Central European Volcanic Province Research Articles

Petrogenesis of Tertiary Hornblende-bearing Lavas in the Rhon, Germany

Hornblende-bearing basanites and alkali basalts from the Rhön area of Germany (part of the Central European Volcanic Province; CEVP) have high TiO2 (3–4 wt %), moderately high Mg# (mostly >0·50), variable Cr (400–30 ppm) and Ni (160–20 ppm) abundances, and are enriched in incompatible trace elements and rare earth elements (REE). In primitive mantle-normalized multi-element diagrams they show a strong depletion in Ba, Rb, and K relative to trace elements of similar incompatibility. Some alkali basalts and more differentiated rocks have lower Mg# and lower abundances of Ni and Cr, and have undergone fractionation of olivine, clinopyroxene, Fe–Ti oxides and amphibole. The trace element constraints (e.g. low Nb/U and Ce/Pb and the Nd–Sr–Pb isotope compositions of some basalts) indicate that assimilation of lower crustal material has modified the composition of the primary mantle-derived magmas. Most of the basanites and alkali basalts approach the Sr–Nd–Pb isotope compositions inferred for the EAR (European Asthenospheric Reservoir) component. Variations in REE abundances and correlations between REE ratios suggest partial melting of amphibole-bearing spinel peridotite containing a significant portion of non-peridotitic material (i.e. pyroxenite). The presence of residual amphibole, indicated by depletion of K and Rb relative to Ba and Nb, requires melting close to the asthenosphere–lithosphere boundary or within the lithospheric mantle, most probably of a veined mantle source. Temperature and pressure estimates indicate a depth of melting for the most primitive lavas at ∼80 km at temperatures of ∼1290°C. Based on Sr–Nd isotope and trace element constraints it is proposed that asthenospheric melts similar in composition to EAR melts observed elsewhere in the CEVP froze at the asthenosphere–lithosphere thermal boundary as veins in the lithospheric mantle. These veins were remelted after only short storage times by ascending asthenospheric melts, imposing the prominent amphibole signature upon the basalts. The fairly radiogenic Pb isotope signatures are expected to originate from melting of enriched, low melting temperature components incorporated in the depleted upper (asthenospheric) mantle and therefore do not require upwelling of deep-seated mantle sources for the Rhön or many other continental alkaline lavas with similar Pb isotope signatures.

Petrogenesis of rift-related tephrites, phonolites and trachytes (Central European Volcanic Province, Rhön, FRG): Constraints from Sr, Nd, Pb and O isotopes

The volcanic rocks of the Rhön area (Central European Volcanic Province, Germany) belong to a moderately alkali basaltic suite that is associated with minor tephriphonolites, phonotephrites, tephrites, phonolites and trachytes. Based on isotope sytematics (87Sr/86Sr: 0.7033–0.7042; 143Nd/144Nd: 0.51279–0.51287; 206Pb/204Pb: 19.1–19.5), the inferred parental magmas formed by variable degrees of partial melting of a common asthenospheric mantle source (EAR: European Asthenospheric Reservoir of Cebriá and Wilson, 1995). Tephrites, tephriphonolites, phonotephrites, phonolites and trachytes show depletions and enrichments in some trace elements (Sr, Ba, Nb, Zr, Y) indicating that they were generated by broadly similar differentiation processes that were dominated by fractionation of olivine, clinopyroxene, amphibole, apatite and titaniferous magnetite±plagioclase±alkalifeldspar. The fractionated samples seem to have evolved by two distinct processes. One is characterized by pure fractional crystallization indicated by increasing Nb (and other incompatible trace element) concentrations at virtually constant 143Nd/144Nd~0.51280 and 87Sr/86Sr~0.7035. The other process involved an assimilation–fractional crystallization (AFC) process where moderate assimilation to crystallization rates produced evolved magmas characterized by higher Nb concentrations at slightly lower 143Nd/144Nd down to 0.51275. Literature data for some of the evolved rocks show more variable 87Sr/86Sr ranging from 0.7037 to 0.7089 at constant 143Nd/144Nd~0.51280. These features may result from assimilation of upper crustal rocks by highly differentiated low-Sr (<100ppm Sr) lavas. However, based on the displacement of the differentiated rocks from this study towards lower 143Nd/144Nd ratios and modeled AFC processes in 143Nd/144Nd vs. 87Sr/86Sr and 207Pb/204Pb vs. 143Nd/144Nd space assimilation of lower crustal rocks seems more likely. The view that assimilation of lower crustal rocks played a role is confirmed by high-precision double-spike Pb isotope data that reveal higher 207Pb/204Pb ratios (15.62–15.63) in the differentiated rocks than in the primitive basanites (15.58–15.61). This is compatible with incorporation of radiogenic Pb from lower crustal xenoliths (207Pb/204Pb: 15.63–15.69) into the melt. However, 206Pb/204Pb ratios are similar for the differentiated rocks (19.13–19.35) and the primitive basanites (19.12–19.55) implying that assimilation involved an ancient crustal end member with a higher U/Pb ratio than the mantle source of the basanites. In addition, alteration-corrected δ18O values of the differentiated rocks range from c. 5 to 7‰ which is the same range as observed in the primitive alkaline rocks. This study confirms previous interpretations that highlighted the role of AFC processes in the evolution of alkaline volcanic rocks in the Rhön area of the Central European Volcanic Province.

