Quaternary West Eifel Volcanic Field Research Articles

New evidence for upper Permian crustal growth below Eifel, Germany, from mafic granulite xenoliths

Abstract. Granulite xenoliths from the Quaternary West Eifel Volcanic Field in Germany record evidence of magmatism in the lower crust at the end of the Permian. The xenoliths sampled two distinct bodies: an older intrusion (ca. 264 Myr old) that contains clinopyroxene with flat, chondrite-normalised rare earth element (REE) profiles and a younger (ca. 253 Myr old) intrusion that crystallised middle-REE-rich clinopyroxene. The younger body is also distinguished based on the negative Sr, Zr and Ti anomalies in primitive mantle-normalised multi-element plots. REE-in-plagioclase–clinopyroxene thermometry records the magmatic temperature of the xenoliths (1100–1300 ∘C), whereas Mg-in-plagioclase and Zr-in-titanite thermometry preserve an equilibration temperature of ca. 800 ∘C. These temperatures, together with a model of the mineral assemblages predicted from the composition of one of the xenoliths, define the pressure of crystallisation as ∼1 GPa. The xenoliths also preserve a long history of reheating events whose age ranges from 220 to 6 Myr. The last of these events presumably led to breakdown of garnet; formation of symplectites of orthopyroxene, plagioclase and hercynite; and redistribution of heavy rare earth elements into clinopyroxene. The data from the West Eifel granulite xenoliths, when combined with the existing data from granulites sampled in the East Eifel, indicate that the lower crust has a long a complex history stretching from at least 1.6 Ga with intrusive events at ca. 410 and 260 Ma and reheating from the Triassic to late Miocene.

Pleniglacial sedimentation process reconstruction on laminated lacustrine sediments from lava-dammed Paleolake Alf, West Eifel Volcanic Field (Germany)

Differentiating between regularly seasonal, irregular and event-based clastic sedimentation is difficult if sedimentation structures resemble and dating methods are imprecise. In this study - clastic light and dark laminae from lava-dammed Paleolake Alf in the Late Pleistocene in the Quaternary West Eifel Volcanic Field are analyzed to clarify how they formed and if they are of annual origin and comparable to assumed periglacial varves from neighboring Lake Holzmaar. Therefore, a multiproxy approach is applied combining sediment thin section analysis which focuses on composition and structure with 14C dates. The results are compared to recently-formed annually-laminated clastic sediments of, e.g., the High Canadian Arctic. Observed sedimentation structures reveal sediment delivery by over- and interflows and deposition from suspension forming two characteristic microfacies: Type I graded laminae and Type II laminae with graded sublayers. Additionally, erosional bases and event deposits indicate episodic underflows. Thus, lamination is potentially seasonal but is significantly veiled by extreme runoff causing erosion and resuspension processes or a mixed water body preventing sediment delivery into the lake basin. However, sedimentation processes between watershed and lake could be reconstructed by comparing recent and paleosediment structures.

Palaeobotanical evidence for agricultural activities in the Eifel region during the Holocene: plant macro-remain and pollen analyses from sediments of three maar lakes in the Quaternary Westeifel Volcanic Field (Germany, Rheinland-Pfalz)

As a part of the ELSA-project (Eifel Laminated Sediment Archive) new pollen and plant macro-remain analyses have been carried out on a series of Holocene lacustrine sediments from three open maar lakes of the Quaternary Westeifel Volcanic Field. In combination with already existing pollen analyses, the archaeological record and written sources, the present study casts new light on settlement activities and henceforth the development of agriculture from the prehistoric to historic times in this region. While there are clues that wood pasturing was practised in the Eifel region from the Michelsberg Culture onwards (c. 4300 cal. b.c.), the Vulkaneifel is a remote area with relatively poor soils and a humid climate and was not constantly settled until the Late Neolithic/Early Bronze Age, when cereal pollen was found regularly in the deposits. Plant macro-remains (chaff), which give us direct evidence for arable agriculture in the surroundings of the maars, were also found in layers belonging to the Early Bronze Age (c. 1900 cal. b.c.). At the same time we can observe the massive spread of Fagus sylvatica (beech) in all pollen diagrams, which was most probably caused by a combination of climatic, anthropogenic and competitive factors. Later impacts of agriculture were an abundance of crop weeds and pollen in the following Middle Bronze Age. Nevertheless human impact remained discontinuous until the Urnfield Culture (1200–800 cal. b.c.). A layer of weeds dating at the end of the Urnfield Culture was found and also flax (Linum usitatissimum) cultivation first becomes apparent. However, the subsequent Iron Age and Roman Period reveal only crop weeds and cereal pollen in slightly higher concentrations, but the abundance of Poaceae pollen at this time is most probably consistent with grazing activities. There follows compelling evidence of the importance of flax cultivation and processing at the maars from the Merovingian Period (5th century a.d.) onwards. A detailed insight into the agriculture of the High Medieval comes from flash flood layers of the 14th century a.d., where remains of Secale cereale (rye) and crop weeds reflect winter-sown cultivation of rye. Cannabis sativa (hemp) was also cultivated and processed during the medieval. Finally we can trace the Prussian reforestation in the 19th century a.d., with an increase in Pinus sylvestris (pine) and Picea abies (fir), by both pollen and plant macro-remains.

