Central European Volcanic Province Research Articles

Two Distinct Metasomatized Mantle Sources Produced Two Groups of Alkaline SiO2-Undersaturated Rocks in the Southern Central European Volcanic Province

Abstract Upper Cretaceous–Miocene alkaline SiO2-undersaturated volcanic rocks in the southern Central European Volcanic Province (CEVP) comprise two distinct rock series: (i) Upper Cretaceous–Eocene (~73–47 Ma) olivine nephelinites, basanitic nephelinites, and nepheline basanites have moderate to high MgO (8–16 wt. %), CaO, Ni, Co, Cr, Nb, and Ba, coupled with low F and SiO2 concentrations. These rocks contain abundant clinopyroxene and variable amounts of olivine macrocrysts as well as nepheline, K-dominated F-poor mica, and hydroxyapatite. Evolved and less common apatite-rich (phonolitic) haüynites/noseanites and haüyne nephelinites (~68–62 Ma) represent differentiated counterparts within this older group, showing higher alkali, Al2O3, P2O5, Nb, Zn, REE, and SO3 concentrations at low MgO (4–6 wt. %), CaO, Ni, Co, and Cr contents. (ii) Oligocene–Miocene (~27–9 Ma) olivine melilitites and melilite-bearing olivine nephelinites are characterized by even higher MgO (10–22 wt. %), CaO, Ni, Co, Cr, Nb, Ba, and high F contents at lower SiO2 concentrations, as reflected by the presence of abundant olivine macrocrysts, melilite, perovskite, Cr-rich spinel, F- and Ba-rich mica, and fluorapatite in addition to clinopyroxene and nepheline. Distinct mineral assemblages, crystallization trends, and various xenocrysts indicate different melt sources, a varying extent of enrichment, retention, and loss of volatiles (including timing of H2O and CO2 saturation), and limited wall rock interaction for the two rock groups. Partly resorbed, Fo-depleted olivine cores in the younger rocks and green-core pyroxenes in the older ones suggest early magma mixing. The nephelinitic–basanitic magmas derived from up to 6% partial melting of amphibole-bearing garnet/spinel lherzolite at or just above the lithosphere–asthenosphere boundary. This source was metasomatized involving hydrous melts or fluids. On the other hand, the melilite-bearing rocks probably originated in the upper asthenosphere by less than 3.5% partial melting of amphibole ± phlogopite-bearing garnet wehrlite, previously generated by subduction-related metasomatism with high CaO/MgO and CO2/(CO2 + H2O) ratios. Infiltration and storage of the metasomatic agents occurred in the former lower lithosphere, following continuous recycling of oceanic crust, comprising the release of Ca, CO2, H2O, further volatiles, and incompatible elements. Both volcanic episodes coincide with topographic uplift, erosion, rifting, and reactivation of lithosphere-scale faults, probably related to phases of strong mechanical coupling between Alpine orogen and European foreland. The first period overlapped with an era of prolonged N-directed intraplate compressional stress due to the Adriatic-Eurasian collision, provoking large-scale deformation, isostatic compensation, erosion, and consequent lithosphere thinning in the future CEVP. The second period is associated with the Oligocene–Miocene main stage of the European Cenozoic Rift System. Onset of volcanism was accompanied by a change in deformation in the Upper Rhine Graben from (W)NW extension to (E)NE extension and transtension by a complex interplay of evasive movements responding to shortening in Alps and Jura. Magma compositions, barely magmatic graben structures, volcanic activity outside rifts, and extensive exhumation suggest that in response to rifting, passive asthenospheric doming also contributed to magmatism by causing strong lithosphere–asthenosphere interaction and providing heat.

Petrogenesis of volcanic rocks from the Quaternary Eifel volcanic fields, Germany: detailed insights from combined trace-element and Sr–Nd–Hf–Pb–Os isotope data

Quaternary rocks from the East and West Eifel volcanic fields in western Germany are a key suite of intraplate volcanic rocks that can provide insights into volcanism of the Central European Volcanic Province (CEVP) and into continental intraplate volcanism in general. We present a comprehensive dataset for Eifel lavas including isotope as well as major and trace element data for 59 samples covering representative compositions of the different volcanic fields. In line with previous studies, the lavas are all SiO2-undersaturated, alkaline-rich and mainly comprise primitive basanites, melilitites, and nephelinites (Mg# ≥ 57). Geochemical compositions of samples from both volcanic subfields display distinct differences in their trace-element as well as radiogenic isotope compositions, largely confirming previous subdivisions. Coupled trace-element and radiogenic Sr–Nd–Hf–Pb–Os isotope compositions can now provide firm evidence for spatially heterogeneous mantle sources and compositionally distinct magmatic pulses. Within the West Eifel Field, Sr–Nd–Pb isotope compositions of the younger (≤80 ka), ONB-suite (olivine-nephelinite-basanite) are similar to FOZO (FOcal ZOne) or the EAR (European Asthenospheric Reservoir) and resemble compositions that have been previously reported from plume-sourced ocean island basalts (OIB). In marked difference, older (700 Ma to 80 ka) volcanic rocks from the F-suite (Foidite) in the West Eifel field and from the entire east Eifel Field tap a more enriched mantle component, as illustrated by more radiogenic Sr isotope (86Sr/87Sr up to 0.705408) and variable Pb isotope compositions (206Pb/204Pb = 18.61–19.70, 207Pb/204Pb = 15.62–15.67 and 208Pb/204Pb = 38.89–39.76). Combined trace-element compositions of ONB-suite samples are in good agreement with results from batch melting models suggesting a hybrid composition of Eifel magmas formed through mixing 10% of a FOZO-like melt with 90% of a DMM-like melt, similar to melts from the Tertiary HEVF. However, radiogenic Sr–Nd–Pb isotope compositions of F-suite and EEVF and some ONB lavas require the admixture of melts from lithospheric mantle sources. Elevated Nb/Ta and Lu/Hf ratios in combination with variable 187Os/188Os ratios can now demonstrate the presence of residual carbonated eclogite components, either in the lithosphere or in the asthenospheric mantle. Finally, by combining geochemical and temporal constraints of Tertiary and Quaternary volcanism it becomes evident that CEVP volcanism in central and western Germany has resulted from compositionally distinct magmatic pulses that tap separate mantle sources. Although the presence of a mantle plume can neither be fully confirmed nor excluded, plume-like melt pulses which partially tap carbonated eclogite domains that interact to variable extents with the lithosphere provide a viable explanation for the temporal and compositional cyclicity of CEVP volcanism.