Plio-Pleistocene basanitic and melilititic series of the Bohemian Massif: K-Ar ages, major/trace element and Sr–Nd isotopic data

The Plio-Pleistocene volcanic rocks of the Bohemian Massif comprise a compositional spectrum involving two series: an older basanitic series (6.0–0.8Ma) and a younger, melilititic series (1.0–0.26Ma). The former consists of relatively undifferentiated basaltic rocks, slightly silica-undersaturated, with Mg# ranging from 62 to almost primitive mantle-type values of 74. The major and trace element characteristics correspond to those of primitive intra-plate alkaline volcanic rocks from a common sub-lithospheric mantle source (European Asthenospheric Reservoir – EAR) including positive Nb, and negative K and Pb anomalies. 87Sr/86Sr ratios of 0.7032–0.7034 and 143Nd/144Nd of 0.51285–0.51288 indicate a moderately depleted mantle source as for other mafic rocks of the central European volcanic province with signs of HIMU-like characteristics commonly attributed to recycling of subducted oceanic crust in the upper mantle during the Variscan orogeny. The melilititic series is characterized by higher degrees of silica-undersaturation, and high Mg# of 68–72 values, compatible with primitive-mantle-derived compositions. The high OIB-like Ce/Pb (19–47) and Nb/U (32–53) ratios indicate that assimilation of crustal material was negligible. In both series, concentrations of incompatible elements are mildly elevated and 87Sr/86Sr ratios (0.7034–0.7036) and 143Nd/144Nd ratios (0.51285–0.51288) overlap. Variations in incompatible element concentrations and isotopic compositions in the basanitic series and melilititic series can be explained by a lower degree of mantle melting for the latter with preferential melting of enriched mantle domains. The Sr and Nd isotopic compositions of both rock series are similar to those of the EAR. Minor differences in geochemical characteristics between the two series may be attributed to: (i) to different settings with respect to crust and lithospheric mantle conditions in (a) Western Bohemia (WB) and (b) Northeastern Bohemia (NEB) and the Northern Moravia and Silesia (NMS) areas, (ii) a modally metasomatized mantle lithosphere in WB in contrast to cryptically metasomatized domains in the NEB and NMS, (iii) different degrees of partial melting with very low degrees in WB but higher degrees in NEB and NMS. The geochemical and isotopic similarity between the Plio-Pleistocene volcanic rocks and those of the late Cretaceous and Cenozoic (79–6Ma) suggests that their magmas came from compositionally similar mantle sources, that underwent low degrees of melting over an interval of ∼80Ma. The Oligocene to Miocene basanitic series that accompanied the Plio-Pleistoicene basanitic series in the NMS region indicate that they shared a common mantle source. There is no geochemical evidence for thermal erosion of the lithospheric mantle or significant changes in mantle compositions within the time of a weak thermal perturbation in the asthenospheric mantle. These perturbations were caused by a dispersed mantle plume or passively upwelling asthenosphere in zones of lithospheric thinning.

Geochemistry and petrology of pyroxenite xenoliths from Cenozoic alkaline basalts, Bohemian Massif

Pyroxenites occur as rare, but important, mantle xenoliths within Cenozoic volcanic rocks of the Central European Volcanic Province (CEVP). We report the petrography, geothermobarometry, mineral chemistry as well as clinopyroxene trace-element and Sr-Nd isotopic compositions of six pyroxenite xenoliths hosted by Tertiary-Quaternary volcanic rocks from Kozakov (NE Bohemia), Dobkovicky and Kuzov (Ohre/Eger Rift), and Lutynia (SW Poland). Three Kozakov xenoliths record a complex nature and evolution. As suggested by contrasting estimated temperatures (596-663 degrees C and 1008-1067 degrees C) and textures, olivine clinopyroxenite and one websterite were likely derived from different depths and may represent crystallization products of transient melts. Another websterite contains symplectite pseudomorphs after garnet which originated by exsolution from highly aluminous clinopyroxene, and the garnet breakdown was associated with melt introduction. This websterite P-T conditions (17.3 to 21.4 kbar and 1080-1200 degrees C) correspond to derivation from depths of 55-69 km. The composition of websterite from Dobkovicky suggests its origin as a cumulate from melt derived from a highly depleted mantle source with a composition similar to Depleted MORB Mantle (DMM). The position and petrological significance of websterite from Kuzov remains unclear because of a lack of data from associated peridotites. At Lutynia, cryptic metasomatism of peridotites by a CO2-bearing alkaline melt was described previously, and the composition of melt calculated to be in equilibrium with Lutynia websterite clinopyroxene is very similar to that in equilibrium with metasomatized amphibole-bearing peridotite. Therefore, the Lutynia websterite may represent a cumulate from percolating melt that metasomatized the lithospheric mantle in this area.

Petrogenesis of Tertiary Alkaline Magmas in the Siebengebirge, Germany

Basanites from theTertiary Siebengebirge area of Germany (part of the Central European Volcanic Province; CEVP) have high Mg# (40·60), moderate to high Cr (4 300 ppm) and Ni (4 200 ppm) contents and strong light rare earth element enrichment, but systematic depletion in Rb and K relative to trace elements of similar compatibility in anhydrous mantle. Rare earth element melting models can explain the petrogenesis of these basanites in terms of partial melting of a spinel peridotite source containing residual amphibole. It is inferred that amphibole, indicated by the relative K and Rb depletion and the melting model, was precipitated in the spinel peridotite lithospheric mantle beneath the Siebengebirge, by metasomatic fluids or melts from a rising mantle diapir or plume. Alkali basalts and more differentiated rocks have lower Mg# and lower abundances of Ni and Cr, and have undergone fractionation of mainly olivine, clinopyroxene, Fe^Ti oxides, amphibole and plagioclase. Most of the basanites and alkali basalts approach the Sr^Nd^Pb isotope compositions inferred for the European Asthenospheric Reservoir component. Trace element constraints (i.e. low Nb/U and Ce/Pb ratios) and the Sr^Nd^Pb isotope composition of the differentiated rocks indicate that assimilation of lower crustal material has modified the composition of the primary mantle-derived magmas. High 207 Pb/ 204 Pb ratios in the differentiated lavas point to assimilation of ancient lower crustal components having high U/Pb and Th/Pb ratios. Relatively shallow melting of inferred amphibole-bearing spinel peridotite sources may suggest an origin from the metasomatized part of the thermal boundary layer. Application of new thermobarometric equations for the basaltic magmas indicates relatively normal mantle potential temperatures (1300^14008C); thus the inferred mantle ‘baby plume’ or ‘hot finger’ is not thermally anomalous.

