Journal of Petrology | Volume 45 | Number 5 | Pages 883-905 | 2004
Journal of Petrology 45(5) © Oxford University Press 2004; all rights reserved.
Petrogenesis of Tertiary Continental Intra-plate Lavas from the Westerwald Region, Germany
INSTITUT FÜR GEOWISSENSCHAFTEN DER UNIVERSITÄT KIEL, OLSHAUSENSTRASSE 40, D-24118 KIEL, GERMANY
RECEIVED SEPTEMBER 23, 2002; ACCEPTED OCTOBER 1, 2003
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONGEOLOGICAL SETTINGSAMPLING AND ANALYTICAL METHODSRESULTSDISCUSSIONCONCLUSIONSSUPPLEMENTARY DATAREFERENCES |
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Tertiary volcanic rocks from the Westerwald region range from basanites and alkali basalts to trachytes, whereas lavas from the margin of the Vogelsberg volcanic field consist of more alkaline basanites and alkali basalts. Heavy rare earth element fractionation indicates that the primitive Westerwald magmas probably represent melts of garnet peridotite. The Vogelsberg melts formed in the spinel–garnet peridotite transition region with residual amphibole for some magmas suggesting melting of relatively cold mantle. Assimilation of lower-crustal rocks and fractional crystallization altered the composition of lavas from the Westerwald and Vogelsberg region significantly. The contaminating lower crust beneath the Rhenish Massif has a different isotopic composition from the lower continental crust beneath the Hessian Depression and Vogelsberg, implying a compositional boundary between the two crustal domains. The mantle source of the lavas from the Rhenish Massif has higher 206Pb/204Pb and 87Sr/86Sr than the mantle source beneath the Vogelsberg and Hessian Depression. The 30–20 Ma volcanism of the Westerwald apparently had the same mantle source as the Quaternary Eifel lavas, suggesting that the magmas probably formed in a pulsing mantle plume with a maximum excess temperature of 100°C beneath the Rhenish Massif. The relatively shallow melting of amphibole-bearing peridotite beneath the Vogelsberg and Hessian Depression may indicate an origin from a metasomatized portion of the thermal boundary layer.
KEY WORDS: continental rift volcanism; basanites; trachytes; assimilation; fractional crystallization; partial melting
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTINGSAMPLING AND ANALYTICAL METHODSRESULTSDISCUSSIONCONCLUSIONSSUPPLEMENTARY DATAREFERENCES |
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Continental intra-plate volcanism occurs within lithosphere of all ages from Archaean to Phanerozoic; generation of the most primitive magmas has been explained either by lithospheric extension inducing decompression melting or by a rise in the mantle temperature within a deep mantle plume (Turcotte & Emerman, 1983
). Volcanic rocks of Tertiary and Quaternary age are abundant in Central Europe, occurring in Germany, France, Hungary, the Czech Republic, and Poland. Early models attributed the volcanic activity to the formation of a large rift system caused by the Alpine collision, with magmas formed by adiabatic melting as a result of the extension of the lithosphere (Illies & Greiner, 1978; Sengör et al., 1978; Dewey & Windley, 1988). More recently, however, a number of workers have suggested that a mantle plume may underlie the Rhenish Massif (Granet et al., 1995; Hoernle et al., 1995; Goes et al., 1999; Ritter et al., 2001), and that the volcanism is thus due to adiabatic melting of anomalously hot mantle. For example, Ritter et al. (2001) suggested excess temperatures of 150–200°C for a 100 km wide deep mantle plume situated beneath the Eifel, some 100 km to the west of the Westerwald region.
The composition and the origin of the magma sources of the Tertiary Central European volcanic province have been extensively debated. Three models have been proposed: (1) the magmas are partial melts of metasomatically enriched asthenospheric mantle (Wedepohl et al., 1994; Hegner et al., 1995); (2) the magmas form at the base of the lithosphere in a thermal boundary layer (TBL) that was enriched by a mantle plume (Wilson et al., 1995); (3) the magmas represent partial melts of a deep mantle plume (Granet et al., 1995; Hoernle et al., 1995; Goes et al., 1999). The effect of lithospheric contamination has been noted in several studies of lavas from the German volcanic province and this process further complicates the definition of possible magma sources (Wilson & Downes, 1991; Wedepohl et al., 1994; Hoernle et al., 1995; Jung & Masberg, 1998).
In this paper, we present new geochemical and Sr, Nd, and Pb isotopic data for a suite of Tertiary volcanic rocks from the Westerwald and Vogelsberg areas. We show that crustal assimilation and fractional crystallization are important processes affecting the lavas and that the mantle-derived melts show evidence for contamination by crustal rocks of regionally different compositions. Petrological data suggest a maximum excess temperature of 100°C in the mantle beneath the Westerwald in the Tertiary, which we relate to the activity of a mantle plume, whereas the eastern lavas formed from cooler mantle, probably the thermal boundary layer at the base of the lithosphere, and erupted in a sedimentary basin. The mantle plume source has a different isotopic composition from the thermal boundary layer source and the two sources produced magma at different times.
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GEOLOGICAL SETTING |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTINGSAMPLING AND ANALYTICAL METHODSRESULTSDISCUSSIONCONCLUSIONSSUPPLEMENTARY DATAREFERENCES |
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Volcanic rocks of Tertiary age occur in the northern Rhine Graben region in a 50 km wide belt between the Eifel and Siebengebirge in the west and the Vogelsberg, Hessian Depression and the Rhön in the east (Fig. 1a). The Westerwald volcanic field is the second largest occurrence of Tertiary volcanic rocks in Germany after the Vogelsberg volcanic field and lies between the Eifel and Vogelsberg regions. Geophysical studies suggest that the volcanic regions lie north of a triple junction situated near the city of Frankfurt (Fig. 1a) where the Upper Rhine Graben splits into the NW-trending Lower Rhine embayment and the NE-trending Hessian Depression (Illies & Greiner, 1978
; Fairhead & Stuart, 1982; Ziegler, 1992). The Hessian Depression forms a continuous sedimentary basin with the Upper Rhine Graben, whereas the Lower Rhine embayment is connected with the Upper Rhine Graben by a system of faults running through the Rhenish Massif (Ziegler, 1992). Seismic activity has been limited to the Upper Rhine Graben and Lower Rhine embayment whereas the Hessian Depression has been largely inactive (Bonjer et al., 1984; Bonjer, 1997). Earthquake fault-plane solutions indicate NE–SW-directed extension for the whole Rhine Graben (Plenefisch & Bonjer, 1997), which is probably due to the combined forces of Mid-Atlantic Ridge push in the north and the Alpine collision in the south (Müller et al., 1992). Uplift of the Rhenish Massif probably started in the Eocene and has continued in the northern region of the Upper Rhine Graben to recent times (Sengör et al., 1978; Ziegler, 1992). The formation of the Rhine Graben rift basins commenced in the late Eocene with maximum subsidence phases from the late Eocene to early Oligocene (42–31 Ma) and late Oligocene to early Miocene (25–20 Ma) (Ziegler, 1992). During the Oligocene and Miocene marine transgressions occurred and the total thickness of Tertiary sediments in the northern rift arms ranges from less than 1000 m (Teichmüller, 1974) in the Lower Rhine embayment to more than 3000 m in the northern Rhine Graben (Meier & Eisbacher, 1991).
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, Vogelsberg region. The dotted line indicates the approximate location of a zone of major faults between the uplifted Palaeozoic sedimentary cover of the Rhenish Massif and the Mesozoic sedimentary basin of the Hessian Depression.The Westerwald volcanic field covers about 800 km2 and consists of (1) a larger part (
500 km2) in the NE that comprises several mafic lava flows and (2) a smaller part in the SW (280 km2) dominated by trachytic to phonolitic lavas, intrusions and volcaniclastic rocks (Schreiber et al., 1999). The lavas of the Westerwald region overlie Devonian and Carboniferous sedimentary and volcanic rocks of the Rhenish Massif. The crustal thickness beneath the Westerwald is about 30 km and several low-velocity layers have been identified in both the crust and the mantle beneath the region (Prodehl et al., 1992). K–Ar age dating of the SW Westerwald volcanic rocks has suggested three phases of volcanism with the main activity at about 25 ± 3 Ma and two later phases at 5·6 Ma and 0·4–0·8 Ma (Lippolt & Todt, 1978). Isolated volcanic plugs and remnants of lava flows occur east of the Westerwald; the basaltic rocks in the area of the Westerwald have been dated at 32–21 Ma whereas the eastern lavas at the margin of the Vogelsberg erupted between 19 and 9 Ma (Turk et al., 1984). The latter ages are comparable with the age dates for the Vogelsberg lavas, which are significantly younger than most Westerwald lavas. Thus, the two adjacent volcanic regions show different times of activity and two groups of lavas can be distinguished based on their location and age. |
SAMPLING AND ANALYTICAL METHODS |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTINGSAMPLING AND ANALYTICAL METHODSRESULTSDISCUSSIONCONCLUSIONSSUPPLEMENTARY DATAREFERENCES |
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Sixty samples were taken in the Westerwald volcanic field and from small occurrences (remnants of lava flows, plugs and necks) in the region surrounding the Westerwald (Fig. 1b). Another 23 samples were collected from small outcrops west and NW of the Vogelsberg volcanic field (Fig. 1b). In the following discussion the samples are grouped according to their geographical occurrence into Westerwald and Vogelsberg region lavas. The sample suite consists mainly of mafic lavas and intrusive rocks; however, samples from the southwestern part of the Westerwald are mainly felsic lavas. An extensive programme of K–Ar dating has been carried out in this region (Lippolt & Todt, 1978
; Turk et al., 1984) and we re-sampled several of the previously dated outcrops to study both the temporal as well as the regional variation in lava composition. We tried to collect fresh samples in the field and from these samples the interiors were sawn. The sawn pieces were washed with water and then crushed to coarse sand size, which was washed again with deionized water. Then the samples were reduced to powder in an agate ball mill.