Two distinct age groups of melilitites, foidites, and basanites from the southern Central European Volcanic Province reflect lithospheric heterogeneity

Petrographic observations and in situ U–Pb ages of melilitites, foidites, basanites, phonolites, and trachytes from the southern part of the Central European Volcanic Province (CEVP) and related plutonic inclusions therein reveal two distinct age groups separated by a gap of ~ 20 Myr. A late Cretaceous to early Eocene group (~ 73–47 Ma; Taunus, Lower Main plain, Odenwald and Kraichgau area, Bonndorfer Graben and Freiburger Bucht area, Vosges and Pfälzerwald) is characterized by nephelinites and basanites mostly devoid of melilite and perovskite, and by rare haüynites, and trachytes. In contrast, a late Oligocene to late Miocene group (~ 27–9 Ma; Lorraine, southern Upper Rhine Graben, Urach, Hegau area) is dominated by melilitites, melilite-bearing nephelinites (both carrying perovskite), and phonolites. Both magmatic episodes are related to domal topographic uplift, erosion, and formation of major angular unconformities in the Upper Rhine Graben, suggesting an association with dynamic topography interrupted by phases of subsidence (or abatements of uplift). The investigated rocks in the southern CEVP (south of a line Eifel–Vogelsberg–Rhön–Heldburg), except for the Kaiserstuhl volcanic complex, mostly comprise small and isolated occurrences or monogenetic volcanic fields, whereas the northern CEVP is dominated by large volcanic complexes and dyke swarms, which are mostly SiO2-saturated to weakly SiO2-undersaturated. In the northern CEVP, evidence of spatially varying but recurrent volcanic activity exists since the Eocene, lacking the distinct 20 Myr gap as documented from the southern CEVP. While the temporal and spatial distribution of volcanism are a result of the Cretaceous to Miocene tectonic evolution in Central Europe, further studies are needed to explain the petrographic differences between the two age groups in the south.

Melting and metasomatism in West Eifel and Siebengebirge Sub-Continental Lithospheric Mantle: Evidence from concentrations of volatiles in fluid inclusions and petrology of ultramafic xenoliths

The possibility of constraining the composition and evolution of specific portions of the Sub-Continental Lithospheric Mantle (SCLM) by means of an integrated study of petrography, mineral chemistry, and concentrations of volatiles in fluid inclusions (FI) is a novel approach that can provide clues on the recycling of volatiles within the lithosphere. This approach is even more important in active or dormant volcanic areas, where the signature of the gaseous emissions at the surface can be that of the underlying lithospheric mantle domains. In this respect, the ultramafic xenoliths brought to the surface in West Eifel (~0.5–0.01 Ma) and Siebengebirge (~30–6 Ma) volcanic fields (Germany) are ideal targets, as they provide direct information on one of the most intriguing portions of SCLM beneath the Central European Volcanic Province (CEVP). Five distinct populations from these localities were investigated using petrographic observations, mineral phase analyses and determination of He, Ne, Ar and CO2 contents in olivine-, orthopyroxene-, and clinopyroxene-hosted FI. The most refractory Siebengebirge rocks have highly forsteritic olivine, high-Mg#, low-Al pyroxene, and spinel with high Cr#, reflecting high extents (up to 30%) of melt extraction. In contrast, xenoliths from West Eifel are modally and compositionally heterogeneous, as indicated by the large forsterite range of olivine (Fo83–92), the Cr# range of spinel (0.1–0.6), and the variable Al and Ti contents of pyroxene. Equilibration temperatures vary from 870 °C to 1070 °C in Siebengebirge, and from ⁓900 °C to ⁓1190 °C in West Eifel xenoliths, at oxygen fugacity values generally between −0.5 and + 1.3 ΔlogƒO2 [FMQ]. In both areas, the FI composition was dominated by CO2, with clinopyroxene, and most of the orthopyroxene had the highest concentrations of volatiles, while olivine was gas-poor. The noble gas and CO2 distributions suggest that olivine is representative of a residual mantle that experienced one or more melt extraction episodes. The 3He/4He ratio corrected for air contamination (Rc/Ra values) varied from 6.8 Ra in harzburgitic lithotypes to 5.5 Ra in lherzolites and cumulate rocks, indicating that the original MORB-like mantle signature was progressively modified by interaction with crustal-related components and melts having 3He/4He and 4He/40Ar* values consistent with those published for magmatic gaseous emissions. The Ne and Ar isotope systematics indicated that most of the data were consistent with mixing between a recycled atmospheric component and a MORB-like mantle, which does not necessarily require the involvement of a lower mantle plume beneath this portion of the CEVP. The major element distribution in mineral phases from West Eifel and Siebengebirge, together with the systematic variations in FI composition, the positive correlation between Al enrichment in pyroxene and equilibration temperatures, and the concomitant Rc/Ra decrease with increasing temperature, suggest that the SCLM beneath Siebengebirge represented the Variscan lithosphere in CEVP prior to the massive infiltration of melts/fluids belonging to the Quaternary Eifel volcanism. In contrast, West Eifel xenoliths reflect multiple heterogeneous metasomatism/refertilisation events that took place in the regional SCLM between ~6 and ~ 0.5 Ma.