Upper mantle structure below the European continent: Constraints from surface‐wave tomography and GRACE satellite gravity data

We here exploit fundamental mode Rayleigh and Love seismic wave information and the high resolution satellite global gravity model GGM02C to obtain a 1° × 1° 3‐D image of: (a) upper‐mantle isotropic shear‐wave speeds; (b) densities; and (c) density‐vScoupling below the European plate (20°N–90°N) (40°W–70°E). The 3‐D image of the density‐vScoupling provides unprecedented detail of information on the compositional and thermal contributions to density structures. The accurate and high‐resolution crustal model allows us to compute a reliable residual topography to understand the dynamic implications of our models. The correlation between residual topography and mantle residual gravity anomalies defines three large‐scale regions where upper mantle dynamics produce surface expression: the East European Craton; the eastern side of the Arabian Plate; and the Mediterranean Basin. The effects of mantle convection are also clearly visible at: (1) the Eastern Sirt Embayment; (2) the West African Craton northern margins; (3) the volcanically active region of the Canarian Archipelago; (4) the northern edge of the Central European Volcanic Province; and (5) the Northeastern part of the Atlantic Ocean, between Greenland and Iceland. Strong connections are observed among areas of weak radial anisotropy and areas where the mantle dynamics show surface expression. Although both thermal and additional dependencies have been incorporated into the density model, convective down‐welling in the mantle below the East European Craton is required to explain the strong correlation between the estimated negative mantle residual anomalies and the negative residual topography.

Petrogenesis of Mafic to Felsic Lavas from the Oligocene Siebengebirge Volcanic Field (Germany): Implications for the Origin of Intracontinental Volcanism in Central Europe

Magmatism in the Cenozoic Central European Volcanic Province (CEVP) has been related to two geodynamic scenarios, either extensional tectonics in the north Alpine realm or upwelling of deep mantle material. The Oligocene ( 30^19 Ma) Siebengebirge Volcanic Field (SVF) is a major part of the German portion of the CEVP and consists of erosional remnants of mafic to felsic volcanic edifices. It covers an area of 35 km (NW^SE) by 25 km (SW^NE) with eruptive centres concentrated near the eastern shore of the Rhine river in the vicinity of the city of Bonn. Mafic rocks in the SVF comprise strongly SiO2-undersaturated basanites to alkaline basalts. Occurrences of alkaline basalts are confined to an inner NW^SE-striking zone, whereas the more SiO2-undersaturated basanites dominate the western and eastern periphery of the SVF. Radiogenic isotope compositions (Sr/Sr 0·70335^ 0·70371; eNd þ3·1 to þ4·5; eHf þ6·5 to þ8·0; Pb/Pb 19·46^19·69; Pb/Pb 15·63^15·66; Pb/Pb 39·34^ 39·62) indicate a common asthenospheric mantle end-member with HIMU-like characteristics for all mafic rocks, similar to the European Asthenospheric Reservoir (EAR). A lithospheric mantle source component with a residual K-bearing phase (phlogopite or amphibole) is inferred from negative K anomalies. Incompatible trace element modelling indicates that melting took place in the spinel^garnet transition zone with low degrees of melting at higher pressures generating the basanitic magmas (LaN/YbN1⁄4 20^25), whereas the alkaline basalts (LaN/YbN1⁄414^18) are the result of higher melting degrees at shallower average melting depths. Differentiation of basanitic primary melts generated tephritic to tephriphonolitic magmas that, for instance, erupted at the Lo« wenburgVolcanic Complex in the central SVF. Latites and trachytes, such as the prominent Drachenfels andWolkenburg protrusions, are more common in the central portion of the SVF.These compositions originate from parental alkaline basaltic melts. All differentiated samples show evidence for crustal contamination, possibly with lowerto mid-crustal material comprising mafic granulites as found in Eifel basalt xenoliths and metapelites. Based on the spatial and temporal distribution of the various volcanic rock types, a model for the temporal evolution of the SVF can be proposed. During the initial phase of volcanism, low-degree basanitic melts were generated as a result of decompression following tectonic rifting and formation of the Cologne Embayment, a northward extension of the Rhine Graben. In a second stage, alkali basalts were generated at shallower depths and higher degrees of melting as a result of continued lithospheric thinning and passive upwelling of asthenospheric mantle. These conclusions strengthen previous models suggesting

Reconstruction of the Landeskrone Scoria Cone in the Lusatian Volcanic Field, Eastern Germany — Long-term degradation of volcanic edifices and implications for landscape evolution