The petrography of several samples was studied macroscopically (Table 1) and in thin section, and the mineral phases of representative samples were analysed by electron microprobe. Whole-rock major element analyses were obtained by X-ray fluorescence spectrometry (XRF) with a Philips PW1400 system at the Institut für Geowissenschaften, Universität Kiel, using international rock standards for calibration and data quality control. Average results for the international rock standard BHVO-1 are presented in Table 2 together with the major element data for the samples. Trace elements were analysed by inductively coupled plasma mass spectrometry (ICP-MS) with an upgraded PlasmaQuad PQ1 system at the Institut für Geowissenschaften, Universität Kiel, following the method of Garbe-Schönberg (1993). The reproducibility of replicate analyses of the samples is better than 4% and the accuracy of the data based on the analysis of international rock standard JB-1a (Table 2) is better than 5% for most elements.
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For isotopic determinations, the rock powders were leached for 1 h in hot ultrapure 6N HCl before dissolution. The ion exchange techniques used to produce Sr, Nd and Pb separates were described by Hoernle & Tilton (1991)
. Strontium and Pb isotope ratios were analysed using a Finnigan MAT 262 mass spectrometer in static mode at GEOMAR, Kiel. The Nd isotope compositions were analysed in dynamic mode on the same machine. Applied isotope fractionation corrections for Sr were 86Sr/88Sr = 0·1194 and 146Nd/144Nd = 0·7219, with repeated measurements of NBS 987 (n = 12) yielding 87Sr/86Sr = 0·710218 (2
= 0·000024). Repeat measurements (n = 10) of the Nd Spex standard gave an average of 0·511710 (15) and of the La Jolla standard (n = 3) gave 143Nd/144Nd = 0·511827 (2 = 0·000007). Our reported Sr and Nd analyses (Table 3) are normalized to values of NBS 987 = 0·71025 and La Jolla of 0·511855, respectively. For Pb, the analyses were fractionation-corrected using repeated measurements of NBS 981 (n = 13; errors are 2 values; 206Pb/204Pb = 16·909 ± 0·017, 207Pb/204Pb = 15·455 ± 0·022, 208Pb/204Pb = 36·584 ± 0·069) normalized to its accepted values (Todt et al., 1996). The relative precision per mass unit of the NBS 981 runs was <1
(2), and Pb blanks were negligible (<50 pg). As a result of the relatively high Rb/Sr, U/Pb and Th/Pb ratios in the samples, significant age corrections are necessary and the results are shown in Table 3 using the concentrations determined by ICP-MS (Table 2). The age-corrected isotope compositions are used in the figures.
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RESULTS |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTINGSAMPLING AND ANALYTICAL METHODSRESULTSDISCUSSIONCONCLUSIONSSUPPLEMENTARY DATAREFERENCES |
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Petrography and mineralogy of the Westerwald region lavas
The mineralogy of the samples from the various locations is given in Table 1. Lava samples are generally fresh; however, olivine phenocrysts often show iddingsite rims and in several samples vesicles are filled with carbonate. The most primitive lavas contain phenocrysts of olivine, clinopyroxene and spinel, with crystals reaching sizes up to 5 mm. Peridotitic xenoliths up to 2 cm in diameter, as well as olivine and orthopyroxene xenocrysts several millimetres in diameter, are observed in several samples. The matrix olivines as well as olivine phenocrysts contain 70–85% Fo whereas the olivine in the xenoliths and the olivine xenocrysts have higher Fo contents of 88–91%. Clinopyroxenes in the more primitive rocks are brownish Ti-augites with strong zoning and the large crystals often show euhedral growth rims surrounding rounded lighter-coloured cores. The compositions are similar to the Ti-augites in the Eifel lavas (Duda & Schmincke, 1985
) and are in the range of 0·62–0·85 atoms Mg per formula unit (p.f.u.) and 0·45–0·15 atoms Al p.f.u. The Ti/Al ratios range from 0·12 in the cores of phenocrysts to 0·30 in rims of phenocrysts and matrix crystals. Matrix plagioclase in the primitive lavas ranges from An64 to An87. The more evolved lavas often contain large volumes of phenocrysts of plagioclase, anorthoclase, sanidine and kaersutite, as well as less abundant aegirine– augite, Ti-magnetite and apatite. The plagioclase phenocrysts have compositions ranging from An26 to An45. Typically, the euhedral kaersutite crystals are surrounded by thick opacitized rims or are completely altered. Biotite occurs more rarely in the evolved rocks. Several samples (e.g. samples 010 and P19) contain xenoliths of country rocks; for example, fragments of the underlying Tertiary claystones.
Geochemical compositions of the Westerwald region lavas
The lavas from the Westerwald region span a range of compositions from basanites and picrobasalts to trachytes, whereas the samples from the Vogelsberg margin consist of basanites and alkali basalts (Fig. 2a). The most primitive samples (SiO2 <45 wt %) from the region surrounding the Vogelsberg are generally more alkaline than the lavas from within the Vogelsberg volcanic field and from the Westerwald. Many Westerwald region lavas lie on a trend with higher Al2O3 contents for a given SiO2 content than most of the Vogelsberg lavas (Fig. 2b). The primitive lavas show significant differences in their incompatible element compositions with, for example, TiO2 ranging between 2·2 and 4·0 wt % (Fig. 2c). The variation in TiO2 content (Fig. 2c) is similar to that of lavas from the Vogelsberg volcanic field (Bogaard & Wörner, 2003); we also find high-TiO2 basanites in the Vogelsberg margin of the Westerwald. Most of the primitive Vogelsberg region samples have relatively low TiO2 contents and thus probably represent the volumetrically most abundant lava type in the Vogelsberg volcanism. FeOT and CaO contents decrease with increasing SiO2 concentration, paralleling the trend for the Vogelsberg (Fig. 2d and f). The majority of the basanitic and alkali basaltic lavas have relatively high MgO contents between 8 and 14 wt % and exhibit a large variation in SiO2 content (40–50 wt %) (Fig. 2e). Many primitive Vogelsberg lavas have higher SiO2 contents for a given MgO than the mafic rocks from the Westerwald region.
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Two trends are observed in Fig. 2e for the more evolved rocks with more than 45 wt % SiO2: (1) a trend defined by Westerwald region plus some Vogelsberg lavas with low MgO contents; (2) a linear trend at higher MgO contents mostly defined by Vogelsberg samples (Fig. 2e). Na2O and K2O concentrations increase with increasing SiO2 up to
63 wt % SiO2; several of the Vogelsberg margin lavas have higher contents in both alkali elements than the Westerwald lavas and the most primitive rocks from the Vogelsberg volcano (Fig. 2g and h).
The (Ce/Yb)N ratios of the Westerwald basanites and alkali basalts with <50 wt % SiO2 range between 10 and 20 whereas those in the trachytes (SiO2 63 wt %) are 18–20 (Fig. 3a). The mafic rocks from the Vogelsberg margin resemble the published Vogelsberg volcanic field lavas of Bogaard & Wörner (2003). Most of the data from the Westerwald and the Vogelsberg lie on a negative trend between (Ce/Yb)N and SiO2 (Fig. 3a). To determine the possible influence of crustal contamination on the magmas we have also plotted data for lower-crustal rocks from the Eifel in Fig. 3, which are characterized by low (Ce/Yb)N and show some overlap with the Vogelsberg lavas.
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The Westerwald and Vogelsberg lavas show a broad negative correlation between Ce/Pb and SiO2 content (Fig. 3b). The majority of the basalts have Ce/Pb ratios between 25 and 35; however, three primitive Vogelsberg margin samples with SiO2 <50 wt % and about 12 wt % MgO have Ce/Pb of 15–20 and resemble the Eifel granulite xenoliths (Fig. 3b). The trachytes have low Ce/Pb of about 13, which is also comparable with the Ce/Pb composition of the lower continental crust.