Variable origin of clinopyroxene megacrysts carried by Cenozoic volcanic rocks from the eastern limb of Central European Volcanic Province (SE Germany and SW Poland)

Clinopyroxene megacrysts occurring in Cenozoic mafic alkaline volcanic rocks from the northern margin of the Bohemian Massif (SW Poland, SE Germany) could be subdivided by colour and Mg# (Mg/(Mg + Fetot)) in three groups. Megacryst with the highest Mg# = 90.0–91.5 (“high Mg#”, HMg#) is transparent and strongly Light Rare Earth Element (LREE) depleted and contains abundant orthopyroxene lamellae. The clinopyroxene megacrysts with “medium Mg#” (MMg#) values (76.8–83.4) are transparent to light grey and are all LREE-enriched. The group with the lowest Mg# (LMg#; 62.2–74.6) is intensively coloured (from grey to green) and may enclose apatite, Ti-magnetite or pseudomorphs after amphibole. The “low Mg#” (LMg#) clinopyroxenes are LREE-enriched, and some display positive Zr–Hf anomalies.The single HMg# megacryst records pressures ~1GPa and temperature of 1280 °C, pointing to its mantle origin. It is the first megacryst described in European lavas and possibly worldwide, which shows affinity to Depleted MORB Mantle (DMM). The MMg# megacrysts formed from fractionating mafic magmas at variable pressures and temperatures – from those corresponding to mantle depths (>1 GPa, 1230–1350 °C) to lower/middle crustal values (0.5–0.9 GPa, 1120–1150 °C). The parental melts for this group are isotopically related to the Cenozic volcanic rocks from the study area. The LMg# megacrysts crystallized from strongly alkaline melts, mostly at lower- to middle-crustal pressures (0.4–1.0 GPa). Their parental melts were also related to Cenozoic volcanism, but their strongly evolved nature resulted in local, but significant enrichment in Zr and Hf. The LMg# megacrysts from three localities in Poland are the first evidence of strongly alkaline magmatism in the north-easternmost part of Cenozoic European Volcanic Province.

Novel insights into the degassing history of Earth's mantle from high precision noble gas analysis of magmatic gas

The noble gas isotope composition of the mantle can provide unique insights into the origin and evolution of volatile elements on Earth. Xenon isotopes combine primordial signatures with contributions from extinct and extant radionuclides, therefore offering the potential to set constraints on both the nature of Earth's planetary precursor(s) and the timing of their contributions. However, measuring the Xe isotope composition of mantle-derived samples to sufficiently high-precision has proven difficult due to (i) large occurrence of a modern-like atmospheric component in the mantle, and (ii) contribution from shallow and post-eruptive atmospheric contamination. Mantle-derived samples therefore exhibit only small deviations from the modern atmospheric composition, making the identification and deconvolution of mantle-derived Xe signals challenging. Here, we use the Giggenbach sampling method to concentrate magmatic noble gases from the Eifel volcanic area (Germany) into glass bottles in order to conduct high-precision analyses of Ne, Ar and Xe isotopes. The three samples collected from Victoriaquelle and Schwefelquelle wells (South East Eifel) show variable contributions from atmospheric contamination, with the least contaminated sample reaching 40Ar/36Ar ∼8,300. Our data indicate that the mantle beneath the Eifel volcanic area, and by extension the Central European Volcanic Province, resembles the convective upper mantle reservoir with limited evidence for an OIB-like deep plume source contribution. It has a geochemical signature that is similar (e.g. in Ne isotopic composition, 40Ar/36Ar, 129Xe/130Xe and 129Xe/136Xe) to the mantle source of the so-called popping rocks (thought to best represent the upper mantle), with an additional source of 238U-derived Xe and low 3He/4He that we attribute to the influence of an ancient subducted component (HIMU). A dichotomy exists between the main sources of fissiogenic xenon isotopes measured in popping rocks and Eifel gas, which appear to be mainly derived from 244Pu and 238U, respectively. According to their respective ratios of 244Pu- to 238U-derived Xe, the mantle sources for Eifel volcanism and popping rocks would have experienced extensive and limited degassing, respectively. In this regard, high Pu–Xe/(Pu+U)–Xe may no longer be considered as being indicative of a mantle deep origin, therefore calling for the geochemical differences between plume and MORB sources to be redefined, with the possibility that volatile signatures within the solid Earth may be more heterogeneously distributed than previously thought.