Remnants of numerous monogenetic volcanoes are preserved in the Central European Volcanic Province (CEVP). The Landeskrone Hill, a monogenetic scoria cone in the Lusatian Volcanic Field, is reconstructed here. This was done using measurements of the dip of columnar jointing of lava lake basalts and the detailed mapping of the volcaniclastic rocks. The reconstruction implies a large scoria cone and a lava lake; filling the crater with a thickness of more than 110m. Volcanic activity was characterized by strombolian eruptions possibly after an opening phreatomagmatic phase. A scoria cone was developed in an initial maar-diatreme volcano. A late phreatomagmatic phase could explain the unusual modified crater of the scoria cone. The conduit was emptied and so the crater was widened in this late eruption stage. An increased magma flux probably induced a change in the eruptive style and, finally, the crater was filled in a single event by a lava lake. This effusive phase completed the multi-stage volcano development.The 34Ma-old volcano is an excellent example for the persistence of relicts of monogenetic volcanoes over a long period of time. There are some other remnants of monogenetic scoria cones that survived degradation processes in the area. With respect to the present surface, the reconstruction of the paleosurface implies low uplift and erosion rates of about 3mm/ka since the Oligocene. These denudation values support and expand on the previously published fission track data (prior to the Upper Cretaceous) and glacial-sedimentological data on neotectonic movements since the Middle Pleistocene. The erosion rate estimated by physical volcanological data implies stagnation of tectonic uplift from the Upper Paleogene to the Middle Neogene and a reactivation of tectonic movement for the Lusatian Massif in the Middle Pleistocene. Thus, the reconstructed edifice provides a powerful tool for the study of landscape evolution by clearly defining the characteristics of the paleosurfaces at certain times.

Lithospheric Mantle Modification during Cenozoic Rifting in Central Europe: Evidence from the Księginki Nephelinite (SW Poland) Xenolith Suite

The Miocene Księginki nephelinite in SW Poland is part of the Central European Volcanic Province and occurs at the NE termination of the Cenozoic Ohře (Eger) Rift in the NW part of the Bohemian Massif. It contains numerous pyroxenite and peridotite xenoliths and clinopyroxene megacrysts. Pyroxenites (mostly olivine clinopyroxenites with no primary orthopyroxene) commonly have cumulate textures. Wehrlite and websterite xenoliths, texturally and chemically related to the pyroxenite suite, contain volcanic-textured intercumulus material similar to that of the host nephelinite. The Mg/(Mg + Fe) ratios of clinopyroxenes from the pyroxenite suite are 0·81–0·93; these are low-calcium]Ca 0·72–0·82 atoms per formula unit (a.p.f.u.)] and contain 0·22–0·37 a.p.f.u. Al. Clinopyroxene rare earth element (REE) patterns are slightly enriched in middle REE (MREE) and light REE (LREE) relative to heavy REE (HREE) (SmN/YbN = 2·4–4·3, LaN/YbN = 1·7–3·0) and are deflected downwards from Pr/Nd to La. The whole-rock REE patterns of the pyroxenite xenoliths are identical and show slight enrichment in LREE relative to HREE (LaN/Ybn = 3–5). The wehrlite cumulates are more LREE-enriched (LaN/Ybn = 13) and their REE patterns are identical to those of the host nephelinite. The peridotite xenoliths exhibit a range of textures from protogranular to mylonitic. Typically the peridotites (mostly harzburgites) consist of domains of olivine I (Fo88·9–91·9) and orthopyroxene I plus interstitial, texturally later clinopyroxene I [both with Mg/(Mg + Fe) = 0·89–0·93] and rare spinel [Cr/(Cr + Al) = 0·35–0·62]. The REE patterns of clinopyroxene I are similar to those of the pyroxenite suite. Fine grains of olivine II, commonly with forsterite contents identical to those of olivine I, but enriched in Ca, occur at the contacts between olivine I grains and in interstices or pools with glass. Clinopyroxene I commonly has spongy rims of clinopyroxene II, which are richer in Mg and poorer in Na. The peridotite xenoliths contain sparse fine-grained micrometre- to millimetre-scale aggregates consisting of (1) spinel, olivine II, clinopyroxene III and altered glass ± titanian biotite or (2) plagioclase, olivine II and clinopyroxene III ± glass. Olivine I in xenoliths containing these aggregates is typically Fo85·6–89·4, and the Mg/(Mg + Fe) ratio of pyroxenes is similar to that of olivine. The lithospheric peridotitic mantle beneath Księginki was infiltrated by alkaline silicate melts similar to the nephelinite lava prior to and during the volcanism. Pyroxenite cumulates crystallized from the melt in places where the flow was channelized. Locally melt infiltration was pervasive, leading to ‘Fe-metasomatism’ of the peridotites, which lowered the Fo content of the olivine to c. 86 %. The metasomatism was cryptic, and only the clinopyroxene trace element content was modified in peridotites not affected by pervasive melt infiltration or not in contact with pyroxenites. The mantle section affected by the metasomatism was located at a depth of ~35–50 km and was thermally equilibrated (temperatures of 1060–1120°C are recorded in both the pyroxenite and peridotite suites). The fine-grained aggregates are interpreted as remnants of the interstitial melt frozen after entrainment of the xenoliths during the eruption of the host nephelenite; they show that some of the peridotites resided at pressures allowing plagioclase crystallization, whereas the others were brought to the surface directly from spinel peridotite facies depths. The Księginki xenoliths provide a complete snapshot of the various effects of alkaline melt migrating through the lithospheric mantle during the initial stages of continental rifting.