With the exception of one trachyte sample all the lavas from the Westerwald and Vogelsberg lie on a negative trend of Ce/Pb vs Ba/La (Fig. 4a); all lavas with low Ce/Pb lie close to the compositions of the Eifel lower-crustal granulites. The volcanics from the Vogelsberg (Bogaard & Wörner, 2003) plot on the same trend as the Westerwald lavas but have even higher Ce/Pb than the Westerwald samples (Fig. 4a). Generally, the Ce/Pb and the Nb/U ratios of continental crustal rocks are significantly lower than those of mantle-derived magmas (Hofmann et al., 1986). Most of the Westerwald lavas, even some with low Ce/Pb, have high Nb/U within the range suggested for the mantle (Fig. 4b). The Vogelsberg lavas of Bogaard & Wörner (2003) lie on a positive trend between the two ratios extending towards the crustal granulites. Because most of our samples lie in the typical Nb/U range of mantle-derived basaltic rocks (47 ± 10; Hofmann et al., 1986) we consider that crustal contamination or alteration of the samples is negligible and that U (as well as Rb and K) has not been mobilized. The Eifel granulites define two groups; one lies on the elongation of the Vogelsberg lava trend but at even lower Ce/Pb and Nb/U, whereas the other group of lower-crustal rocks has mantle-like high Nb/U but low Ce/Pb similar to several of our samples (Fig. 4b).
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Isotopic compositions of the Westerwald and Vogelsberg margin lavas
Most of the Westerwald and Vogelsberg margin lavas from this study have relatively high 143Nd/144Nd (>0·5128) compared with many volcanic rocks from the Eifel, Hessian Depression and Vogelsberg. However, a number of samples (e.g. a mugearite and a trachyte) trend towards higher Sr (
0·7047) and lower Nd isotope ratios (Fig. 5a). Vogelsberg volcanic field lavas with the same 143Nd/144Nd have lower 87Sr/86Sr than Westerwald, Eifel and Siebengebirge samples. Some of the Westerwald region lavas lie at the radiogenic (high 206Pb/204Pb) end of the highly variable Pb isotope compositions observed in the Tertiary volcanic rocks from the Eifel to the west of the Vogelsberg (Fig. 5b and c). However, several of the Westerwald samples from the Vogelsberg margin plot within the mid-ocean ridge basalt (MORB) array but have lower 143Nd/144Nd than MORB (Figs 5a and 6). The Sr–Nd–Pb isotopic compositions of volcanic rocks from the region between the Eifel in the west and the Hessian Depression in the east (Fig. 1) show that two groups of lavas exist. Relative to the eastern lavas from the Hessian Depression and Vogelsberg, the western lavas from the Eifel and Siebengebirge have high 87Sr/86Sr for a given 143Nd/144Nd or 206Pb/204Pb (Figs 5a and 6a) and higher 206Pb/204Pb for a given 143Nd/144Nd, although there is some overlap (Fig. 6b). Consequently, we define an ‘Eifel’ group consisting of lavas from the Eifel, Siebengebirge and Westerwald region in contrast to the lavas with lower 87Sr/86Sr and 206Pb/204Pb from the Vogelsberg region and Hessian Depression, which form the ‘Vogelsberg’ group (Figs 5 and 6). The samples from the Westerwald region fall into the Eifel group whereas the samples from the Vogelsberg margin are similar to the Vogelsberg volcanic rocks (Fig. 5).
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The lavas with SiO2 contents higher than 48 wt % have significantly lower 143Nd/144Nd (<0·51275) than the basanites and alkali basalts (Fig. 7a). Several lava compositions overlap with the compositional field of Eifel granulites. The Sr concentrations in the lavas vary by a factor of three at approximately constant 87Sr/86Sr (Fig. 7b). Lavas from the Westerwald (Eifel group) with high Ce/Pb of about 30 have 206Pb/204Pb between about 19·4 and 19·6, whereas Vogelsberg group lavas with high Ce/Pb have lower 206Pb/204Pb between 19·0 and 19·4 (Fig. 8a). For both groups, the volcanic rocks with low Ce/Pb also have lower 206Pb/204Pb and overlap with the lower-crustal granulites from beneath the Eifel. Thus, two distinct positive trends between Ce/Pb and 206Pb/204Pb can be defined for the Vogelsberg group and the Eifel group. The Vogelsberg group samples have a positive correlation between (Ce/Yb)N and 206Pb/204Pb, and lavas with the lowest 206Pb/204Pb show similar low (Ce/Yb)N to Eifel granulites (Fig. 8b). The Westerwald lavas have lower (Ce/Yb)N and higher 206Pb/204Pb than the Vogelsberg group samples with the highest 206Pb/204Pb (Fig. 8b).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTINGSAMPLING AND ANALYTICAL METHODSRESULTSDISCUSSIONCONCLUSIONSSUPPLEMENTARY DATAREFERENCES |
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Fractional crystallization and crustal contamination of the lavas
The large range of MgO contents in the lavas from the Westerwald and Vogelsberg region indicates that fractional crystallization processes have affected the magmas during ascent. It has been shown previously that comparable alkaline magmas from the Eifel stagnate in the lower crust at pressures of about 0·65 GPa close to the brittle–ductile boundary (Duda & Schmincke, 1985
; Sachs & Hansteen, 2000). The Westerwald and Vogelsberg region mafic magmas contain Ti-augites with cores having relatively high Al contents and low Ti/Al ratios similar to the high-pressure clinopyroxenes in the alkaline magma series from the Eifel and Slovakia (Duda & Schmincke, 1985; Dobosi & Fodor, 1992). Consequently, the Westerwald and Vogelsberg region magmas are likely to have stagnated in the lower crust at comparable depths to the Eifel magmas, and assimilation of crustal wall rocks during the time of stagnation and crystallization is possible.
The lower crust beneath the Rhenish Massif consists of mafic to felsic granulites, whereas the upper crust is composed of Palaeozoic sediments and volcanics (Mengel et al., 1991). Detailed geochemical studies with trace element and isotope data exist only for the lower-crustal rocks from the Eifel (Stosch & Lugmair, 1984; Stosch et al., 1986, 1991; Loock et al., 1990; Rudnick & Goldstein, 1990) and these data are used to investigate the influence of assimilation of crustal rocks on the Tertiary magmas. The lower continental crustal rocks have Ce/Pb below 20 and generally high but variable Sr, and low Nd isotopic compositions (Figs 4 and 5). In contrast, oceanic basalts [(MORB and ocean island basalts (OIB)] have high Ce/Pb of 25 ± 5, reflecting the composition of the Earth's mantle (Hofmann et al., 1986), and are similar to the basanites and alkali basalts from the Westerwald and Vogelsberg region, which have Ce/Pb between 20 and 40 (Fig. 3b). The low Ce/Pb in some lavas, the observed correlations of Ce/Pb and 143Nd/144Nd with SiO2 (Figs 3b and 7a), and the correlation of Ce/Pb with Ba/La (Fig. 4a) suggest assimilation of lower-crustal rocks with comparable compositions to the Eifel granulites. We suggest that the lavas with low Ce/Pb and relatively high SiO2 contents have assimilated significant amounts of lower-crustal material. The two different trends of Ce/Pb vs 206Pb/204Pb of the Vogelsberg group and the Eifel group (Fig. 8a) and the overlap of the lavas with low Ce/Pb and the Eifel granulites suggest that two regionally distinct crustal end-members are present. However, most of the Tertiary lavas with low Ce/Pb have higher SiO2 contents than the analysed granulites, indicating that assimilation and fractional crystallization processes (AFC) occurred together. The energy required for the melting of country rocks is released by the crystallization processes of the magma, and the recent models of Spera & Bohrson (2001) suggest energy-constrained AFC (EC-AFC).
To test the influence of assimilation and fractional crystallization we performed calculations for both the Eifel and the Vogelsberg groups using the EC-AFC model with the parameters listed in Table 4. The temperature of the lower crust beneath the Eifel has been estimated at about 800°C (Sachs & Hansteen, 2000) and thermodynamic models show that at this temperature high rates of assimilation relative to fractional crystallization (r) of 2·0–2·7 can occur (Reiners et al., 1995). For the uncontaminated magma end-member we use the incompatible element composition of basanite sample 010-4, which is primitive with 13·3 wt % MgO and which has a Ce/Pb of 29. Together with these concentrations we use average isotope compositions of the uncontaminated Eifel and Westerwald group lavas, respectively (Table 4). The Eifel granulites have very variable compositions and in our EC-AFC model we use two granulites (S32 and S35) having the approximate isotopic compositions of the two end-members suggested by the Eifel and the Vogelsberg groups in isotope–isotope and isotope–incompatible element diagrams (Figs 5, 6 and 8). The trace element concentrations of these two granulites differ significantly (Table 4) and their Ce/Pb ratios are too high to represent the exact end-members (Fig. 8a), but the range of Pb isotope variations in the two observed lava groups can be reproduced (Figs 5a and 6). Furthermore, it is possible largely to reproduce the variation of Ce/Pb, Ba/La and Sr concentrations with this model and we conclude that EC-AFC played an important role in the genesis of the more evolved Tertiary lavas. For example, the Westerwald region lavas may have formed by up to 35% fractionation and 1–5% assimilation of a granulite with a composition like that of sample S32 (Stosch & Lugmair, 1984; Loock et al., 1990; Rudnick & Goldstein, 1990). On the other hand, if the magmas of the Vogelsberg group assimilated granulite with lower concentrations of Sr, Nd and Pb, comparable with sample S35, up to 50% assimilation and extremely high degrees of fractionation (to 90%) are required to generate the most evolved lavas (Figs 5 and 6). However, the composition of the lower crust beneath the north German volcanic fields has to be determined much better in order to better define and quantify the AFC processes in the Tertiary magmas of the Rhenish Massif, Vogelsberg and Hessian Depression.
