaneritic rock types have been identified, including clinopyroxenite (both the clinopyroxenite-dominated breccia and xenoliths in younger intrusive phases), essexite, sodalite-bearing monzodiorite, sodalite monzosyenite to nepheline–sodalite syenite, and highly mafic to felsic nephelinolites (melteigite–ijolite–urtite). These rocks are accom panied by vent breccias, and a thermal and metasomatic aureole with zones of pervasive phlogopitization. Phaneritic intrusions and their host rocks are cut by thin aphanitic dykes ranging from mafic alkaline lamprophyres to felsic trachytes and phonolites. All the intrusions are characterized by undersaturated alkaline compositions with high abundances of incompatible elements. Geochemical variations and also subtle differences in the isotopic compositions of Sr and Nd suggest that the less undersaturated essexitic rocks and sodalite syenitoids do not form a common evolutionary line with highly undersaturated nephelinolites. Nephelinolites are isotopically close to the “European Asthenospheric Reservoir” or the “Common Mantle Reservoir” compositions and interaction of their parental magmas with the continental crust was negligible. Magmatic evolution of nephelinolites with “dry” mineral assemblages was dominated by fractionation of clinopyroxene accompanied at the early stage by olivine, and later by titanite, magnetite and apatite. Essexite is isotopically more different though still within the field of mafic lavas from DHVC and other volcanic complexes in Central and Western Europe. Either its parental magma originated in heterogeneous mantle with more distinct lithospheric signature or this magma reacted with the continental crust during its ascent and fractionation. Sodalite monzosyenite displays significantly higher 87 Sr/ 86 Sr ratios and its parental magma probably interacted even more intensively with the crustal rocks. The K–Ar ages of phaneritic intrusive rocks from Doupov are c. 30–29 Ma. Intrusions are older than the tephrite–basanite and foidite lavas forming the upper parts of the DHVC. The rock association resembles that accompanying many carbonatite intrusions around the world. However, brecciation and high-temperature metasomatic overprint including the extensive phlogopitization may be related to ijolitic magma. Nevertheless, the presence of carbonatite at some deeper level cannot be excluded.

Dioritic intrusions of the Slavkovský les (Kaiserwald), Western Bohemia: their origin and significance in late Variscan granitoid magmatism

Mafic and intermediate intrusions occur in the Slavkovský les as dykes, sills and minor tabular bodies emplaced in metamorphic rocks or enclosed in late Variscan granites near the SW contact of the Western Krusne hory/Erzgebirge granite pluton. They are similar in composition and textures to the redwitzites defined in NE Bavaria. Single zircon Pb-evaporation analyses constrain the age of a quartz monzodiorite at 323.4 ± 4.4 Ma and of a granodiorite at 326.1 ± 5.6 Ma. The P–T range of magma crystallization is estimated at ~1.4–2.2 kbar and ~730–870°C and it accords with a shallow intrusion level of late Variscan granites but provides lower crystallization temperatures compared to the Bavarian redwitzites. We explain the heterogeneous composition of dioritic intrusions in the Slavkovský les by mixing between mafic and felsic magmas with a minor effect of fractional crystallization. Increased K, Ba, Rb, Sr and REE contents compared to tholeiitic basalts suggest that the parental mafic magma was probably produced by melting of a metasomatised mantle, the melts being close to lamprophyre or alkali basalt composition. Diorites and granodiorites originated from mixed magmas derived by addition of about 25–35 and 50 vol.%, respectively, of the acid end-member (granite) to lamprophyre or alkali-basalt magma. Our data stress an important role of mafic magmas in the origin of late Variscan granitoids in NW Bohemian Massif and emphasize the effect of mantle metasomatism on the origin of K-rich mafic igneous rocks.