Crustal contamination and mantle source characteristics in continental intra-plate volcanic rocks: Pb, Hf and Os isotopes from central European volcanic province basalts

We report new Os–Pb–Hf isotope data for a suite of alkaline to basaltic (nephelinites, basanites, olivine tholeiites to quartz-tholeiites) lavas from the Miocene Vogelsberg (Germany), the largest of the rift-related continental volcanic complexes of the Central European Volcanic Province (CEVP). 187Os/ 188Os in primitive (high-MgO) alkaline lavas show a much wider range than has been observed in alkaline basalts and peridotite xenoliths from elsewhere in the CEVP, from ratios similar to those in modern MORB and OIB (0.1260–0.1451; 58.9–168 ppt Os) to more radiogenic ratios (0.1908 and 0.2197; 27.6–15.1 ppt Os). Radiogenic Os is associated with high ε Hf and ε Nd, low 87Sr/ 86Sr and does not correlate with Mg ∗ or incompatible trace elements (e.g. Ce/Pb), suggesting the presence of a radiogenic endmember in the mantle rather than crustal contamination as the source of radiogenic Os. This contrasts with another high-Mg alkaline lava characterized by highly radiogenic 187Os/ 188Os (0.4344, 10.3 ppt Os), lower ε Hf and ε Nd, higher 87Sr/ 86Sr, and Pb isotope signatures than the other alkaline lavas with similar trace element composition suggestive of contamination with crustal material. Hafnium (ε Hf: +8.9 to +5.0) and Pb isotope compositions ( 206Pb/ 204Pb: 19.10–19.61; 207Pb/ 204Pb: 15.56–15.60) of the alkaline rocks fall within the range of enriched MORB and some OIB. The Vogelsberg tholeiites show even more diverse 187Os/ 188Os, ranging from 0.1487 in Os-rich olivine tholeiite (31.7 ppt) to ratios as high as 0.7526 in other olivine-tholeiites and in quartz-tholeiites with lower Os concentrations (10.3–2.0 ppt). Low- 187Os/ 188Os tholeiites show Pb–Hf isotope ratios ( 206Pb/ 204Pb:18.81; 207Pb/ 204Pb: 15.61; ε Hf: +2.7) that are distinct from those in alkaline lavas with similar 187Os/ 188Os and originate from a different mantle source. By contrast, the combination of radiogenic Os and low 206Pb/ 204Pb and ε Hf in the other tholeiites probably reflects crustal contamination. The association at Vogelsberg of primitive alkaline and tholeiitic lavas with a range of MORB- to OIB-like Os–Pb–Hf–Nd–Sr isotopic characteristics requires at least two asthenospheric magma sources. This is consistent with trace element modelling which suggests that the alkaline and tholeiitic parent magmas represent mixtures of melts from garnet and spinel peridotite sources (both with amphibole), implying an origin of the magmas in the garnet peridotite-spinel peridotite transition zone, probably at the asthenosphere–lithosphere interface. We propose that uncontaminated Vogelsberg lavas originated in ‘metasomatized’ mantle, involving a 3-stage model: (1) early carbonatite metasomatism several 10–100 Ma before the melting event (2) deposition of low-degree asthenospheric melts from carbonated peridotite at the lithosphere–asthenosphere thermal boundary produces hydrous amphibole-bearing veins or patches, and (3) remobilization of this modified lithospheric mantle into other asthenospheric melts passing through the same area later. In keeping with ‘metasomatized’ mantle models for other continental basalt provinces, we envisage that stage (2) is short-lived (few Ma), thus producing a prominent lithospheric trace element signature without changing the asthenospheric isotopic signatures. Models of this type can explain the peculiar mix of lithospheric (prominent depletions of Rb and K) and asthenospheric (OIB-like high 187Os/ 188Os, 143Nd/ 144Nd and 176Hf/ 177Hf) signatures observed in the Vogelsberg and many other continental basalt suites.

Recurrent Cenozoic volcanic activity in the Bohemian Massif (Czech Republic)

Cenozoic anorogenic volcanism of the Bohemian Massif is an integral part of the Central European Volcanic Province. The temporal and spatial distribution of volcanic rocks in the Bohemian Massif, their geochemistry and mineralogy as well as their tectonic setting and paleostress data are used to characterize and classify this volcanic activity. Three main volcanic periods can be distinguished based on K–Ar data and known paleostress fields: (i) pre-rift (79–49 Ma), (ii) syn-rift (42–16 Ma) and (iii) late-rift (16–0.3 Ma), with the youngest period further subdivided into three episodes. The dominant mafic rock types (> 7 wt.% MgO) of all periods are of nephelinite–basanite/tephrite composition. The exceptions are suites of melilitic ultramafic rocks of the pre-rift period in northern Bohemia and of the final episode of the late-rift period in western Bohemia. The most voluminous are volcanic rocks of the syn-rift period occurring in the Ohře Rift Graben. The initial 87Sr/ 86Sr (0.7032 to 0.7050) and 143Nd/ 144Nd (0.51264 to 0.51301) ratios of the mafic volcanic rocks of the Bohemian Massif are characteristic of magmas derived from a sub-lithospheric mantle source. The isotopic ratios resemble those of the HIMU mantle source ( 206Pb/ 204Pb ca. 19 to 20). These rocks have the most isotopically depleted compositions among the Central European Volcanic Province volcanics.