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In conclusion, those lavas with high SiO2 and Ba/La but low Ce/Pb and Nd isotope ratios have assimilated significant amounts of lower-crustal material and concurrently underwent fractional crystallization processes, in agreement with the results of Jung & Masberg (1998)
. The significant crustal contamination of the Westerwald and Vogelsberg basalts with more than 48 wt % SiO2 implies that there are no primary SiO2-rich basaltic (tholeiitic) magmas which would reflect shallow and high-degree partial melts of the mantle. The only lavas with high Ce/Pb and SiO2 contents of 47–48 wt % in Fig. 3b are alkali basalts and hawaiites.
Evidence from the lavas for regionally distinct lower-crustal compositions
The observed variations in Pb and Sr isotope composition for the crustal end-members that have contaminated the two lava groups imply that there are significant differences in crustal composition between the western region of the Eifel, Siebengebirge and Westerwald and the eastern volcanic region of the Vogelsberg and Hessian Depression. Crustal assimilation by the magmas may average out the large compositional variation of the crustal rocks and so the contaminated lavas can be used to define a representative crustal composition for a large region. The lava compositions require that the lower crust below the Rhenish Massif probably has 87Sr/86Sr >0·7060 and 206Pb/204Pb >19·2 whereas the crust contaminating the Vogelsberg group melts has 87Sr/86Sr <0·7045 and 206Pb/204Pb <18·6 as well as lower 143Nd/144Nd and 207Pb/204Pb (Figs 5 and 6). We speculate that a distinct isotopic boundary occurs in the lower crust between the area of the Eifel, Siebengebirge and Westerwald in the west and the eastern region of the Vogelsberg and Hessian Depression. The existence of a relatively sharp boundary in the crustal composition east of the Westerwald coincides with changes in the seismic and magnetotelluric properties of the middle and lower crust (Prodehl et al., 1992) and with the location of a zone of north–south-trending faults marking the eastern boundary of the Rhenish Massif (Fig. 1). A tectonic boundary between two crustal blocks of different composition appears possible. Unfortunately, no isotopic data exist for crustal xenoliths of the eastern region but it is known that there are significant lithological differences between the two areas. Xenoliths from the Hessian Depression and available seismic data indicate that the lower crust beneath the Hessian Depression consists of mafic granulites whereas the lower crust beneath the Eifel contains largely meta-granitic and tonalitic rocks (Mengel et al., 1991). We suspect that the felsic lower-crustal rocks beneath the Eifel have significantly higher 87Sr/86Sr and 207Pb/204Pb than the more mafic granulites and amphibolites beneath the Hessian Depression, which could explain the higher Sr and 207Pb/204Pb isotope ratios in the contaminated lavas of the Eifel group compared with the eastern Tertiary volcanic region.
Magma generation beneath the Westerwald region
The relatively uncontaminated lavas from the Vogelsberg region generally show higher (Ce/Yb)N than the primitive Westerwald lavas in spite of their lower 206Pb/204Pb (Fig. 8b). Thus, either the mantle beneath the Vogelsberg is more enriched in incompatible elements such as the light rare earth elements (LREE) or the Vogelsberg magmas formed by lower degrees of partial melting of a relatively homogeneous source in terms of incompatible element concentrations. It is generally accepted that the upper mantle is composed dominantly of peridotite and possibly contains minor amounts of pyroxenitic material with or without garnet. Silica-undersaturated melts form at high pressures from garnet peridotite (Kushiro, 1996) but not from garnet pyroxenite or eclogite (Rapp et al., 1991) and thus the most likely magma source of the Westerwald region basanites is garnet lherzolite, which is stable at depths below about 70 km (Robinson & Wood, 1998). Seismic models suggest that the lithosphere has a thickness of about 50–60 km below the Rhenish Massif (Babuska & Plomerova, 1992; Goes et al., 2000), i.e. melting must occur at greater depths. The strong fractionation of the heavy REE (HREE) with (Dy/Yb)N >1·6 (Fig. 9a) indicates that the primitive Westerwald magmas formed in the presence of residual garnet. However, most of the Vogelsberg lavas have lower (Dy/Yb)N for the same range of (Ce/Yb)N than the Westerwald region lavas, indicating less garnet in their source.
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The relatively high Nd isotope ratios of the most primitive magmas suggest that the mantle sources had been depleted for a long period of time. Consquently, we use two model mantle sources that formed from residues after 0·05% (source A) and 0·5% (source B) melting of primitive mantle in our melting model for the petrogenesis of the relatively uncontaminated lavas (with Ce/Pb >20) (Figs 9 and 10). Source A then mixed with 10% of a 1% melt from mantle depleted by 0·5% melting and source B with 10% of a 2% melt from the same depleted mantle. Such mantle sources may form by mixing depleted peridotite with subducted intra-plate basalts (e.g. ocean-island basalt) (Hofmann & White, 1982
) or the depleted mantle may have been metasomatized by small-degree melts; for example, as a former part of the lithosphere (McKenzie & O'Nions, 1983; Hawkesworth et al., 1984). The relatively high K/La ratios of the primitive Westerwald and Vogelsberg basalts allow depletion of the source by only small melt fractions because otherwise the mantle would be too depleted and would require extremely high volumes of a re-enriching melt or the recycled component to generate a significant melt fraction. However, both the Westerwald and Vogelsberg lavas display large ranges of K/La between about 100 and 350, and several of the Vogelsberg region lavas with the highest (Ce/Yb)N and La concentrations have the lowest K/La (Figs 9b and 10). The REE melting model of Fig. 9a suggests that the mantle source of the Westerwald region magmas must have been enriched in the LREE to give degrees of partial melting of about 2–5% melting. Experiments on silicate melt compositions suggest that basanitic and alkali basaltic liquids form at degrees of partial melting higher than 1% (Green, 1973; Mysen & Kushiro, 1977; Kushiro, 1996) and a permeability threshold of 2–3% porosity has to be exceeded to allow silicate melt movement in the mantle (Faul, 1997). The REE composition of the Westerwald region lavas can be modelled by about 4–7% melting of an enriched garnet peridotite (source B). On the other hand, the lower (Dy/Yb)N of the Vogelsberg lavas can be modelled by 3–10% partial melting in the garnet–spinel peridotite transition zone in agreement with the model of Bogaard & Wörner (2003). The transition from garnet to spinel lherzolite occurs at 2·4–2·7 GPa in peridotite of MORB pyrolite composition (Green & Ringwood, 1967; Robinson & Wood, 1998), suggesting that the Vogelsberg magmas formed at 70–80 km depth (Fig. 11). According to the REE model the Westerwald lavas have formed by similar degrees of melting, but significantly deeper in the mantle (>2·7 GPa), than the Vogelsberg lavas. Thus, the solidus beneath the Westerwald must lie deeper than beneath the Vogelsberg, requiring a hotter mantle beneath the Westerwald, whereas the lithosphere must have been thinned as a result of extension beneath the sedimentary basin of the Vogelsberg region (Fig. 11).
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, Melting experiments of peridotite + CO2 from Hirose (1997)
, melting experiments from Mysen & Kushiro (1977)
, Liquids generated from phlogopite–garnet peridotite by Mengel & Green (1989)
, those from Thibault et al. (1992)Most lava compositions in the diagram of K/La vs (Ce/Yb)N (Fig. 9b) can be explained by partial melting of garnet peridotite or garnet–spinel peridotite. However, the low K/La of some of the most LREE-enriched lavas probably did not form by melting of garnet peridotite or spinel peridotite because low-degree melts have both high (Ce/Yb)N and K/La (Fig. 9b). A depletion of the mantle source by more than 0·5% partial melting would lead to lower K/La (Fig. 10) but appears unlikely given the LREE enrichment of the lavas. Thus, a mineral phase that fractionates K/La, probably amphibole, could have been present in the mantle source of some Vogelsberg region magmas and could produce the low K/La at low degrees of melting (Fig. 9b). Residual phlogopite appears less likely than amphibole because phlogopite fractionates K/La even more efficiently than amphibole and also fractionates Ba/La significantly, whereas Ba/La is relatively constant in the uncontaminated Westerwald and Vogelsberg lavas (Fig. 4a). Amphibole is known from metasomatized mantle peridotite xenoliths in all regions of the Eifel, Vogelsberg and Hessian Depression (Stosch & Seck, 1980
; Kramm & Wedepohl, 1990). Because the Vogelsberg group magmas with low K/La apparently formed in the garnet–spinel peridotite transition zone in the presence of residual amphibole (Fig. 9b) we infer that their melting region lies at about 1250°C and 2·5 GPa (Fig. 11). Consequently, the Vogelsberg group magmas probably represent melts from a metasomatized part of the TBL similar to the source that has been suggested by Wilson et al. (1995) for the melilitites of the Central European volcanic province. The amphibole may have formed by metasomatism of the mantle by migration of small-degree magmas at the margin of a mantle plume beneath the Rhenish Massif; thinning of the TBL during extensional phases in the Oligocene and Miocene (Villemin et al., 1986) could have initiated low degrees of melting.