On the interpretation of normal and inverse magnetic fabric in dikes: Examples from the Eger Graben, NW Bohemian Massif

Recent studies of igneous rocks indicate that the predominant occurrence of normal/inverse fabric in dikes may either reflect the presence of multi-domain (MD)/single-domain (SD) grains or it may result from different orientation mechanisms of magnetic minerals in magmas of different viscosities. The ambiguity in physical vs. geological cause of normal/inverse magnetic fabric must be answered before any successful geological interpretation of magnetic fabric can be made. In order to address this problem, we studied magnetic fabric of selected dikes associated with the SW–NE trending Eger Graben (NW Bohemian Massif). The studied area offered very extensive collection of rock types: basanite, bostonite, camptonite, tinguaite, and trachybasalt. Magnetic susceptibility varies according to rock type and reflects the relative contents of magnetic minerals. In most cases, titanomagnetite with variable Ti content was identified as main magnetic carrier. The degree of anisotropy is relatively low, in most cases less than 10%, the shape of anisotropy ellipsoid ranges from slightly prolate to neutral and oblate. Several different types of magnetic fabric (using anisotropy of low-field magnetic susceptibility, AMS) were observed in studied dikes: so-called normal and inverse magnetic fabrics and anomalous magnetic fabric. Comparing all studied sites it seems that the type of magnetic fabric is lithology-dependent. Normal magnetic fabric with magnetic foliations and subhorizontal magnetic lineations both parallel to the dike margins was found in bostonite and trachybasalt dikes. Inverse magnetic fabric with magnetic lineations and magnetic foliations perpendicular to the dike margins was found in camptonite dikes. Anisotropy of anhysteretic remanent magnetization (AMR) indicate that the observed inverse magnetic fabric may be caused by the presence of SD magnetic grains; AMR fabric being normal with respect to dike margins. In contrast to that no single-domain particles were revealed using frequency dependence and anhysteretic susceptibility measurements. The AMS measured in variable weak magnetic fields is field dependent for camptonite dike and field independent for other rock types, i.e. bostonite, basanite, tinguaite, and trachybasalt. For further flow direction and tectonic interpretations of magnetic fabric in dikes it is suggested to use preferably the AMR fabric (at least for dikes which demonstrate significant field dependence of AMS) as it reflects the ‘true’ rock fabric more accurately than AMS fabric.

Petrology, geochemistry and zircon age for redwitzite at Abertamy, NW Bohemian Massif (Czech Republic): tracing the mantle component in Late Variscan intrusions

A small body of mafic texturally and compositionally varied igneous intrusive rocks corresponding to redwitzites occurs at Abertamy in the Western pluton of the Krušné hory/Erzgebirge granite batholith (Czech Republic). It is enclosed by porphyritic biotite granite of the older intrusive suite in the southern contact zone of the Nejdek-Eibenstock granite massif. We examined the petrology and geochemistry of the rocks and compared the data with those on redwitzites described from NE Bavaria and Western Bohemia. The redwitzites from Abertamy are coarse- to medium-grained rocks with massive textures and abundant up to 2 cm large randomly oriented biotite phenocrysts overgrowing the groundmass. They are high in MgO, Cr and Ni but have lower Rb and Li contents than the redwitzites in NE Bavaria. Compositional linear trends from redwitzites to granites at Abertamy indicate crystal fractionation and magma mixing in a magma chamber as possible mechanisms of magma differentiation. Plots of MgO versus SiO 2, TiO 2, Al 2O 3, FeO, CaO, Na 2O, and K 2O indicate mainly plagioclase and orthopyroxene fractionation as viable mechanisms for in situ differentiation of the redwitzites. The porphyritic biotite monzogranite enclosing the redwitzite is the typical member of the early granitic suite (Older Intrusive Complex, OIC ) with strongly developed transitional I/S-type features. The ages of zircons obtained by the single zircon Pb-evaporation method suggest that the redwitzites and granites at Abertamy originated during the same magmatic period of the Variscan plutonism at about 322 Ma. The granitic melts have been so far mainly interpreted to be formed by heat supply from a thickened crust or decompression melting accompanying exhumation and uplift of overthickened crust in the Krušné hory/Erzgebirge due to a previous collisional event at ca. 340 Ma. The presence of mafic bodies in the Western pluton of the Krušné hory/Erzgebirge batholith confirms a more significant role of mantle-derived mafic magmas in heating of the sources of granitic melts than previously considered.