Caught in the act — The first few hours of xenolith assimilation preserved in lavas of the Rockeskyllerkopf volcano, West Eifel, Germany

Lithic sedimentary xenoliths in magmatic pyroclastic deposits in the Rockeskyllerkopf Volcanic Complex of the Quaternary West Eifel volcanic field preserve evidence of the earliest stages of melting, reaction with and assimilation into their silica-undersaturated basanite host. The xenoliths have a 0.5–1.5 mm thick surface glaze of rhyolitic glass and internally have both silica-undersaturated and silica-saturated glasses that are the result of in situ melting of micas and clay minerals and reaction of these melts with the surrounding quartz matrix. Mixing between the rhyolite glaze and host basanite resulted in the formation of a clinopyroxene- and glass-rich hybrid zone. The glazed xenoliths are commonly found with either no coating of lava or with void space between them and the host. This unusual occurrence is due to vesiculation of the glaze during eruption which led to physical separation of the xenolith from its surrounding melt. Simple calculations indicate that assimilation occurred at ~ 1180 °C over a period of ~ 12 h and that all the textural features including deformation of the partially molten xenoliths, occurred during transport and eruption rather than after deposition.

The temporal evolution of three magmatic systems in the West Eifel volcanic field, Germany

The Quaternary West Eifel volcanic field is the surface expression of a mantle plume that is thought to contain ∼1% melt. The volcanoes range in age from 0.7 Ma to ∼10 000 yr BP and the average recurrence rate of volcanism is around 3.5×10 −4 eruptions per year, giving a mean time between eruptions of 2875 years, assuming that the field is monogenetic. The presence of mafic and ultramafic xenoliths is taken as evidence for rapid, single-stage emplacement of magmas at velocities around 0.5 km/h. However, the presence of clinopyroxene that preserves evidence of polybaric crystallisation belies this simple model. This study uses variations in the Fo content of olivine in mantle-derived xenoliths and in olivine xenocrysts enclosed in cumulate xenoliths to calculate the time scale of evolution of three volcanic centres in the Eifel field. Core–rim variations in olivine are interpreted to result from Fe–Mg interdiffusion between it and a surrounding melt. Using experimental determinations of the interdiffusion coefficient and approximations of magma temperature, contact times have been calculated using planar and radial diffusion models. The three volcanic centres studied were Meerfelder Maar, Gemündener Maar and Baarley. The cumulate xenoliths at Meerfelder Maar formed at a depth of ∼13 km and record a magma emplacement event at between 1 and 3 years prior to eruption. Two suites of mantle-derived olivine clinopyroxenites record a further two events at 1–8 days and ∼3 h prior to eruption. The final event is interpreted to record the eruption of this volcano and the 3-h contact time records the time required for transport of peridotite xenoliths from the mantle to surface, giving an average emplacement velocity of around 15 km/h. The cumulate xenoliths at Gemündener Maar crystallised at depths of between 4 and 22 km. There are four distinct events recorded by the zoned olivine grains in the clinopyroxene-rich cumulates from this locality. The maximum residence time is similar to that at Meerfelder Maar. However, in this case the final magma resided in the chamber for between 23 days and 2.7 months prior to eruption, allowing enough time for any entrained peridotite xenoliths to be filtered out. The cumulate xenoliths from the Baarley locality record a much longer history than those at the other localities. In this case, initially peridotitic olivine has been completely re-equilibrated over >175 years prior to eruption. In common with the other two localities, there is evidence of multiple intrusive events, at depths up to 7 km, prior to final eruption. The duration of the final eruption event and the interpreted depth to source of the xenoliths indicate an emplacement velocity of around 3 km/h. The data derived from modelling of diffusion profiles in olivine from cumulate xenoliths indicate that the magmatic systems feeding these three volcanoes were dynamic and very short-lived. In addition, these data can be used to place constraints on the rate of cumulate formation in subvolcanic magma chambers. On the basis of the data presented here, it would seem that an appreciable thickness of cumulates can form over relatively short time spans (days to a few years).