Two possible models for the generation of the Tertiary magmas can be envisaged: (1) adiabatic melting as a result of thinning of the lithosphere during rifting; (2) a raised mantle temperature of perhaps 200°C (Ritter et al., 2001). Data from experimental petrology on the formation of Si-undersaturated melts are shown in Fig. 11 and can give important insights into the range of pressure and temperature of generation of the Westerwald magmas. Basanitic melts have been shown to form (1) by melting at pressures greater than 2 GPa in the presence of CO2 and residual garnet or (2) by melting of an amphibole- or phlogopite-bearing spinel peridotite or garnet peridotite. For example, magmas with 40–42 wt % SiO2 similar to the Westerwald basanites can form at 3 GPa and 1475°C (Hirose, 1997) and at 2 GPa and 1360°C (Mysen & Kushiro, 1977) with a solidus lowered by low contents of volatiles in the mantle (Fig. 11). On the other hand, experiments on phlogopite-bearing garnet peridotites have also yielded basanitic melts at much lower temperatures of 1200–1250°C at 2·8–3·0 GPa (Mengel & Green, 1989; Thibault et al., 1992). Dry mantle at 3 GPa produces magmas with about 45 wt % SiO2 similar to the uncontaminated alkali basalts of the Westerwald (Jaques & Green, 1980; Kushiro, 1996). The formation of melts with lower SiO2 requires the presence of CO2 + H2O (Brey & Green, 1977; Mengel & Green, 1989; Thibault et al., 1992; Hirose, 1997). Thus, both experimental constraints and our REE model suggest that the highly undersaturated magmas of the Westerwald may have formed in a garnet peridotite mantle source with a potential temperature of around 1400°C (Fig. 11). The average mantle has been inferred to have a potential temperature of 1300°C (McKenzie & Bickle, 1988), implying an upper limit of the excess temperature of 100°C for any mantle plume that might have existed beneath the Westerwald 20 Myr ago. We conclude that petrological data do not support the involvement of very hot mantle (with an excess temperature of 200°C) in the petrogenesis of the Tertiary Westerwald magmas as has been suggested on the basis of seismic tomography data for the Quaternary Eifel plume (Ritter et al., 2001). On the other hand, if there is a hydrous phase such as amphibole present in the mantle source of the Vogelsberg group basanites, the potential temperature of parts of the mantle source could have been as low as 1200°C (Fig. 11). This relatively cool mantle must have been a part of the TBL, which melted by adiabatic ascent during rifting and lithospheric thinning. This mantle probably contains high concentrations of incompatible elements and volatiles as a result of metasomatic processes, and relatively high contents of the alkali elements, water and carbon dioxide significantly lower the solidus (Hirschmann, 2000).
The Vogelsberg and Hessian Depression lavas erupted in a sedimentary basin; about 2 km of Mesozoic sediments lie beneath the Hessian Depression volcanics (Mengel, 1990), indicating that this region has been subsiding since the Cretaceous. Similarly, the Vogelsberg volcanic field occurs in a Mesozoic sedimentary basin. The lack of Tertiary uplift in these volcanic areas contrasts with the observation that upwelling mantle plumes should generate a characteristic lithospheric domal uplift both in the oceans and continents (Davies, 1988; Sleep, 1992). We suggest that the absence of such doming indicates average mantle temperatures beneath the Hessian Depression and Vogelsberg. The magmas probably formed by adiabatic decompression melting of the enriched TBL as a result of lithospheric extension and thinning, which also generated the sedimentary basins. The relatively high magma volumes erupted in the Vogelsberg may be due to increased melting at shallower depths in the mantle than beneath the thicker lithosphere beneath the Rhenish Massif. In contrast, magma generation beneath the Westerwald region in Tertiary times may have been due to a mantle plume with an excess temperature of about 100°C, consistent with the velocity anomaly observed at present beneath the Eifel (Ritter et al., 2001).
The mantle sources of the Westerwald and Vogelsberg margin lavas
The basanites, picrobasalts and alkali basalts with Ce/ Pb >25 probably did not assimilate significant amounts of crustal material and thus reflect the Sr, Nd, and Pb isotope composition of their mantle source (Figs 5 and 8). The 206Pb/204Pb of the Westerwald lavas varies between 19·4 and 19·6 and is significantly higher than the 206Pb/204Pb of the Vogelsberg group lavas, which range between 19·0 and 19·4 (Fig. 8a). Several Vogelsberg lavas have relatively low Sr and high Nd isotope ratios (0·7032 and 0·5129, respectively) whereas the generally uncontaminated Eifel group lavas have 87Sr/86Sr and 143Nd/144Nd of 0·7034 and 0·51285, respectively. This implies that the Tertiary volcanoes in the two regions had different mantle sources and each source shows significant heterogeneity. All crustally uncontaminated lavas have lower 87Sr/86Sr and higher 143Nd/144Nd than Bulk Earth, implying a time-integrated depletion of their mantle sources. However, the lower 87Sr/86Sr and higher 143Nd/144Nd of the Vogelsberg group lavas indicates that the eastern mantle source must have been more depleted and/or depleted for a longer period of time than the Eifel group source. The Westerwald lavas have some of the highest Pb isotope ratios found to date in the northern Rhine Graben volcanic fields; only a few Eifel and Siebengebirge lavas have similarly high 206Pb/204Pb (Fig. 5). Comparable radiogenic Pb isotope compositions have also been observed for melilitites from the Tertiary Urach volcanic centre of southern Germany (Hegner et al., 1995; Wilson et al., 1995). The relatively high Pb isotope ratios of the Westerwald basanites indicate higher time-integrated (U + Th)/Pb in the mantle source beneath the Rhenish Massif than beneath the eastern region of the Vogelsberg and Hessian Depression. The high Ce/Pb of 30 in the primitive magmas of both the radiogenic Westerwald and the less radiogenic Vogelsberg source (Fig. 8a) imply either a relative depletion of Pb in the mantle source (Chauvel et al., 1995) or a possible enrichment of Ce during partial melting (Sims & DePaolo, 1997). Because both lava series show the same high Ce/Pb but different Pb isotope ratios the fractionation of Ce/Pb during the low degrees of partial melting required to form the alkali basaltic magmas appears more likely.
The composition of the shallow lithospheric mantle (<2 GPa, <1100°C) beneath the northern Rhine Graben volcanic province is well known, as a result of numerous studies of the elemental and Sr–Nd isotopic composition of spinel lherzolites from the Eifel, Vogelsberg, Rhön, and Hessian Depression (Mengel et al., 1984; Stosch & Lugmair, 1986; Witt-Eickschen & Kramm, 1997, 1998). Most spinel peridotites from this area are LREE depleted and have much higher 143Nd/144Nd (>0·513) than the Westerwald and Vogelsberg region basanites (Witt-Eickschen & Kramm, 1997) and thus cannot represent samples of the source of the mafic magmas. Consequently, the magma sources must lie either in the TBL (McKenzie & Bickle, 1988) or in the asthenosphere.