The causal link between HP-HT metamorphism and ultrapotassic magmatism in collisional orogens: case study from the Moldanubian Zone of the Bohemian Massif

In the Moldanubian Zone of the Bohemian Massif occur Variscan granulites and ultrapotassic magmatites (amphibole-bearing durbachitic suite and two-pyroxene syenitoids) in a close spatial and temporal relationship. The protolith to the most widespread felsic garnet-kyanite-mesoperthite granulites, which equilibrated at P >1.5 GPa and Tc. 1000°C, was analogous to meta-igneous rocks occurring in the Saxothuringian Zone of the NW Bohemian Massif. For the most basic ultrapotassic rocks, the high contents of Cr and Ni as well as high mg# point to derivation from an olivine-rich source (i.e. a mantle peridotite). On the other hand, elevated concentrations of U, Th, light rare earth elements (LREE) and large ion lithophile elements (LILE), pronounced depletion in Ti, Nb and Ta as well as high K 2 O/Na 2 O and Rb/Sr ratios apparently contradict the mantle origin. This dual geochemical character and crustal-like isotopic compositions require melting of anomalous lithospheric mantle sources, metasomatized and contaminated by mature crustal material. Both Moldanubian granulites and ultrapotassic rocks show mutually complementary depletions and enrichments in some trace elements (Cs, Rb, Th, U and Pb). Late Devonian-Early Carboniferous Andean-type subduction is thought to have resulted in continental collision and HP metamorphism of upper crustal, largely meta-igneous lithologies. The subduction of the mature crustal material caused direct contamination and metasomatism in the overlying lithospheric mantle wedge. Shortly after the granulite-facies metamorphic peak (at c . 340 Ma) and the slab break off, these metasomatized and contaminated mantle domains were melted by advected heat from the invading asthenosphere, generating ultrapotassic intrusions, closely related to the granulite occurrences in space and time. This tectonic, petrological and geochemical model is outlined.

Zircon and titanite geochronology of the Furstenstein granite massif, Bavarian Forest, NW Bohemian Massif: Pulses of the late Variscan magmatic activity

The Furstenstein granite massif is the largest late Variscan granitoid body exposed in the Moldanubian basement of the Bavarian Forest. The granite massif comprises different rock types, ranging from diorite to granite in composition. This study provides precise zircon and titanite ages, obtained by the U-Pb isotopic dilution and Pb-Pb evaporation methods, and Nd-Sr isotopic data on this massif. Zircon dating constrains an intrusive sequence as following: Dioritic rocks crystallised about 334 to 332 Ma ago, earlier than the high-temperature peak stage of the Variscan metamorphism of the Moldanubian basement (325 to 320 Ma) in the Bavarian Forest. The medium- to coarse-grained Tittling granite was emplaced about 10 Ma later, between 324 and 321 Ma, while the medium-grained Eberhardsreuth granite crystallised between 316 and 312 Ma, contemporaneously with the coarse-grained porphyritic Saldenburg granite. Titanites from the dioritic rocks give U-Pb ages of about 321 ± 4 Ma, indicating thermal resetting or crystallisation due to the intrusion of the Tittling granite. The diorites have initial ϵ Nd values of −1.3 to −2.9 and relatively low initial 87 Sr/ 86 Sr ratios of 0.7056 to 0.7067, which suggest mantle contribution to melts or melting of young mafic lower crust. The granites have similar initial ϵ Nd values of −3.6 to −4.1 but show some variation in their Sr isotopic composition (initial 87 Sr/ 86 Sr ratios of 0.7063 to 0.7074), probably indicating the involvement of crustal material in their genesis.

A Low-Variance Mineral Assemblage with Talc and Phengite in an Eclogite from the Saxonian Erzgebirge, Central Europe, and its P-T Evolution

A light-coloured, fine-grained eclogite sample from near the village of Hammerunterwiesenthal in the Erzgebirge (NW Bohemian Massif ) preserves the low-variance mineral assemblage of garnet, omphacite, phengite, talc, amphibole, clinozoisite, quartz, rutile, and accessory phases. Porphyroblasts of amphibole, clinozoisite, and phengite formed during a late stage (III) of metamorphism. Paragonite joined the assemblage late in this stage (IIIb). The chemical zonation of the minerals was carefully studied. Various geothermobarometric methods were applied, especially involving phengite and talc. The constrained P–T path for the eclogite starts at about 480 C and 25 kbar (stage Ib), followed by a significant temperature rise (stage II) at slightly increasing pressure. At the peak P–T conditions of 720 C and 27 kbar, blastesis of amphibole, clinozoisite, and phengite was caused by infiltrating hydrous fluids. The resulting density reduction may have allowed buoyant uplift of the eclogite. Subsequently, significant cooling occurred at high pressures. Stage IIIb is characterized by P–T conditions around 520 C and 18 kbar at reduced water activities. This unusual late P–T evolution might explain the freshness of the eclogite, including the preservation of chemical zonation on the micrometre scale.