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The volcanic rocks of the Rhön area (Central European Volcanic Province, Germany) belong to a moderately alkali basaltic suite that is associated with minor tephriphonolites, phonotephrites, tephrites, phonolites and trachytes. Based on isotope sytematics (87Sr/86Sr: 0.7033–0.7042; 143Nd/144Nd: 0.51279–0.51287; 206Pb/204Pb: 19.1–19.5), the inferred parental magmas formed by variable degrees of partial melting of a common asthenospheric mantle source (EAR: European Asthenospheric Reservoir of Cebriá and Wilson, 1995). Tephrites, tephriphonolites, phonotephrites, phonolites and trachytes show depletions and enrichments in some trace elements (Sr, Ba, Nb, Zr, Y) indicating that they were generated by broadly similar differentiation processes that were dominated by fractionation of olivine, clinopyroxene, amphibole, apatite and titaniferous magnetite ± plagioclase ± alkalifeldspar. The fractionated samples seem to have evolved by two distinct processes. One is characterized by pure fractional crystallization indicated by increasing Nb (and other incompatible trace element) concentrations at virtually constant 143Nd/144Nd ~ 0.51280 and 87Sr/86Sr ~ 0.7035. The other process involved an assimilation–fractional crystallization (AFC) process where moderate assimilation to crystallization rates produced evolved magmas characterized by higher Nb concentrations at slightly lower 143Nd/144Nd down to 0.51275. Literature data for some of the evolved rocks show more variable 87Sr/86Sr ranging from 0.7037 to 0.7089 at constant 143Nd/144Nd ~ 0.51280. These features may result from assimilation of upper crustal rocks by highly differentiated low-Sr (< 100 ppm Sr) lavas. However, based on the displacement of the differentiated rocks from this study towards lower 143Nd/144Nd ratios and modeled AFC processes in 143Nd/144Nd vs. 87Sr/86Sr and 207Pb/204Pb vs. 143Nd/144Nd space assimilation of lower crustal rocks seems more likely. The view that assimilation of lower crustal rocks played a role is confirmed by high-precision double-spike Pb isotope data that reveal higher 207Pb/204Pb ratios (15.62–15.63) in the differentiated rocks than in the primitive basanites (15.58–15.61). This is compatible with incorporation of radiogenic Pb from lower crustal xenoliths (207Pb/204Pb: 15.63–15.69) into the melt. However, 206Pb/204Pb ratios are similar for the differentiated rocks (19.13–19.35) and the primitive basanites (19.12–19.55) implying that assimilation involved an ancient crustal end member with a higher U/Pb ratio than the mantle source of the basanites. In addition, alteration-corrected δ18O values of the differentiated rocks range from c. 5 to 7‰ which is the same range as observed in the primitive alkaline rocks. This study confirms previous interpretations that highlighted the role of AFC processes in the evolution of alkaline volcanic rocks in the Rhön area of the Central European Volcanic Province.

Two distinct Miocene age ranges of basaltic rocks from the Rhön and Heldburg areas (Germany) based on 40Ar/ 39Ar step heating data

New 40Ar/ 39Ar age data are determined for Cenozoic basaltic rocks from the Thuringian Rhön and Heldburg Gangschar (area also referred to as Grabfeld), integral parts of the Central European Volcanic Province. Applying the incremental heating technique on groundmass and plagioclase separates provided data which considerably specify our knowledge on the eruption ages in these volcanic fields and narrow down the duration of volcanic activity compared to earlier studies. All data but one outlier range between 20 and 14 Ma, being thus similar to those of the neighbouring Vogelsberg volcanic complex. The spectrum of ages is clearly divided into two distinct subsets: the Rhön ages are between 20 and 18 Ma, those of the Heldburg Gangschar are between 16 and 14 Ma. Thus, the present data clearly indicate a striking regional and temporal division of the Thuringian Miocene volcanism. The composition of the volcanic rocks in the two volcanic fields is remarkably diverse, ranging from tholeiitic basalts over alkali basalts and basanites to nephelinites. However, radiometric ages do not correlate with geochemical or petrological characteristics of the volcanics within each volcanic field, indicating that the different magma types erupted broadly contemporaneously. The outlier in age (29 Ma) is from a volcanic dyke of the NE Rhön area close to the NW end of the Thuringian Forest. However, more data are required to approve the significance of this age value, in particular since the rock showed isotopic age disturbance.

Petrogenesis of Tertiary Mafic Alkaline Magmas in the Hocheifel, Germany

Primitive nephelinites and basanites from the Tertiary Hocheifel area of Germany (part of the Central European Volcanic Province; CEVP) have high Mg-number (>0� 64), high Cr and Ni contents and strong light rare earth element enrichment but systematic depletion in Rb, K and Ba relative to trace elements of similar compatibility in anhydrous mantle. Alkali basalts and more differentiated magmatic rocks have lower Mg-number and lower abundances of Ni and Cr, and have undergone fractionation of mainly olivine, clinopyroxene, Fe–Ti oxide, amphibole and plagioclase. Some nephelinites and basanites approach the Sr–Nd–Pb isotope compositions inferred for the EAR (European Asthenospheric Reservoir) component. The Nd–Sr–Pb isotope composition of the differentiated rocks indicates that assimilation of lower crustal material has modified the composition of the primary mantle-derived magmas. Rare earth element melting models can explain the petrogenesis of the most primitive mafic magmatic rocks in terms of mixing of melt fractions from an amphibole-bearing garnet peridotite source with melt fractions from an amphibole-bearing spinel peridotite source, both sources containing residual amphibole. It is inferred that amphibole was precipitated in the asthenospheric mantle beneath the Hocheifel, close to the garnet peridotite–spinel peridotite boundary, by metasomatic fluids or melts from a rising mantle diapir or plume. Melt generation with amphibole present suggests relatively low mantle potential temperatures (<1200 � C); thus the mantle plume is not thermally anomalous. A comparison of recently published Ar/Ar ages for Hocheifel basanites with the geochemical and isotopic composition of samples from this study collected at the same sample sites indicates that eruption of earlier lavas with an EM signature was followed by the eruption of later lavas derived from a source with EAR or HIMU characteristics, suggesting a contribution from the advancing plume. Thus, the Hocheifel area represents an analogue for magmatism during continental rift initiation, during which interaction of a mantle plume with the overlying lithosphere may have led to the generation of partial melts from both the lower lithosphere and the asthenosphere.