The role of mantle-hybridization and crustal contamination in the petrogenesis of lithospheric mantle-derived alkaline rocks: constraints from Os and Hf isotopes

The Rhon area as part of the Central European Volcanic Province (CEVP) hosts an unusual suite of Tertiary 24-Ma old hornblende-bearing alkaline basalts that provide insights into melting and fractionation processes within the lithospheric mantle. These chemically primitive to slightly evolved and isotopically (Sr, Nd, Pb) depleted basalts have slightly lower Hf isotopic compositions than respective other CEVP basalts and Os isotope compositions more radiogenic than commonly observed for continental intraplate alkaline basalts. These highly radiogenic initial 187Os/188Os ratios (0.268–0.892) together with their respective Sr–Nd–Pb isotopic compositions are unlikely to result from crustal contamination alone, although a lack of Os data for lower crustal rocks from the area and limited data for CEVP basalts or mantle xenoliths preclude a detailed evaluation. Similarly, melting of the same metasomatized subcontinental lithospheric mantle as inferred for other CEVP basalts alone is also unlikely, based on only moderately radiogenic Os isotope compositions obtained for upper mantle xenoliths from elsewhere in the province. Another explanation for the combined Nd, Sr and Os isotope data is that the lavas gained their highly radiogenic Os isotope composition through a mantle “hybridization”, metasomatism process. This model involves a mafic lithospheric component, such as an intrusion of a sublithospheric primary alkaline melt or a melt derived from subducted oceanic material, sometime in the past into the lithospheric mantle where it metasomatized the ambient mantle. Later at 24 Ma, thermal perturbations during rifting forced the isotopically evolved parts of the mantle together with the peridotitic ambient mantle to melt. This yielded a package of melts with highly correlated Re/Os ratios and radiogenic Os isotope compositions. Subsequent movement through the crust may have further altered the Os isotope composition although this effect is probably minor for the majority of the samples based on radiogenic Nd and unradiogenic Sr isotope composition of the lavas. If the radiogenic Os isotope composition can be explained by a mantle-hybridization and metasomatism model, the isotopic compositions of the hornblende basalts can be satisfied by ca. 5–25% addition of the mafic lithospheric component to an asthenospheric alkaline magma. Although a lack of isotope data for all required endmembers make this model somewhat speculative, the results show that the Re–Os isotope system in continental basalts is able to distinguish between crustal contamination and derivation of continental alkaline lavas from isotopically evolved peridotitic lithosphere that was contaminated by mafic material in the past and later remelted during rifting. The Hf isotopic compositions are slightly less radiogenic than in other alkaline basalts from the province and indicate the derivation of the lavas from low Lu–Hf parts of the lithospheric mantle. The new Os and Hf isotope data constrain a new light of the nature of such metasomatizing agents, at least for these particular rocks, which represent within the particular volcanic complex the first product of the volcanism.

High-resolution 40Ar/39Ar geochronology of volcanic rocks from the Siebengebirge (Central Germany)—Implications for eruption timescales and petrogenetic evolution of intraplate volcanic fields

A key parameter in understanding mantle dynamics beneath continents is the temporal evolution of intraplate volcanism in response to lithospheric thinning and asthenospheric uplift. To contribute to a better understanding of how intraplate volcanic fields evolve through time, we present a high precision 40Ar/39Ar age dataset for volcanic rocks from the Siebengebirge volcanic field (SVF) from central Germany, one of the best studied and compositionally most diverse intraplate volcanic fields of the Cenozoic Central European Volcanic Province (CEVP). Petrological and geochemical investigations suggest that the formation of the different rock types that occur in the SVF can be explained by a combination of assimilation and fractional crystallisation processes, starting from at least two different parental magmas with different levels of silica saturation (alkali basaltic and basanitic), and originating from different mantle sources. These evolved along two differentiation trends to latites and trachytes, and to tephrites and tephriphonolites, respectively. In contrast to their petrogenesis, the temporal evolution of the different SVF suites is poorly constrained. Previous K/Ar ages suggested a time of formation between about 28 and 19 Ma for the mafic rocks, and of about 27 to 24 Ma for the differentiated rocks. Our results confirm at high precision that the differentiated lithologies of both alkaline suites (40Ar/39Ar ages from 25.3 ± 0.2 Ma to 25.9 ± 0.3 Ma) erupted contemporaneously within a very short time period of ~ 0.6 Ma, whereas the eruption of mafic rocks (basanites) lasted at least 8 Ma (40Ar/39Ar ages from 22.2 ± 0.2 Ma to 29.5 ± 0.3 Ma). This implies that felsic magmatism in the central SVF was likely a single event, possibly triggered by an intense phase of rifting, and that ongoing melting and eruption of mostly undifferentiated mafic lavas dominate the > 8 Ma long magmatic history of this region. Among the mafic lavas, most basanites and tephrites predate the alkali basalts and hawaiites, suggesting an overall temporal evolution towards less SiO2-undersaturated primary melts and increasing degrees of melting over time. The peak in alkali basaltic to hawaiitic magmatism slightly post dates the flare-up of genetically related felsic magmatism, by no more than ~ 1 Ma. This is consistent with a model in which the magmatic plumbing system erupted successively from upper to lower levels, i.e. from more evolved to more primitive compositions. One young age for a basanitic sample suggests that silica saturation decreased again towards the end of volcanic activity. This chronology of volcanic events is in good agreement with previous models, suggesting continuous lithospheric thinning beneath the SVF as a response to an extensional regime and asthenospheric uplift in the northern alpine realm.