Geochemical and isotopic composition and inherited zircon ages as evidence for lower crustal origin of two Variscan S-type granites in the NW Bohemian massif

Geochemical and isotopic composition and inherited zircon ages as evidence for lower crustal origin of two Variscan S-type granites in the NW Bohemian massif

Geochemical and isotopic composition and inherited zircon ages as evidence for lower crustal origin of two Variscan S-type granites in the NW Bohemian massif

Geochemical and Nd–Sr isotopic compositions and U–Pb zircon ages of two Variscan granites (Neunburg and Oberviechtach) from southern Oberpfalz, NW Bohemian massif, have been investigated in order to place constraints on their formation and on the crustal reworking. Both granites exhibit similar mineralogical, chemical and isotopic characteristics. They have peraluminous compositions (A/CNK ratios 1.2–1.3) and display high K2O/Na2O ratios of 2.2–2.3, consistent with typical S-type granites. In terms of trace elements, they show an enrichment of LREE and strong fractionation between LREE and HREE (LaN/YbN ratios 46 to 60). Compared with the primordial mantle, distinct negative anomalies of several trace elements (Ba, Sr, Nb and Ti) are also observed in both granites. They are further characterised by low initial eNd-values of −6.2 to −8.2 and high initial 87Sr/86Sr ratios of 0.7114 to 0.7147. Zircon U–Pb data indicate that the intrusion of both granites shortly post-dates the HT–LP metamorphism of the Moldanubian basement and crystallised at about 320 Ma. The samples studied contain zircons mostly having xenocrystic cores with diverse morphologies. These inherited zircons have Early Proterozoic to Early Palaeozoic ages. This points to melting of sources comprising substantial sedimentary rocks. The LaN/YbN and TbN/YbN ratios of both granites are the highest so far reported from granitoids within this region. Melting of lower crustal rocks leaving garnet as a restite phase in the source provides a viable mechanism to reproduce the REE characteristics.

Domainal variations of U–Pb monazite ages and Rb–Sr whole‐rock dates in polymetamorphic paragneisses (KTB Drill Core, Germany): influence of strain and deformation mechanisms on isotope systems

Polyphase metamorphic paragneisses from the drill core of the continental deep drilling project (KTB; NW Bohemian Massif) are characterized by peak pressures of about 8 kbar (medium‐P metamorphism) followed by strain accumulation at T &gt;650 °C, initially by dislocation creep and subsequently by diffusion creep. U–Pb monazite ages and Rb–Sr whole‐rock data vary in the dm‐scale, indicating Ordovician and Mid‐Devonian metamorphic events. Such age variations are closely interconnected with dm‐scale domainal variations of microfabrics that indicate different predominant deformation mechanisms. U–Pb monazite age variations dependent on microfabric domains exceed grain‐size‐dependent age variations. In ‘mylonitic domains’ recording high magnitudes of plastic strain, dislocation creep and minor static annealing, monazite yields concordant and near concordant Lower Ordovician U–Pb ages, and the Rb–Sr whole‐rock system shows isotopic disequilibrium at an mm‐scale. In ‘mineral growth/mobilisate domains’, in which diffusive mass transfer was a major strain‐producing mechanism promoting diffusion creep of quartz and feldspar, and in which static recrystallization (annealing) reduced the internal free energy of the strained mineral aggregates, concordant U–Pb ages are Mid‐Devonian. Locally, in such domains, Rb–Sr dates among mm3‐sized whole‐rock slabs reflect post‐Ordovician resetting. In ‘transitional domains’, the U–Pb‐ages are discordant. We conclude that medium‐P metamorphism occurred at 484±2 Ma, and a second metamorphic event at 380–370 Ma (Mid‐Devonian) caused progressive strain in the rocks. Dislocation creep at high rates, even at high temperatures, does not reset the Rb–Sr whole‐rock system, while diffusion creep at low rates and stresses (i.e. low ε/Deff ratios), static annealing and the presence of intergranular fluids locally assist resetting. At temperatures above 650 °C, diffusive Pb loss did not reset Ordovician U–Pb monazite ages, and in domains of overall high imposed strain rates and stresses, resetting was not assisted by dynamic recrystallization/crystal plasticity. However, during diffusion creep at low rates, Pb loss by dissolution and precipitation (‘recrystallization’) of monazite produces discordance and Devonian‐concordant U–Pb monazite ages. Hence, resetting of these isotope systems reflects neither changes of temperature nor, directly, the presence or absence of strain.