The pressure and temperature conditions and timing of glass formation in mantle-derived xenoliths from Baarley, West Eifel, Germany: the case for amphibole breakdown, lava infiltration and mineral - melt reaction

Mantle-derived xenoliths from Baarley in the Quaternary West Eifel volcanic field contain six distinct varieties of glass in veins, selvages and pools. 1) Silica-undersaturated glass rich in zoned clinopyroxene microlites that forms jackets around and veins within the xenoliths. This glass is compositionally similar to groundmass glass in the host basanite. 2) Silica-undersaturated alkaline glass that contains microlites of Cr-diopside, olivine and spinel associated with amphibole in peridotites. This glass locally contains corroded primary spinel and phlogopite. 3) Silica-undersaturated glass associated with diopside, spinel ± olivine and rhonite microlites in partly to completely broken down amphibole grains in clinopyroxenites. 4) Silica-undersaturated to silica-saturated, potassic glass in microlite-rich fringes around phlogopite grains in peridotite. 5) Silica-undersaturated potassic glass in glimmerite xenoliths. 6) Silica-rich glass around partly dissolved orthopyroxene crystals in peridotites.

Isotopic and geochemical investigation of a carbonatite-syenite-phonolite diatreme, West Eifel (Germany)

Abstract The Rockeskyll complex in the north, central part of the Quaternary West Eifel volcanic field encapsulates an association of carbonatite, nephelinite and phonolite. The volcanic complex is dominated by three eruptive centres, which are distinct in their magma chemistry and their mode of emplacement. The Auf Dickel diatreme forms one centre and has erupted the only known carbonatite in the West Eifel, along with a broad range of alkaline rock types. Extrusive carbonatitic volcanism is represented by spheroidal autoliths, which preserve an equilibrium assemblage. The diatreme has also erupted xenoliths of calcite-bearing feldspathoidal syenite, phonolite and sanidine and clinopyroxene megacrysts, which are interpreted as fragments of a sub-volcanic complex. The carbonate phase of volcanism has several manifestations; extrusive lapilli, recrystallized ashes and calcite-bearing syenites, fragmented during diatreme emplacement. A petrogenetic link between carbonatites and alkali mafic magmas is confirmed from Sr and Nd isotope systematics, and an upper mantle origin for the felsic rocks is suggested. The chemistry and mineralogy of mantle xenoliths erupted throughout the West Eifel indicate enrichment in those elements incompatible in the mantle. In addition, the evidence from trace element signatures and melts trapped as glasses support interaction between depleted mantle and small volume carbonate and felsic melts. This close association between carbonate and felsic melts in the mantle is mirrored in the surface eruptives of Auf Dickel and at numerous alkaline-carbonatite provinces worldwide.

Combined paleointensity and 40Ar/39Ar age spectrum data from volcanic rocks of the West Eifel field (Germany): Evidence for an early Brunhes geomagnetic excursion

Results of virtual geomagnetic pole position, paleointensity and isotopic age determinations have been derived from lavas of the Quaternary West Eifel volcanic field (Germany). The characteristic remanent magnetization (CARM) directions and corresponding positions of virtual geomagnetic poles (VGP) have mainly been determined by Böhnel et al. They are exclusively of normal polarity. The VGPs form a strongly non‐Fisherian distribution characterized by about 30% pole positions with latitudes below 60°N, which are mainly confined to the longitude sector from 20°W to 120°E. Eight independent poles are clustered at 40°N and 45°E. Geomagnetic paleointensity values from 37 volcanic units of the West Eifel have been determined. Paleointensities range from 7 to 62 μT with an unexpectedly large number of paleointensity values below 30 μT. About half of the low paleointensity values are associated with intermediate VGP latitudes. Isotopic age determinations from West Eifel volcanic rocks have been published (Fuhrmann and Lippolt). Additional isotopic ages are presented here. The West Eifel volcanic field is usually devided into an older part in the NW and a younger one in the SE, based on petrological models and geological evidence. All available isotopic ages of the West Eifel volcanic rocks which are exclusively from the northwestern part of the field lie in the interval between 460 and 660 kyr. For seven of the clustered low‐latitude VGPs it was possible to determine paleointensities and 40Ar/39Ar ages. Within their error limits six poles have identical virtual dipole moments (VDM) at 3.5×1022A m2. Furthermore, the individual isotopic age intervals of the 40Ar/39Ar data overlap. The assigned age of 510±30 kyr is considered to be the age of a geomagnetic excursion. For higher‐latitude VGP positions (>45°N), VDM values range from 2 to 10×1022A m2. The mean VDM value (about 7.5×1022A m2) of the younger volcanoes (ages estimated to be below 100 kyr) is not remarkably different from the present dipole moment of the Earth. In contrast, the older volcanoes with higher‐latitude VGP positions yielded a mean value of only 5.3×1022A m2 and a bimodal frequency distribution of the VDM values. Isotopic ages determined for two of these volcanic units are close to the inferred excursion age suggesting a decrease of the geomagnetic dipole moment during a time interval which surpassed the duration of the excursion.