Atmospheric outgassing and native-iron formation during carbonaceous sediment–basalt melt interactions

Organic carbon-rich sediment assimilation by basaltic magmas leads to enhanced emission of greenhouse gases during continental flood basalt eruptions. A collateral effect of these interactions is the generation of low oxygen fugacities (fO2) (below the iron-wüstite [IW] buffer curve) during magmatic crystallization, resulting in the precipitation of native-iron. The occurrence of native-iron bearing terrestrial basaltic rocks are rare, having been identified at three locations: Siberia, West Greenland, and Central Germany. We report the first combined study of Re–Os isotopes, highly siderophile element (HSE: Os, Ir, Ru, Pt, Pd, Re), and trace-element abundances for these three occurrences, in addition to host sediments at West Greenland. To quantify the amount of crustal assimilation experienced by the magmas, we present combined crystallization and assimilation models, together with fractional crystallization models, to assess how relative abundances of the HSE have been modified during crystallization. The radiogenic osmium isotopic compositions (γOsinitial +15 to +193) of mafic igneous samples are consistent with assimilation of old high Re/Os crustal contaminants with radiogenic 187Os/188Os, whereas the HSE inter-element fractionations (Pd/Os 2 to >10,000) suggest that some Siberian samples underwent an early stage of sulfide removal.Metalliferous samples from the Siberian intrusions of Khungtukun and Dzhaltul (associated with the Siberian flood basalts) yield internal 187Re–187Os ages of 266±83Ma and 249±50Ma, respectively, reflecting late-Permian emplacement ages. These results imply that crustal assimilation took place prior to crystallization of native-Fe. In contrast, metalliferous samples from Disko Island and Bühl (associated with the West Greenland flood basalts, and the Central European Volcanic Province, respectively) have trends in 187Re/188Os–187Os/188Os space corresponding to apparent ages older than their reported crystallization ages. These anomalous ages probably reflect concurrent assimilation of high Re/Os, radiogenic 187Os crust during crystallization of native-Fe, consistent with the character of local West Greenland sediments. In all three locations, calculations of combined assimilation of crustal sediments and fractional crystallization indicate between 1–7% assimilation can account for the Os-isotope systematics. In the case of Siberian samples, incompatible trace-element abundances indicate that lower crustal assimilation may have also occurred, consistent with the suggestion that crustal assimilation took place prior to native-Fe precipitation. The extent of local crustal contamination at Siberia, West Greenland, and Bühl necessitates that significant quantities of CH4, CO, CO2, SO2 and H2O were released during assimilation of carbonaceous sediments. Consequently, carbonaceous sediment–basalt melt interactions have collateral effects on total gas output from flood basalt volcanism into the atmosphere. However, the amount of carbonaceous sediment contamination experienced by melts forming the Khungtukun and Dzhaltul intrusions alone, cannot explain the major C-isotope excursions at the Permo–Triassic mass-extinction event. Instead, further unsampled intrusions that also experienced significant carbonaceous sediment–melt interactions would be required. Enhanced greenhouse gas-emission during the Permo–Triassic mass extinction may have been facilitated by a combination of mantle melting and carbonaceous sediment–melt interactions, together with other proposed mechanisms, including wildfires, or by microbial metabolic exhalation.

Role of crustal assimilation and basement compositions in the petrogenesis of differentiated intraplate volcanic rocks: a case study from the Siebengebirge Volcanic Field, Germany

The Siebengebirge Volcanic Field (SVF) in western Germany is part of the Cenozoic Central European Volcanic Province. Amongst these volcanic fields, the relatively small SVF comprises the entire range from silica-undersaturated mafic lavas to both silica-undersaturated and silica-saturated differentiated lavas. Owing to this circumstance, the SVF represents a valuable study area representative of intraplate volcanism in Europe. Compositions of the felsic lavas can shed some new light on differentiation of intraplate magmas and on the extent and composition of potential crustal assimilation processes. In this study, we provide detailed petrographic and geochemical data for various differentiated SVF lavas, including major and trace element concentrations as well as Sr–Nd–Hf–Pb isotope compositions. Samples include tephriphonolites, latites, and trachytes with SiO2 contents ranging between 53 and 66 wt%. If compared to previously published compositions of mafic SVF lavas, relatively unradiogenic 143Nd/144Nd and 176Hf/177Hf coupled with radiogenic 87Sr/86Sr and 207Pb/204Pb lead to the interpretation that the differentiated volcanic rocks have assimilated significant amounts of lower crustal mafic granulites like the ones found as xenoliths in the nearby Eifel volcanic field. These crustal contaminants should possess unradiogenic 143Nd/144Nd and 176Hf/177Hf, radiogenic 87Sr/86Sr, and highly radiogenic 207Pb/204Pb compositions requiring the presence of ancient components in the central European lower crust that are not sampled on the surface. Using energy-constrained assimilation–fractional crystallisation (EC-AFC) model calculations, differentiation of the SVF lithologies can be modelled by approximately 39–47 % fractional crystallisation and 6–15 % crustal assimilation. Notably, the transition from silica-undersaturated to silica-saturated compositions of many felsic lavas in the SVF that is difficult to account for in closed-system models is also well explained by such amounts of crustal assimilation.

From mantle roots to surface eruptions: Cenozoic and Mesozoic continental basaltic magmatism

Basaltic volcanism is an important process in shaping large areas of the Earth’s surface, not only in continental extensional environments and at the ocean floor. This special issue contains a collection of fifteen papers that are dedicated to recent researches on various aspects of continental basaltic magmatism from its mantle roots via the ascent paths of the melt to the surface where different styles of volcanism take place erupting lavas or through explosive volcanism depositing various types of pyroclasts. Two of the fifteen were published earlier (Downes et al. 2015; Herrero-Hernandez et al. 2015). Continental basaltic volcanism also contributes to the total terrestrial sedimentary budget not only by its primary pyroclastic deposits but also their reworked varieties. Most of the papers result from presentations at the BASALT 2013 conference, which took place from April 18–24, 2013, in Gorlitz, Germany (Buchner et al. 2013). The conference was organized by the Senckenberg Museum of Natural History Gorlitz and co-organized by the International Association of Volcanology and Chemistry of the Earth´s Interior (IAVCEI)—particularly its Commission on Monogenetic Volcanism—and the “Sachsische Landesstiftung Natur und Umwelt” LaNU Academy). Since the conference was held in the heart of Europe in Germany with accompanied field excursions to Poland and the Czech Republic, many contributions are related to the Cenozoic Central European Volcanic Province (Fig. 1). However, there was also a variety of contributions about Mesozoic and Cenozoic basaltic rocks worldwide. This variety is reflected in this issue. The issue brings together studies on different aspects of basaltic magmatism. Thus, this volume contains petrological and geochemical studies spanning from studies of mantle peridotites to those on volcanic rocks as well as papers presenting geophysical data and interdisciplinary interpretation.