Several workers have suggested that a mantle source component with radiogenic Pb isotope characteristics represents a widespread component in the asthenosphere beneath the Central European volcanic province; this has been termed European Asthenospheric Reservoir (EAR; Granet et al., 1995) or the low-velocity component (LVC; Hoernle et al., 1995). This component is probably transported to the surface in small upper-mantle plumes (Granet et al., 1995), one of which appears to be the source of the Quaternary Eifel volcanism (Ritter et al., 2001). These small upper-mantle plumes may be fed by a lower-mantle plume (Goes et al., 1999) and mix with variable sources from within the TBL to form the observed range of lavas in Central Europe (Granet et al., 1995; Hoernle et al., 1995). Because the Tertiary Westerwald lavas have the same mantle source (with high 206Pb/204Pb) as the Quaternary Eifel lavas (Figs 5, 6, and 12) they may have formed either from the same mantle plume or from a small mantle plume similar to that at present underneath the Eifel. The deep formation of the magmas in the garnet peridotite stability field (Figs 9 and 11) supports a plume origin of the Westerwald lavas. However, the source of the Eifel group lavas shows some heterogeneity in terms of its Pb isotope composition; the most radiogenic lavas have a 206Pb/204Pb of about 19·6 (Fig. 5) and it is not clear whether this represents a mixture between the proposed EAR/LVC end-member with a 206Pb/204Pb of 20 (Hoernle et al., 1995) and a less radiogenic source. The mantle source of the Vogelsberg lava group is clearly distinct from that of the Eifel group lavas. In our opinion there is no evidence that the postulated uniform mantle end-member (EAR/LVC) with a very radiogenic Pb isotope composition occurs everywhere beneath the northern Rhine Graben area. Instead, the two mantle sources beneath the northern Rhine Graben are regionally distinct and show only limited variation. The relatively unradiogenic Pb isotope composition of the Vogelsberg group lavas indicates that the more radiogenic end-member had only a minor influence on the source mantle of the Vogelsberg and Hessian Depression volcanism. However, more data for unaltered mantle-derived magmas are required to determine any mixing relationships between the Eifel and Vogelsberg group mantle sources. We conclude that the Eifel group lavas may have formed from a mantle plume source whereas the Vogelsberg group source further to the east is different and probably lies in the TBL in agreement with the magma generation model discussed above.
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Temporal variation of the magmatism
A diagram of 206Pb/204Pb vs age of the volcanism (Fig. 12) indicates not only that the Eifel and Vogelsberg lava groups show differences in composition and regional occurrence, but also that the lavas formed at different times. Thus, the main activity in the Westerwald and Siebengebirge occurred between approximately 20 and 30 Ma (Lippolt, 1982
) and these melts tapped a mantle source with relatively high 206Pb/204Pb isotope composition (Fig. 12). It is possible that the lavas from the Hocheifel also belong into this phase of activity with K/Ar ages of 24–45 Ma (Cantarel & Lippolt, 1977) and 206Pb/204Pb of about 19·5 (Wedepohl & Baumann, 1999) but unfortunately few isotope data exist for this lava group. The Quaternary lavas from the Eifel also have 206Pb/204Pb of about 19·5 (Fig. 12), implying that the source of the youngest magmas was similar to that of the early volcanic phase of the Westerwald–Siebengebirge and possibly Hocheifel. Thus, the isotopic signature of the magma source beneath the Rhenish Massif has apparently been constant between 25 ± 5 Ma and Quaternary times. A constant source is in agreement with an origin from a homogeneous mantle plume beneath the Rhenish Massif; the break in the volcanic activity between about 20 and 5 Ma may imply a pulsing of this plume. Results from a recent seismic tomographic study beneath the Eifel show a ‘hole’ between 170 and 240 km depth in the mantle velocity anomaly (Keyser et al., 2002), which could indicate that the mantle plume consists of several separate blobs rather than being continuous. Between about 20 and 10 Ma volcanic activity occurred further east in the Vogelsberg and Hessian Depression region and the magmas probably formed from a TBL source with less radiogenic Pb isotope ratios (Fig. 12). The enriched TBL source may have melted as a result of TBL thinning during a major rifting episode lasting from the Oligocene to early Miocene with increased subsidence in the northern Rhine Graben region (Villemin et al., 1986; Ziegler, 1992). |
CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTINGSAMPLING AND ANALYTICAL METHODSRESULTSDISCUSSIONCONCLUSIONSSUPPLEMENTARY DATAREFERENCES |
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(1) The composition of the Westerwald lavas ranges from basanitic to trachytic whereas the lavas from the margin of the Vogelsberg volcanic field are more alkaline basanites and alkali basalts.
(2) The stronger fractionation of the HREE in the Westerwald lavas indicates formation of the most primitive magmas in the garnet peridotite stability field at high temperature. In contrast, the Vogelsberg region melts have formed shallower in the spinel–garnet peridotite transition region and some also in the presence of residual amphibole, suggesting relatively low temperatures, possibly within the thermal boundary layer.
(3) The isotopic composition of the uncontaminated Tertiary to Quaternary lavas from the northern Rhine Graben region indicates that lavas from the Rhenish Massif (Eifel, Westerwald, Siebengebirge = Eifel group) have a different mantle source from the eastern lavas of the Vogelsberg and Hessian Depression (= Vogelsberg group). The Eifel group source has higher Sr and Pb isotope ratios than the Vogelsberg group mantle.
(4) Several lavas from the Westerwald and Vogelsberg region have assimilated significant amounts of continental crustal material during fractional crystallization in the lower crust. A significant isotopic compositional boundary exists between the lower continental crust beneath the Rhenish Massif in the west and the Hessian Depression and Vogelsberg in the east.
(5) The Eifel group volcanic activity apparently occurred between 40 and 20 Ma and again in the Quaternary, possibly suggesting a pulsing mantle plume beneath the Rhenish Massif. In contrast, the shallow melting in the presence of residual amphibole of the Vogelsberg group melts may indicate an origin from a metasomatized portion of the thermal boundary layer.
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SUPPLEMENTARY DATA |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTINGSAMPLING AND ANALYTICAL METHODSRESULTSDISCUSSIONCONCLUSIONSSUPPLEMENTARY DATAREFERENCES |
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Supplementary data for this paper are available on Journal of Petrology online.
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ACKNOWLEDGEMENTS |
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We thank P. Appel, H. Blaschek, F. Hauff, B. Mader, N. Stroncik, S. Vetter and M. Weinkauf for the help during the analytical work of this project. The constructive reviews of K. Hoernle, S. Jung, M. Wilson and G. Wörner are gratefully acknowledged and helped to significantly improve the quality of the paper.
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FOOTNOTES |
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* Corresponding author. Telephone: (49) (0)431 880 2865. Fax: (49) (0)431 880 4376. E-mail: kh{at}gpi.uni-kiel.de
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REFERENCES |
|---|
|
TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTINGSAMPLING AND ANALYTICAL METHODSRESULTSDISCUSSIONCONCLUSIONSSUPPLEMENTARY DATAREFERENCES |
|---|
Babuska, V. & Plomerova, J. (1992). The lithosphere in central Europe—seismological and petrological aspects. Tectonophysics 207, 141–163.[CrossRef][Web of Science]
Bogaard, P. J. F. & Wörner, G. (2003). Petrogenesis of basanitic to tholeiitic volcanic rocks from the Miocene Vogelsberg, Central Germany. Journal of Petrology 44, 569–602.
Bonjer, K.-P. (1997). Seismicity pattern and style of seismic faulting at the eastern borderfault of the southern Rhine Graben. Tectonophysics 275, 41–69.[CrossRef][Web of Science]
Bonjer, K.-P., Gelbke, C., Gilg, B., Rouland, D., Mayer-Rosa, D. & Massinon, B. (1984). Seismicity and dynamics of the Upper Rhinegraben. Journal of Geophysics 55, 1–12.
Brey, G. & Green, D. H. (1977). Systematic study of liquidus phase relations in olivine melilitite + H2O + CO2 at high pressures and petrogenesis of an olivine melilitite magma. Contributions to Mineralogy and Petrology 61, 141–162.[CrossRef][Web of Science]
Cantarel, P. & Lippolt, H. J. (1977). Age and sequence of Tertiary volcanism in the Hocheifel area, Germany. Neues Jahrbuch für Geologie und Paläontologie, Monatshefte 1977, 600–612.
Chauvel, C., Goldstein, S. L. & Hofmann, A. W. (1995). Hydration and dehydration of oceanic crust controls Pb evolution in the mantle. Chemical Geology 126, 65–75.[CrossRef][Web of Science]
Davies, G. F. (1988). Ocean bathymetry and mantle convection 1. Large-scale flow and hotspots. Journal of Geophysical Research 93, 10467–10480.[CrossRef]
Dewey, J. F. & Windley, B. F. (1988). Palaeocene–Oligocene tectonics of NW Europe. In: Morton, A. C. & Parson, L. M. (eds) Early Tertiary Volcanism and the Opening of the NE Atlantic. Geological Society, London, Special Publications 39, 25–31.
Dobosi, G. & Fodor, R. V. (1992). Magma fractionation, replenishment, and mixing as inferred from green-core clinopyroxenes in Pliocene basanite, southern Slovakia. Lithos 28, 133–150.[CrossRef][Web of Science]
Dosso, L., Hanan, B. B., Bougault, H., Schilling, J.-G. & Joron, J.-L. (1991). Sr–Nd–Pb geochemical morphology between 10° and 17°N on the Mid-Atlantic Ridge: a new MORB isotope signature. Earth and Planetary Science Letters 106, 29–43.[CrossRef][Web of Science]
Dosso, L., Bougault, H., Langmuir, C., Bollinger, C., Bonnier, O. & Etoubleau, J. (1999). The age and distribution of mantle heterogeneity along the Mid-Atlantic Ridge (31–41°N). Earth and Planetary Science Letters 170, 269–286.[CrossRef][Web of Science]
Duda, A. & Schmincke, H.-U. (1985). Polybaric differentiation of alkali basaltic magmas: evidence from green-core clinopyroxenes (Eifel, FRG). Contributions to Mineralogy and Petrology 91, 340–353.[CrossRef][Web of Science]
Dupré, B. & Allègre, C. J. (1980). Pb–Sr–Nd isotopic correlation and the chemistry of the North Atlantic mantle. Nature 286, 17–21.[CrossRef]
Fairhead, J. D. & Stuart, G. W. (1982). The seismicity of the East African Rift system and comparison with other continental rifts. In: Palmason, G. (ed.) Continental and Oceanic Rifts. Geophysical Monograph, American Geophysical Union 8, 41–61.