Sources of primitive alkaline volcanic rocks from the Central European Volcanic Province (Rhön, Germany) inferred from Hf, Os and Pb isotopes

Basanites and nephelinites from the Tertiary Rhon area (Germany), which are part of the Central European Volcanic Province (CEVP), have high MgO, Ni and Cr contents and prominent garnet signatures indicating that they represent near-primary magmas formed by melting of a CO2-bearing peridotitic mantle source at high pressure. The Pb and Hf isotope (and previously published Nd and Sr isotope) ratios of the Rhon lavas are rather uniform, whereas the Os isotope composition is highly variable. For the most primitive basanites, Pb, Os and Hf isotope compositions fall within the range of enriched MORB and some OIB. Other basanites and nephelinites with low Os concentrations have distinctly more radiogenic Os (187Os/188Os: 0.160–0.469) isotope compositions, which are inferred to originate from crustal contamination. The samples with the highest Os concentrations have the lowest Os isotope ratios (187Os/188Os(23 Ma): 0.132–0.135), and likely remain unaffected by crustal contamination. Together with their fairly depleted Sr, Nd and Hf isotope ratios, the isotopic composition of the Rhon lavas suggests derivation from an asthenospheric mantle source. Prominent negative K and Rb anomalies, however, argue for melting amphibole or phlogopite-bearing sources, which can only be stable in the cold lithosphere. We therefore propose that asthenospheric melts precipitated at the asthenosphere-lithosphere thermal boundary as veins in the lithospheric mantle and were remelted or incorporated after only short storage times (about 10–100 million years) by ascending asthenospheric melts. Due to the short residence time incorporation of the vein material imposes the prominent phlogopite/amphibole signature of the Rhon alkaline basalts but does not lead to a shift in the isotopic signatures. Melting of the lithospheric mantle cannot strictly be excluded, but has to be subordinate due to the lack of the respective isotope signatures, in good agreement with the fairly thin lithosphere observed in the Rhon area. The fairly radiogenic Pb isotope signatures are expected to originate from melting of enriched, low melting temperature portions incorporated in the depleted upper (asthenospheric) mantle and therefore do not require upwelling of deep-seated mantle sources for the Rhon or many other continental alkaline lavas with similar Pb isotope signatures.

Osmium isotopic systematics of melilitites from the Tertiary Central European Volcanic Province in SW Germany

Combined Os–Pb–Sr–Nd isotopic compositions of melilitite samples from SW Germany provide new constraints on the origin of melts derived by low degrees of mantle melting in the course of Tertiary–Quaternary volcanism in Central Europe. The first Os isotopic compositions reported here show an unusually wide range in initial 187Os/ 188Os isotopic ratios of 0.119 to 0.225. The low ratios overlap those in the lithospheric mantle and the highly radiogenic ratios exceed those in ocean island basalts. Pb and Sr isotopic compositions show small variations ( 206Pb/ 204Pb=19.19–19.57; 87Sr/ 86Sr=0.7034–0.7039) whereas Nd isotopes ( ε Nd 3.4–4.2) indicate melting of overall uniform sources. A positive correlation between initial 187Os/ 188Os and 207Pb/ 204Pb isotopic ratios is interpreted as a mixing trend between melts with low and lithospheric mantle-like 187Os/ 188Os isotopic ratios and an old crustal component. Thus, we interpret the low 187Os/ 188Os isotopic ratios in some of the samples as primary mantle source-derived signatures and propose that the melilitites originated from the lithospheric mantle. Independent evidence for an lithospheric mantle source is provided by earlier reported trace element data indicating melting of metasomatized sources with residual K-bearing phases whose stability is confined to the lithospheric mantle rather than thermal plume. A contribution from deeper mantle sources is not required to explain the data; however, the lack of Os data from other more typical basalts of the Central European Volcanic Province (CEVP) precludes an evaluation of the extent of plume involvement. Remarkable characteristics of the melilitites are their high Re/Os and U/Pb ratios. It follows that overprinting of the lithospheric mantle by melts derived by low degrees of melting of metasomatized sources will eventually produce domains with evolved Os and Pb isotopic compositions as required as source material for basalts of high 238U/ 204Pb (HIMU) affinity.