Determination of geomagnetic palaeointensities from the Quaternary West Eifel volcanic field, Germany

SUMMARY Determination of geomagnetic palaeointensities has been attempted for 418 specimens from 51 volcanic units of the Quaternary West Eifel volcanic field. This paper describes the measuring procedure using a modification of the Thellier method, the analysis of the derived data points, the determination of linear segments and the quality of the individual palaeointensity values calculated from the corresponding slopes. According to a series of acceptance and rejection criteria, finally 312 reliable individual palaeointensity results have been obtained. About 73 per cent of the accepted regression lines have correlation coefficients better than 0.990, and less than 10 per cent of the accepted Thellier experiments have quality factors less than 1.0. The carriers of magnetization of the West Eifel volcanic rocks are titanomagnetites with a broad range of titanium contents and oxidation states in a more or less unweathered, mostly foiditic rock matrix. Accompanying rock magnetic experiments allowed a rough classification of the specimens corresponding to the oxidation state of the titanomagnetites. Specimens with highly oxidized titanomagnetites have been the most successful candidates for the Thellier experiments. However, a prediction of the suitability of the specimens for the Thellier experiments by sample selection according to this classification was not always appropriate. Instead, for most volcanic units pilot specimens were first measured to determine the suitability of a specific sample type, the temperature interval from which the evaluation of palaeointensity was possible, and the best laboratory field intensity to keep the slope errors of the Thellier experiments to a minimum. Palaeointensity determination was not possible for 14 volcanic units because only unsuitable specimens were available. For each of the 37 remaining volcanic units at least five successful Thellier experiments have been carried out, leading to a final mean palaeointensity result. These 37 values have relative standard errors of between 4 and 23 per cent, mostly around 10 per cent. Specimens with differing rock magnetic properties from the same volcanic unit yielded consistent palaeointensity values. Therefore, all West Eifel palaeointensity results have to be regarded as reliable estimates of the Earth’s magnetic palaeofield strength. Mean palaeointensities range from 7 to 62 pT with an unexpectedly large number of palaeointensity values less than 30 pT. About half of the low palaeointensity values are associated with intermediate virtual geomagnetic pole latitudes, which may be due to a geomagnetic excursion.

Paleointensity in the Quaternary West Eifel volcanic field, Germany: preliminary results

Preliminary paleointensity results are presented from 36 sites with virtual geomagnetic pole (VGP) latitudes of about 30–90° normal polarity in the Quaternary West Eifel volcanic field. A strong correlation between VGP latitude and the Earth's virtual magnetic dipole moment (VDM) is observed, with low intensities for low VGP latitudes indicating possibly an emplacement during an excursion or event of the Brunhes epoch. The age distribution of the West Eifel volcanics is, as yet, poorly known. Also, the mean VDM value for sites with high VGP latitudes is considerably lower than the present day dipole moment of the Earth.

Mafic potassic lavas of the Quaternary West Eifel volcanic field

Major and trace element analyses for 103 volcanoes of the Quaternary West Eifel volcanic field show the lavas to be dominantly primitive (MgO>7 wt.%) and potassic (Na2O/K2O∼1). The rocks are divided into (1) a foidite (F)-suite, volumetrically dominant and consisting of four types: leucitites and nephelinites, melilite-bearing foidites, olivine-free foidites, sodalite-bearing melilite-free foidites, and (2) a younger olivine-nephelinite and basanite (ONB)-suite, concentrated in the southeastern part of the field. Dominantly cpx-phyric F-suite magmas differ from the dominantly ol-phyric ONB-suite mainly in higher K2O/ Na2O and CaO/Al2O3-ratios, higher Rb, Cu, H2O, CO2 and LREE concentrations and slightly lower Sr, Ni and Y contents. Most magmas have fractionated small amounts of olivine, clinopyroxene, and minor phlogopite. Systematic compositional variations within volcanoes or volcano groups are rare. Five more differentiated volcanoes (2 tephrites, 3 phonolites) occur in the center of the field. Their magmas are interpreted to have formed by fractionation within crustal magma chambers.