Falloon, T. J. & Green, D. H. (1990). Solidus of carbonated fertile peridotite under fluid-saturated conditions. Geology 18, 195–199.
Faul, U. H. (1997). Permeability of partially molten upper mantle rocks from experiments and percolation theory. Journal of Geophysical Research 102, 10299–10311.[CrossRef][Web of Science]
Foley, S. (1991). High-pressure stability of the fluor- and hydroxy-endmembers of pargasite and K-richterite. Geochimica et Cosmochimica Acta 55, 2689–2694.[CrossRef][Web of Science]
Garbe-Schönberg, C.-D. (1993). Simultaneous determination of thirty-seven trace elements in twenty-eight international rock standards by ICP-MS. Geostandards Newsletters 17, 81–97.
Goes, S., Spakman, W. & Bijward, H. (1999). A lower mantle source for Central European volcanism. Science 286, 1928–1931.
Goes, S., Govers, R. & Vacher, P. (2000). Shallow mantle temperatures under Europe from P and S wave tomography. Journal of Geophysical Research 105, 11153–11169.[CrossRef]
Granet, M., Wilson, M. & Achauer, U. (1995). Imaging a mantle plume beneath the French Massif Central. Earth and Planetary Science Letters 136, 281–296.[CrossRef][Web of Science]
Green, D. H. (1973). Conditions of melting of basanite magma from garnet peridotite. Earth and Planetary Science Letters 17, 456–465.[CrossRef][Web of Science]
Green, D. H. & Ringwood, A. E. (1967). The stability fields of aluminous pyroxene peridotite and garnet peridotite and their relevance in upper mantle structure. Earth and Planetary Science Letters 3, 151–160.[CrossRef][Web of Science]
Hawkesworth, C. J., Rogers, N. W., van Calsteren, P. W. C. & Menzies, M. A. (1984). Mantle enrichment processes. Nature 311, 331–335.[CrossRef]
Hegner, E., Walter, H. J. & Satir, M. (1995). Pb–Sr–Nd isotopic compositions and trace element geochemistry of megacrysts and melilitites from the Tertiary Urach volcanic field: source composition of small volume melts under SW Germany. Contributions to Mineralogy and Petrology 122, 322–335.[CrossRef][Web of Science]
Hirose, K. (1997). Partial melt compositions of carbonated peridotite at 3 GPa and role of CO2 in alkali-basalt magma generation. Geophysical Research Letters 24, 2837–2840.[CrossRef][Web of Science]
Hirschmann, M. M. (2000). Mantle solidus: experimental constraints and the effects of peridotite composition. Geochemistry, Geophysics, Geosystems 1, 10.1029/2000GC000070.
Hoernle, K., Zhang, Y.-S. & Graham, D. (1995). Seismic and geochemical evidence for large-scale mantle upwelling beneath the eastern Atlantic and western and central Europe. Nature 374, 34–39.[CrossRef]
Hoernle, K. A. & Tilton, G. R. (1991). Sr–Nd–Pb isotope data for Fuerteventura (Canary Islands) basal complex and subaerial volcanics: applications to magma genesis and evolution. Schweizerische Mineralogische und Petrographische Mitteilungen 71, 3–18.
Hofmann, A. W. & White, W. M. (1982). Mantle plumes from ancient oceanic crust. Earth and Planetary Science Letters 57, 421–436.[CrossRef][Web of Science]
Hofmann, A. W., Jochum, K. P., Seufert, M. & White, W. M. (1986). Nb and Pb in oceanic basalts: new constraints on mantle evolution. Earth and Planetary Science Letters 79, 33–45.[CrossRef][Web of Science]
Illies, J. H. & Greiner, G. (1978). Rhinegraben and the Alpine system. Geological Society of America Bulletin 89, 770–782.
Ito, E., White, W. M. & Göpel, C. (1987). The O, Sr, Nd and Pb isotope geochemistry of MORB. Chemical Geology 62, 157–176.[CrossRef][Web of Science]
Jaques, A. L. & Green, D. H. (1980). Anhydrous melting of peridotite at 0–15 Kb pressure and the genesis of tholeiitic basalts. Contributions to Mineralogy and Petrology 73, 287–310.[CrossRef][Web of Science]
Johnson, K. T. M. (1998). Experimental determination of partition coefficients for rare earth and high-field-strength elements between clinopyroxene, garnet, and basaltic melt at high pressures. Contributions to Mineralogy and Petrology 133, 60–68.[CrossRef][Web of Science]
Jung, S. & Masberg, P. (1998). 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. Journal of Volcanology and Geothermal Research 86, 151–177.[CrossRef][Web of Science]
Kelemen, P. B., Shimizu, N. & Dunn, T. (1993). Relative depletion of niobium in some arc magmas and the continental crust: partitioning of K, Nb, La and Ce during melt/rock reaction in the upper mantle. Earth and Planetary Science Letters 120, 111–134.[CrossRef][Web of Science]
Keyser, M., Ritter, J. R. R. & Jordan, M. (2002). 3D shear-wave velocity structure of the Eifel plume, Germany. Earth and Planetary Science Letters 203, 59–82.[CrossRef][Web of Science]
Kramm, U. & Wedepohl, K. H. (1990). Tertiary basalts and peridotite xenoliths from the Hessian Depression (NW Germany), reflecting mantle compositions low in radiogenic Nd and Sr. Contributions to Mineralogy and Petrology 106, 1–8.[CrossRef][Web of Science]
Kushiro, I. (1996). Partial melting of a fertile mantle peridotite at high pressures: an experimental study using aggregates of diamond. In: Basu, A. & Hart, S. R. (eds) Earth Processes: Reading the Isotopic Code. Geophysical Monograph, American Geophysical Union 95, 109–122.
LaTourrette, T., Hervig, R. L. & Holloway, J. R. (1995). Trace element partitioning between amphibole, phlogopite, and basanite melt. Earth and Planetary Science Letters 135, 13–30.[CrossRef][Web of Science]
Lippolt, H. J. (1982). K/Ar age determinations and the correlation of Tertiary volcanic activity in Central Europe. Geologisches Jahrbuch D52, 113–135.
Lippolt, H. J. & Todt, W. (1978). Isotopische Altersbestimmungen an Vulkaniten des Westerwaldes. Neues Jahrbuch für Geologie und Palaontologie, Monatshefte 1978, 332–352.
Loock, G., Stosch, H.-G. & Seck, H. A. (1990). Granulite facies lower crustal xenoliths from the Eifel, West Germany: petrological and geochemical aspects. Contributions to Mineralogy and Petrology 105, 25–41.[CrossRef][Web of Science]
McKenzie, D. & Bickle, M. J. (1988). The volume and composition of melt generated by extension of the lithosphere. Journal of Petrology 29, 625–679.
McKenzie, D. & O'Nions, R. K. (1983). Mantle reservoirs and ocean island basalts. Nature 301, 229–231.[CrossRef]
Meier, L. & Eisbacher, G. H. (1991). Crustal kinematics and deep structure of the northern Rhine Graben, Germany. Tectonics 10, 621–630.[Web of Science]
Mengel, K. (1990). Crustal xenoliths from Tertiary volcanics of the Northern Hessian Depression. Contributions to Mineralogy and Petrology 104, 8–26.[CrossRef][Web of Science]
Mengel, K. & Green, D. H. (1989). Stability of amphibole and phlogopite in metasomatized peridotite under water-saturated and water-undersaturated conditions. In: Ross, J. (ed.) Proceedings of the 4th International Kimberlite Conference. Perth: Geological Society of Australia, pp. 571–581.
Mengel, K., Kramm, U., Wedepohl, K. H. & Gohn, E. (1984). Sr isotopes in peridotite xenoliths and their basalitc host rocks from the northern Hessian Depression (NW Germany). Contributions to Mineralogy and Petrology 87, 369–375.[CrossRef][Web of Science]
Mengel, K., Sachs, P. M., Stosch, H. G., Wörner, G. & Loock, G. (1991). Crustal xenoliths from Cenozoic volcanic fields of West Germany: implications for structure and composition of the crust. Tectonophysics 195, 271–289.[CrossRef][Web of Science]
Mertes, H. (1983). Aufbau und Genese des Westeifeler Vulkanfeldes. Bochumer Geologische und Geotechnische Arbeiten 9, 415.