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).

Petrogenesis of Tertiary continental intra-plate lavas between Siebengebirge and Westerwald, Germany: Constraints from trace element systematics and Nd, Sr and Pb isotopes

New 39Ar/40Ar ages and major- and trace-element and radiogenic isotope data are presented for basanites and alkali basalts from the transition area between the Westerwald and Siebengebirge volcanic fields (Germany) that belongs to the Central European Volcanic Province (CEVP). The 39Ar/40Ar ages indicate ages of c. 24 and c. 5Ma which are fully compatible with previous K/Ar ages indicating that the evolution of this volcanic field belongs to the Westerwald area (28–22Ma and 5Ma) rather than to the Siebengebirge area (26–23Ma). Based on the occurrence of >30 isolated volcanic plugs with a simple igneous history, this volcanic field can be viewed as a monogenetic volcanic field. Compositions of some basanites are primitive, whereas others and the alkali basalts show decreasing Cr and Ni contents and CaO/Al2O3 ratios. However, increasing TiO2, Al2O3 and incompatible elements (Sr, Zr, Y, Hf, Ta) concentrations with decreasing MgO indicating fractionation of mainly olivine with minor amounts of clinopyroxene and spinel can be noticed. Rare earth element systematics suggest that most of the alkaline rocks are generated by different degrees of melting (5%–10%) of a garnet-bearing peridotite containing some residual amphibole. Negative anomalies of Rb and K in primitive mantle-normalized diagrams and a lack of Ba/Rb fractionation suggest that amphibole was the major OH-bearing mineral phase in the mantle. The alkaline rocks have a restricted range in 87Sr/86Sr and 143Nd/144Nd ratios ranging from 0.7033 to 0.7044 and from 0.51275 to 0.51285, respectively. Lead isotope compositions (206Pb/204Pb: 19.21–19.65; 207Pb/204Pb: 15.62–15.67; 208Pb/204Pb: 39.10–39.46) of the alkaline rocks are within the range of most OIB in which the higher values approach the composition of the European Asthenospheric Reservoir (EAR). The correlation between Sr and Nd isotopes and trace element constraints (Ce/Pb; Nb/U) indicates that for some samples interaction with crustal rocks during fractionation has occurred. Miocene intraplate basaltic volcanism in the area probably occurred as a result of minor “baby plume” activity. Each volcanic plug records evidence of a specific stage of fractionation with or without assimilation; however, in summary the lavas plot on a single fractionation path. This implies that during evolution of the volcanic field initial melting took place in the asthenosphere or at the lithosphere–asthenosphere interface. The melts moved through the lithospheric mantle and stagnated at crustal levels, however the observed fractionation paths suggest that they were fed from a single reservoir. This model, which involves small-scale plume impact followed by asthenosphere–lithosphere interaction together with minor crustal contamination, should also be applicable to other intra-continental rift-related areas.

Geochemical composition, petrography and 40Ar/39Ar age of the Heldburg phonolite: implications on magma mixing and mingling

Differentiated magmatic rocks such as trachyte and phonolite are volumetrically subordinate to mafic volcanic rocks within the Cenozoic Central European Volcanic Province (exceptions are the East Eifel and the Rhon volcanic fields). Within the volcanic field of the “Heldburg dike swarm” (Heldburger Gangschar), the phonolite of the Burgberg near Heldburg represents the only known occurrence of differentiated magmatic rocks. However, the Heldburg phonolite is famous foremost for containing mantle xenoliths (spinel lherzolite). Former studies proposing a cogenetic relationship between the phonolite and the peridotites concluded that the phonolite magma must have evolved under upper mantle conditions. Herewith, we present petrographic and geochemical evidence for magma mixing and mingling in the Heldburg phonolite melt due to the intrusion of mantle-derived basanitic magma, which is exposed today as dikes at the foot of the Heldburg Burgberg. During this process, the mantle xenoliths were introduced into the phonolite melt as they all contain rims of basanitic magma. Extensive mingling features (e.g., schlieren layers, load casts, flame structures, mafic enclaves) are developed, indicating that the basanite and the zoned phonolitic body were melts at the time of mixing. These petrographic and geochemical indications of two coeval melts of different composition are substantiated by 40Ar/39Ar dating, revealing identical ages of ca. 15 Ma.

Upper Cretaceous to Pleistocene melilitic volcanic rocks of the Bohemian Massif: petrology and mineral chemistry