Extending a thickened crustal bulge: toward a new geodynamic evolution model of the paleozoic NW Bohemian Massif, German Continental Deep Drilling site (SE Germany)

Fault-bounded (tectonic) metamorphic complexes assembling the NW Bohemian Massif around the German Continental Deep Drilling (KTB) site are seen to be extremely heterogeneous in tectonic and metamorphic histories. In current models, the different complexes were supposed to reflect a puzzle of small pre-Devonian microplates, and the related collision events supposedly lasted until the Carboniferous. Opposed to these models, it will be shown that all the boundaries among the complexes were formed by detachment, late in a prolonged overall geodynamic history of a thickened crustal bulge, during extensional tectonics and associated thermal events that outlasted the onset of collision in the Silurian/Lower Devonian by about 70–80 Ma. (Micro-)structures, petrological and geochronological data of individual complexes predominantly preserve the late stages rather than the unbroken record of their tectonometamorphic histories. Such partial histories strongly different among individual complexes, depict diverse snapshots taken at different places in the evolving thickened crustal bulge and at different instants in its overall evolution, and do not define different precollisional microplates. Predominantly P– T and deformation episodes after terrane juxtaposition are preserved. This article presents an integrated view of the structural geology, microscopic fabrics, P– T data and geochronology of such diverse metamorphic complexes. This integrated view provides a new understanding of (1) the tectonic evolution during Upper Silurian/Devonian collision of the Gondwana-derived Central European lithosphere with Laurussia, (2) the postaccretionary events that lasted through the Upper Carboniferous and (3), the earlier (Lower Ordovician) metamorphic and magmatic history, which is only locally recorded. Metamorphic complexes occupying the structurally highest position (upper tectonic complexes) record Devonian and earlier tectonometamorphic and magmatic events. After the Mid-Devonian, such complexes did not experience any metamorphism. The recorded Devonian events consist of subduction and exhumation of HP-rocks and their exhumation involved thrusting and extensional tectonics. Upper tectonic complexes comprise fragments of both the plate that was subducted in this period and the overriding plate. In the footwall, Carboniferous extension has brought upper tectonic complexes against metamorphic complexes that essentially record Lower and Upper Carboniferous tectonometamorphic and magmatic events (lower tectonic complexes). In the lower tectonic complexes, such events are (1) consecutive extensional stages that created at least three sets of ductile normal detachment systems intersecting each other, and various associated thermal pulses, as well as (2) predetachment events that are only recorded as petrologic and structural relics transposed during extension. Inferred as predetachment events are crustal subduction as well as stacking that outlasted thrusting and exhumation of the upper tectonic complexes. In the deeper portion of the thickened crustal bulge represented by the lower tectonic complexes, spatial variations of P– T– t– d histories during decompression occurred as a result of continued differential exhumation on the three sets of detachment systems, and also from various thermal pulses that have affected different parts of this section during progressive shearing and exhumation to various degrees. Whereas the lower tectonic complexes of the Erzgebirge Gneiss Dome preserve the record of a Lower Carboniferous history of HP-metamorphism and crustal thickening followed by extension, in those of the Oberpfalz region (KTB site), a major Upper Carboniferous thermal pulse mostly erased the pre-Upper Carboniferous strain fabrics and metamorphic record. In the Oberpfalz region, ongoing extension emplaced mid-crustal rocks that pervasively equilibrated in the Upper Carboniferous at high temperatures and low pressures (low P/T ratio) against rocks not exposed to high temperatures at that time. In summary, a prolonged postaccretionary history concealed large-scale structural relationships linked to crustal subduction and thickening associated with former juxtaposition of Gondwana lithosphere to Laurussia. Polyphase crustal extension and strike slip created the variety of different metamorphic tectonic complexes observed in the NW Bohemian Massif.