The source of Cenozoic volcanism in the Ceske stredohori Mts., Bohemian Massif

The Ceske středohoři Mts. represent the largest and most complicated relict of volcanic complex within the Ohre (Eger) Rift forming an integral part of the Central European Volcanic Province of Cenozoic age. Therift-related (extrusive and intrusive) alkaline volcanic series are exclusively of mantle-derived origin. Using the stratigraphic model of effusive basaltic rock superposition in the central part of the Ceske středohoři Mts. (CAJZ et al. 1999), that relates the volcanics to the Lower Formation (LF), Upper Formation (UF), Uppermost Formation (UMF) and the Group of Late Miocene Intrusives (GLMI), geochemical characteristics were established for the individual formations. Only trachybasaltic rocks of the UF substantially differ from mutually geochemically indistinguishable basanitic rock groups comprising the LF, UMF and GLMI formations. Geochemical characteristics of the LF rocks indicate the influence of several different sources on the parental magma. The trace element and isotopic data, and modelling of magma evolution during assimilation-fractional crystallisation (AFC) process imply, that rocks of the LF, UMF and GLMI represent more primitive magma derived from depleted mantle and influenced by HIMU-like component. Rocks of the UF crystallised from more evolved magma that formed from the primitive one by the AFC process, where the upper crust was assimilated and clinopyroxene, olivine and in later stages also Fe-oxides fractionated. Two magmatic chambers were probably present beneath the Ohre Rift during the Tertiary; the deeper one closely associated with the plume component and the shallower one in deeper parts of the upper crust. According to the K-Ar dating volcanic activity in the LF, UF and UMF overlapped in the time lasting from 44 Ma to 21 Ma. Both chambers must have been active simultaneously. Intrusion of the GLMI was younger, starting at ca 13 Ma. Geochemical similarity of this rock group to the LF and UMF points to reactivation of the deeper chamber. Comparing the studied Tertiary rocks to the Permo-Carboniferous volcanics of the Bohemian Massif, a similarity was found indicating a common source material and existence of the HIMU-like source already in the Permo-Carboniferous time. Derivation of the initial magma at that time from depleted mantle is confirmed by the Nd-model ages of the most primitive samples.

Major- and trace-element systematics and isotope geochemistry of Cenozoic mafic volcanic rocks from the Vogelsberg (central Germany): Constraints on the origin of continental alkaline and tholeiitic basalts and their mantle sources

The Cenozoic basaltic province of the Vogelsberg area (central Germany) is mainly composed of intercalated olivine to quartz tholeiites and near-primary nephelinites to basanites. The inferred mantle source for the alkaline and tholeiitic rocks is asthenospheric metasomatized garnet peridotite containing some amphibole as the main hydrous phase. Trace element modelling indicates 2 to 3% partial melting for the alkaline rocks and 5 to 7% partial melting for the olivine tholeiites. Incompatible trace element abundances and ratios as well as Nd and Sr radiogenic isotope compositions lie between plume compositions and enriched mantle compositions and are similar to those measured in Ocean Island Basalts (OIB) and the Central European Volcanic Province elsewhere. The mafic olivine tholeiites have similar Ba/Nb, Ba/La and Nd–Sr isotope ratios to the alkaline rocks indicating derivation of both magma types from chemically comparable mantle sources. However, Zr/Nb ratios are slightly higher in olivine tholeiites than in basanites reflecting some fractionation of Zr relative to Nb during partial melting. Quartz tholeiites have higher Ba/Nb, Zr/Nb, La/Nb, but lower Ce/Pb ratios and lower Nd isotope compositions than the alkaline rocks which can be explained by interaction of the basaltic melt with lower (granulite facies) crustal material or partial melts thereof during stagnation within the lower crust. It appears most likely that upwelling of hot, asthenospheric material results in the generation of primitive alkaline rocks at the base of the lithosphere at depths of 75–90 km. Lithospheric extension together with minor plume activity and probably lower lithosphere erosion induced melting of shallower heterogenous upper mantle generating a spectrum of olivine tholeiitic melts. These olivine tholeiitic rocks evolved via crystal fractionation and probably limited contamination to quartz tholeiites.

Nd-Sr isotopic and trace element study of rocks and fluids from the continental deep drilling Project (KTB), Germany

Geochemical analyses were interpreted on the dominant lithological units and on a deep crustal fluid from the Continental Deep Drilling Project (KTB) Pilot Hole, situated at the western margin of the Variscan Bohemian Massif. The biotite gneiss (from 384 m depth) shows a rare earth element pattern very similar to the European shale composite with Nd model ages of 940 Ma (CHUR) and 1.4 Ga (DM). The lamprophyre dike in the upper profile (1549 m), a nepheline and olivine normative basalt, is geochemically and isotopically similar to rocks from the Tertiary Central European Volcanic Province. The lower metabasite sequence (3575–4000 m), with an intrusion age of approximately 500 Ma, is made up primarily of highly metamorphosed subalkalic olivine basalts. The geochemical characteristics of the metabasites are a (La/Yb)N of 5–10, an La concentration of 20–50 times chondrite as well as (87Sr/86Sr)i of 0.7035–0.7038 and ɛeNd(T) of 4–6. These values suggest a depleted mantle source for the igneous precursors, evolving by assimilation-fractional crystallization processes with up to 25% of upper crust into the ultramafic, basaltic, and intermediate rock types of the metabasite sequence. The strong geochemical and chronological similarities between the KTB metabasites and rocks from the Munchberg Massif suggest that these units belong to the same lithological complex. The high salinity as well as the radiogenic 87Sr/86Sr ratio of 0.709413 in the KTB fluid from 4000 m depth might be the result of migrating fluids reacting with the regional Permo-Mesozoic evaporite deposits, followed by extensive Sr isotopic exchange with the upper crust.

Strontium and neodymium isotope study of Bohemian basalts

The Cenozoic alkaline basalts of northern and western Bohemia are part of the Central European Volcanic Province (CEVP) which extends from the Rhineland (Eifel, Germany) to Moravia (Czechoslovakia) and the Lover Silesia (Poland). Seven samlpes from locations within Czechoslovakia have been analyzed isotopically for the Rb-Sr and the Sm-Nd systems. Present-day, normalised 87Sr/86Sr ratios range from 0.7031 to 0.7036, and the corresponding 143Nd/144Nd ratios range from 0.51279 to 0.51286. An eigth sample from the Silurian basalts of the ‘St. Jan’ type (K-Ar age: 420 Ma) occuring in the Barrandian basin in Central Bohemia is also analysed. Its Present-day, normalised, 87Sr/86Sr ratio is 0.7031, and the corresponding 143Nd/144Nd ratio 0.51288.