Abstract Upper Cretaceous to Pleistocene volcanic rocks of the Bohemian Massif represent the easternmost part of the Central European Volcanic Province. These alkaline volcanic series include rare melilitic rocks occurring as dykes, sills, scoria cones and flows. They occur in three volcanic periods: (i) the Late Cretaceous to Paleocene period (80–59 Ma) in northern Bohemia including adjacent territories of Saxony and Lusatia, (ii) the Mid Eocene to Late Miocene (32.3–5.9 Ma) period disseminated in the Ohře Rift, the Cheb–Domažlice Graben, Vogtland, and Silesia and (iii) the Early to Late Pleistocene period (1.0–0.26 Ma) in western Bohemia. Melilitic magmas of the Eocene to Miocene and Pleistocene periods show a primitive mantle source [(143Nd/144Nd)t=0.51280–0.51287; (87Sr/86Sr)t=0.7034–0.7038)] while those of the Upper Cretaceous to Paleocene period display a broad scatter of Sr–Nd ratios. The (143Nd/144Nd)t ratios (0.51272–0.51282) of the Upper Cretaceous to Paleocene rocks suggest a partly heterogeneous mantle source, and their (87Sr/86Sr)t ratios (0.7033–0.7049) point to an additional late- to post-magmatic hydrothermal contribution. Major rock-forming minerals include forsterite, diopside, melilite, nepheline, sodalite group minerals, phlogopite, Cr- and Ti-bearing spinels. Crystallization pressures and temperatures of clinopyroxene vary widely between ~1 to 2 GPa and between 1000 to 1200 °C, respectively. Nepheline crystallized at about 500 to 770 °C. Geochemical and isotopic similarities of these rocks occurring from the Upper Cretaceous to Pleistocene suggest that they had similar mantle sources and similar processes of magma development by partial melting of a heterogeneous carbonatized mantle source.

Temporal–spatial evolution of low-SiO2 volcanism in the Pleistocene West Eifel volcanic field (West Germany) and relationship to upwelling asthenosphere

The temporal–spatial evolution of low-SiO2 lavas from the Pleistocene West Eifel volcanic field (Central European Volcanic Province) and linked petrogenetic variations are evaluated using 40Ar/39Ar age and geochemical data. Geochronological and petrological evidence is related to the physical structure of the previously established seismologically anomalous asthenosphere interpreted as thermally upwelling mantle (Eifel Plume).Lava flows >480ka (Middle Pleistocene) occur exclusively in the NW of the volcanic field. After a time span of ca. 400ka lacking significant activity, volcanism has migrated to the SE generating flows <80ka (Late Pleistocene). Middle Pleistocene occurrences form a bilaterally structured, SE-striking zone comprising an internal domain of intense activity between ca. 560 and 480ka and an external domain of less intense preceding activity initiated at ca. 720ka. The tectonic pattern corresponds to recent stress field conditions probably analogous to the Pleistocene setting, which may have reactivated major transverse structures of Hercynian origin providing preferred pathways for melt transfer through the brittle crust.At melting depth >70km of parental asthenospheric melts in garnet–spinel peridotite the surface-projected contour of the low-velocity P-wave anomaly coincides with the geographical boundary separating >480ka volcanism in the NW from <80ka volcanism in the SE. Thus, the melting zones of <80ka and >480ka volcanism reside within and beyond the P-wave velocity anomaly, respectively. The coupling between time-space pattern of volcanism and seismological contrast in the mantle sources indicates that volcanic activity is linked to a highly dynamic low-velocity anomaly with lateral and vertical motion rates of 4–5cm/year and up to 6cm/year, respectively. The change in seismological contrast is accompanied by a transition in the petrogenetic style resulting from differently intense thermal erosion of multiply metasomatized lithosphere by upwelling asthenosphere. Asthenosphere-lithosphere interaction is widespread in the NW and subordinate in the SE of the volcanic field, where melts ascended through a more refractory lithosphere which has been affected by preferential melting of hydrous portions by pre-80ka thermal exposure.

Volcanology, geochemistry and age of the Lausitz Volcanic Field

The Lausitz (Lusatia) Volcanic Field is part of the Central European Volcanic Province, and its magmas represent an alkaline trend from olivine nephelinites and basanites to trachytes and phonolites, typical for intraplate settings. Neighbouring volcanic fields are the Ceske Středohoři Mountains to the south-west and the Fore-Sudetic Basin in Lower Silesia to the east. More than 1000 volcanic structures associated with approximately 500 vents have been located within this volcanic field. Residuals of scoria cones, lava lakes, lava flows and maar-diatreme in filling occur in situ near the level of the original syn-volcanic terrain. In more deeply eroded structures, volcanic relicts outcrop as plugs or feeders. Evolved rocks occur as monogenetic domes or intrusions in diatremes, while their volcaniclastic equivalents are rare. Twenty-three localities were dated using the 40Ar/39Ar method. The ages range from 35 to 27 Ma, with a focus around 32–29 Ma, indicating Late Eocene and mainly Oligocene volcanism for the LVF. Differentiated rocks appear to be slightly younger than less differentiated. No geographical age clusters are apparent.

The Fossil-Lagerstätte Enspel — a crater lake in a volcanic-influenced terrestrial environment of the Westerwald Basin (late Oligocene, Western Germany)