Granitoid magmatism of the NW Bohemian massif revealed: gravity data, composition, age relations and phase concept

The late Variscan granitoids of the NW Bohemian massif (northeast Bavaria, west Bohemia) constitute four partly contiguous granitoid complexes: Fichtelgebirge, northern Oberpfalz, Waidhaus-Rozvadov and Bor, incorporating more than 20 intrusive units. Based on gravity data, the granites can be modeled as steeply inclined slab- and wedge-like bodies with thicknesses between 2 and 8 km. A rough estimate of the total volume of the granites is approximately 18 000 km3. Within the four areas named above, composition ranges from less evolved dioritic rocks, known as the redwitzite suite, to highly evolved granites. The redwitzites comprise metaluminous rocks with dominant I type features. These rocks yield aberrantly old Rb–Sr ages (545–415 Ma), low initial Sr ratios (0.706–0.708) and high and variable ɛNd(T) values (1 to –4). Sr–Nd isotopes of the redwitzites show contamination trends towards the granites suggesting mixing between mantle magma and crustal granitic melts. An older plutonic association (granites of Bor, Leuchtenberg, Weissenstadt-Marktleuthen, Zainhammer) is mildly peraluminous, displaying features of both I and S type granitoids. These granites are characterized by Lower Carboniferous ages (Rb–Sr, K–Ar, U–Pb), low to intermediate initial Sr ratios (0.707–0.708) and high ɛNd(T) values (–2 to –4) which overlap with those of paragneisses from the Zone of Erbendorf-Vohenstrauss (ZEV) and from the western part of the Tepla Barrandian. It is postulated that the older granites were formed either by partial melting of ZEV or Tepla Barrandian crust, or alternatively, of preexisting mature crust contaminated by mantle material. The younger granites are strongly peraluminous and of S type. They yield Upper Carboniferous Rb–Sr and K–Ar ages and exhibit a range towards high initial Sr ratios (0.710–0.720) and low ɛNd(T) values (–4 to –8). Similar values are found in Moldanubian paragneisses and in Saxothuringian metasediments, both of which provide potential source–rock lithologies for these granites.

Eclogite facies relics and a multistage breakdown in metabasites of the KTB pilot hole, NE Bavaria: implications for the Variscan tectonometamorphic evolution of the NW Bohemian Massif

Complex reaction textures in coronitic metagabbros and retrograded eclogites of the KTB pilot and an adjacent drilling provide evidence for a multistage metamorphic history in the Variscan basement of the NW Bohemian Massif. The eclogites show complete metamorphic recrystallization leaving no textural or mineral relics of their igneous precursors. In contrast, textural relics of the igneous protolith are still preserved in the metagabbros where the metamorphic overprint under high pressure conditions achieved only partial replacement of the initial assemblage plagioclase + augite + amphibole (+olivine or orthopyroxene?) + ilmenite to form the eclogite facies assemblage garnet + omphacite + kyanite + zoisite + quartz+rutile. The garnets in the metagabbros occur in the typical ‘necklace’ fashion at the borders between the original plagioclase and mafic phase domains. In the same rocks, omphacite formed by a topotactic reaction mechanism replacing igneous augite as well as in smaller grains at the margins of the texturally igneous clinopyroxene where it occurs without fixed orientation with respect to the relict phase. Both eclogites and metagabbros show a partial breakdown under high pressure granulite (transitional to high pressure amphibolite) facies conditions during which omphacite broke down to vermicular symplectites of diopside + plagioclase. A later pervasive medium pressure metamorphism under amphibolite facies conditions led to the development of assemblages dominated by hornblende + plagioclase+titanite: phases prevailing in the overwhelming majority of the surrounding metabasites. Subsequent vein-associated retrogression produced minerals typical of the greenschist to zeolite facies. All metamorphic stages may be represented in a single thin section but although the overall reaction sequence is apparent, the obvious disequilibrium in the rocks makes the use of conventional geothermobarometry difficult. However, calculations made by assuming an approach to domainal equilibrium show that both the eclogite facies and early breakdown occurred above 10 kb. As the metamorphic unit hosting these particular metabasites is generally characterized by pressures below 10 kb these results have important implications for understanding the tectonometamorphic evolution of the region. The relationship between the studied rocks and other units in the NW Bohemian Massif exhibiting a multistage metamorphic evolution is discussed and possible tectonic models evaluated.