The Westerwald region belongs to the Central European Volcanic Province (CEVP). As seen here, theWesterwald Basin is an aggregation of small basins (the smallest ones cover an area as little as 10,000 m), mainly situated within the Variscan Mosel, Lahn and Dill Synclines. Sedimentation started in the middle Eocene (Hottenrott 2002). A pre-existing landscape was filled first by coarse sediments, followed by clays and brown coal (Schafer et al. 2011). Tectonic activity seems to have increased during the middle late Oligocene, resulting in an increasing relief, displayed by sandy sediments (Butz and Vortisch 1987). Two watersheds separate the Westerwald Basin from the Lower Rhine Embayment (Schindler and Wuttke 2013) (Fig. 1). In the late Oligocene, volcanic activity commenced with trachytic intrusions, phreatomagmatic maar eruptions (Enspel) and pyroclastic eruptions. They were followed by basic shallow intrusions, lava flows, and continued maar formation (Elbtal, ?Elz) and the deposition of thick pyroclastics (Schreiber 1996; Haase et al. 2004). Locally, the eruptions and minor intercalated sediments covered the landscape up to 50 m high (Schafer et al. 2011). Reischmann (2011) relates the Oligocene volcanism to the Alpine orogeny and North Atlantic rifting; the main reason is extension and thinning of the lithosphere, resulting in a passive uplift of the asthenosphere; partial melting was caused by pressure release, which resulted in the eruptions in the CEVP. The volcanic activity led to changes in the local drainage systems, e.g. dammed valleys with subsequent lake formation. Caused by contemporaneous temporarily high Chattian sea level, palustrine facies became widespread, so that thick lignite seams were formed along with laminated lake sediments (e.g. Lake Norken, Uhl et al. 2011; fossil site Breitscheid, Schindler and Kolb 2010). Hygrophilous plant communities covered the lowlands. The macroflora of this riparian vegetation is dominated by leaves of Acer cf. tricuspidatum BRONN, twigs of Glyptostrobus europaeus (BRONGNIART) UNGER and charcoalified wood of Cupressaceae (Taxodiaceae s.l.), as seen in the lacustrine to swampy sediments of Lake Norken (Uhl et al. 2011). Upland or dryland floras and faunas were also influenced by volcanic activity. Forests died under heavy ash load, and their remnants were washed into the lowlands and partly preserved as silicified trunks. Isolated reptile and mammal bones and teeth (e.g. Teike and Tobien 1950) can be found within coarse volcanic ash (personal field observation, and unprepared material of the collections of the Museums of Breitscheid, Hachenburg, Wiesbaden and the Hessisches Landesamt fur Umwelt und Geologie, Wiesbaden, all Germany), demonstrating the influence of pyroclastic and/or lahar events on the biota. Floral recovery starts with pioneering plants like Cyperaceae, Poaceae and Betulaceae. This pioneering flora was succeeded by zonal assemblages of a mesophytic forest with strong East Asian influence (Herrmann et al. 2003, 2010; Kohler and Uhl 2014). This article is a contribution to the special issue “The Fossil-Lagerstatte Enspel reconstructing the palaeoenvironment with new data on fossils and geology” M. Wuttke (*) : T. Schindler Direktion Landesarchaologie, Referat Erdgeschichte, Generaldirektion Kulturelles Erbe Rheinland-Pfalz, Grose Langgasse 29, 55116 Mainz, Germany e-mail: michael.wuttke@gdke.rlp.de

Geochemical and petrological constraints on mantle composition of the Ohře(Eger) rift, Bohemian Massif: peridotite xenoliths from the České Středohoří Volcanic complex and northern Bohemia

The Ohře(Eger) rift in the Bohemian Massif, which represents part of the Cenozoic Central European Volcanic Province, contains abundant mantle xenoliths of spinel peridotite. Petrography, geothermometry, mineral compositions and whole-rock and clinopyroxene trace element compositions of xenolith suites from four localities in the central part Ceske Středohoři Volcanic complex (CSVC) and one locality in the eastern continuation (northern Bohemia) of the rift system display important differences that reveal small-scale compositional heterogeneity of the mantle. The xenoliths from the CSVC are mostly harzburgite that experienced high degrees of partial melting from ~17 to 21 %. However, xenoliths from one locality, Medvědický vrch, are predominantly fertile lherzolite. Subsequent pervasive metasomatism produced enrichments in light rare earth elements, large-ion lithophile elements, and U and Th, except for xenoliths from Medvědický vrch, which show marked depletions in all these elements. Such composition most likely reflects refertilization of the source rock by depleted melts. The trace element characteristics of the CSVC xenoliths indicate metasomatism of a depleted protolith by silicate melt at high melt/rock ratios. In contrast, harzburgite xenoliths from northern Bohemia experienced metasomatism at low melt/rock ratios by more evolved, alkaline and carbonate-rich melts.

Alkaline and Carbonate-rich Melt Metasomatism and Melting of Subcontinental Lithospheric Mantle: Evidence from Mantle Xenoliths, NE Bavaria, Bohemian Massif

Peridotite xenoliths hosted by Cenozoic basanite lava flows (21·2–23·5 Ma) occur at several localities in the western continuation of the Ohře/Eger rift (Central European Volcanic Province) in NE Bavaria, Bohemian Massif. Detailed petrography, major and trace element compositions of whole-rock samples and selected mineral phases as well as Sr–Nd–Li isotopic compositions for a suite of mantle xenoliths from Zinst, Hirschentanz and Teichelberg document variable degrees of partial melting and metasomatism. Melting models based on whole-rock major element composition and Cr# of spinel indicate ∼6–30% melting in the spinel stability field. Subsequent metasomatism by alkaline and carbonate-rich melts resulted in modal and cryptic metasomatism, expressed by the presence of carbonate-bearing silicate melt pockets with complex secondary mineral assemblages and by enrichment in light rare earth elements, Li, Rb, U, Pb, high field strength elements and P. The carbonate is most probably associated with fractionation of the Na-rich silicate melt. High P contents, variable but low to negative δ7Li values from +2·5 to −9·7‰, coupled with 87Sr/86Sr ratios between ∼0·7032 and ∼0·7041 may reflect a significant contribution of recycled crustal material such as eclogite in the infiltrating melts responsible for metasomatism, although the Li isotope compositions may reflect kinetic modifications through diffusion. The trace element geochemistry of clinopyroxene, carbonate and melt pockets suggests that clinopyroxene plays a very important role in fractionation of the rare earth elements, high field strength elements and Sr, whereas carbonate does not host large quantities of incompatible trace elements except for Sr, Ba and, to a lesser extent, Th and U.