Müller, B., Zoback, M. L., Fuchs, K., Mastin, L., Gregersen, S., Pavoni, N., Stephansson, O. & Ljunggren, C. (1992). Regional patterns of tectonic stress in Europe. Journal of Geophysical Research 97, 783–803.
Mysen, B. O. & Kushiro, I. (1977). Compositional variations of coexisting phases with degree of melting of peridotite in the upper mantle. American Mineralogist 62, 843–865.[Abstract]
Plenefisch, T. & Bonjer, K. P. (1997). The stress field in the Rhine Graben area inferred from earthquake focal mechanisms and estimation of frictional parameters. Tectonophysics 275, 71–97.[CrossRef][Web of Science]
Prodehl, C., Mueller, S., Glahn, A., Gutscher, M. & Haak, V. (1992). Lithospheric cross sections of the European Cenozoic rift system. Tectonophysics 208, 113–138.[CrossRef][Web of Science]
Rapp, R. P., Watson, E. B. & Miller, C. F. (1991). Partial melting of amphibolite/eclogite and the origin of Archean trondhjemites and tonalites. Precambrian Research 51, 1–25.[CrossRef][Web of Science]
Reiners, P. W., Nelson, B. K. & Ghiorso, M. S. (1995). Assimilation of felsic crust by basaltic magma: thermal limits and extents of crustal contamination of mantle-derived magmas. Geology 23, 563–566.
Ritter, J. R. R., Jordan, M., Christensen, U. R. & Achauer, U. (2001). A mantle plume below the Eifel volcanic fields, Germany. Earth and Planetary Science Letters 186, 7–14.[CrossRef][Web of Science]
Robinson, J. A. C. & Wood, B. J. (1998). The depth of the spinel to garnet transition at the peridotite solidus. Earth and Planetary Science Letters 164, 277–284.[CrossRef][Web of Science]
Rudnick, R. L. & Goldstein, S. L. (1990). The Pb isotopic compositions of lower crustal xenoliths and the evolution of lower crustal Pb. Earth and Planetary Science Letters 98, 192–207.[CrossRef][Web of Science]
Sachs, P. M. & Hansteen, T. H. (2000). Pleistocene underplating and metasomatism of the lower continental crust: a xenolith study. Journal of Petrology 41, 331–356.
Schilling, J.-G., Hanan, B. B., McCully, B., Kingsley, R. H. & Fontignie, D. (1994). Influence of the Sierra Leone mantle plume on the equatorial Mid-Atlantic Ridge: a Nd–Sr–Pb isotopic study. Journal of Geophysical Research 99, 12005–12028.[CrossRef]
Schreiber, U., Anders, D. & Koppen, J. (1999). Mixing and chemical interdiffusion of trachytic and latitic magma in a subvolcanic complex of the Tertiary Westerwald (Germany). Lithos 46, 695–714.[CrossRef][Web of Science]
Sengör, A. M. C., Burke, K. & Dewey, J. F. (1978). Rifts at high angles to orogenic belts: tests for their origin and the upper Rhine Graben as an example. American Journal of Science 278, 24–40.
Shaw, D. M. (1970). Trace element fractionation during anatexis. Geochimica et Cosmochimica Acta 34, 237–243.[CrossRef][Web of Science]
Shirey, S. B., Bender, J. F. & Langmuir, C. H. (1987). Three-component isotopic heterogeneity near the Oceanographer transform, Mid-Atlantic Ridge. Nature 325, 217–223.[CrossRef]
Sims, K. W. W. & DePaolo, D. J. (1997). Inferences about mantle magma sources from incompatible element concentration ratios in oceanic basalts. Geochimica et Cosmochimica Acta 61, 765–784.[CrossRef][Web of Science]
Sleep, N. H. (1992). Hotspot volcanism and mantle plumes. Annual Review of Earth and Planetary Sciences 20, 19–42.[CrossRef][Web of Science]
Spera, F. J. & Bohrson, W. A. (2001). Energy-constrained open-system magmatic processes; I, General model and energy-constrained assimilation and fractional crystallization (EC-AFC) formulation. Journal of Petrology 42(5), 999–1018.
Stosch, H.-G. & Lugmair, G. W. (1984). Evolution of the lower continental crust: granulite facies xenoliths from the Eifel, West Germany. Nature 311, 368–370.[CrossRef]
Stosch, H.-G. & Lugmair, G. W. (1986). Trace element and Sr and Nd isotope geochemistry of peridotite xenoliths from the Eifel (West Germany) and their bearing on the evolution of the sublithosphere. Earth and Planetary Science Letters 80, 281–298.[CrossRef][Web of Science]
Stosch, H.-G. & Seck, H. A. (1980). Geochemistry and mineralogy of two spinel peridotite suites from Dreiser Weiher, West Germany. Geochimica et Cosmochimica Acta 44, 457–470.[CrossRef][Web of Science]
Stosch, H.-G., Lugmair, G. W. & Seck, H. A. (1986). Geochemistry of granulite-facies lower crustal xenoliths: implications for the geological history of the lower continental crust below the Eifel, West Germany. In: Dawson, J. B., Carswell, D. A., Hall, J. & Wedepohl, K. H. (eds) The Nature of the Lower Continental Crust. Geological Society, London, Special Publications 24, 309–317.
Stosch, H.-G., Schmucker, A. & Reys, C. (1991). The nature and geological history of the deep crust under the Eifel, Germany. Terra Nova 4, 53–62.
Sun, S.-s. & McDonough, W. F. (1989). Chemical and isotopic systematics of oceanic basalts: implications for mantle composition and processes. In: Saunders, A. D. & Norry, M. J. (eds) Magmatism in the Ocean Basins. Geological Society, London, Special Publications 42, 313–345.
Teichmüller, R. (1974). Die tektonische Entwicklung der Niederrheinischen Bucht. In: Illies, J. H. & Fuchs, K. (eds) Approaches to Taphrogenesis. Stuttgart: Schweitzerbart, pp. 269–285.
Thibault, Y., Edgar, A. D. & Lloyd, F. E. (1992). Experimental investigation of melts from a carbonated phlogopite lherzolite: implications for metasomatism in the continental lithosphere. American Mineralogist 77, 784–794.[Abstract]
Todt, W., Cliff, R. A., Hanser, A. & Hofmann, A. W. (1996). Evaluation of a 202Pb–205Pb double spike for high-precision lead isotope analysis. Geophysical Monograph, American Geophysical Union 95, 429–437.
Turcotte, D. L. & Emerman, S. H. (1983). Mechanisms of active and passive rifting. Tectonophysics 94, 39–50.[CrossRef][Web of Science]
Turk, P.-G., Lohse, H.-H., Schürmann, K., Fuhrmann, U. & Lippolt, H. J. (1984). Petrographische und Kalium–Argon-Untersuchungen an basischen tertiären Vulkaniten zwischen Westerwald und Vogelsberg. Geologische Rundschau 73, 599–617.
Villemin, T., Alvarez, F. & Angelier, J. (1986). The Rhinegraben: extension, subsidence and shoulder uplift. Tectonophysics 128, 47–59.[CrossRef][Web of Science]
Wedepohl, K. H. (1982). K–Ar-Altersbestimmungen an basaltischen Vulkaniten der nördlichen Hessischen Senke und ihr Beitrag zur Diskussion der Magmengenese. Neues Jahrbuch für Mineralogie, Abhandlungen 144, 172–196.[Web of Science]
Wedepohl, K. H. & Baumann, A. (1999). Central European Cenozoic plume volcanism with OIB characteristics and indications of a lower mantle source. Contributions to Mineralogy and Petrology 136, 225–239.[CrossRef][Web of Science]
Wedepohl, K. H., Gohn, E. & Hartmann, G. (1994). Cenozoic alkali basaltic magmas of western Germany and their products of differentiation. Contributions to Mineralogy and Petrology 115, 253–278.[CrossRef][Web of Science]
Wilson, M. & Downes, H. (1991). Tertiary–Quaternary extension-related alkaline magmatism in western and central Europe. Journal of Petrology 32, 811–849.
Wilson, M., Rosenbaum, J. M. & Dunworth, E. A. (1995). Melilitites: partial melts of the thermal boundary layer? Contributions to Mineralogy and Petrology 119, 181–196.[Web of Science]
Witt-Eickschen, G. & Kramm, U. (1997). Mantle upwelling and metasomatism beneath Central Europe: geochemical and isotopic constraints from mantle xenoliths from the Rhön (Germany). Journal of Petrology 38, 479–493.[CrossRef][Web of Science]
Witt-Eickschen, G. & Kramm, U. (1998). Evidence for the multiple stage evolution of the subcontinental lithospheric mantle beneath the Eifel (Germany) from pyroxenite and composite pyroxenite/peridotite xenoliths. Contributions to Mineralogy and Petrology 131, 258–272.[CrossRef][Web of Science]
Wittenbecher, M. (1992). Geochemie tholeiitischer und alkaliolivinbasaltischer Gesteine des Vogelsberges. Geologische Abhandlungen Hessen 97, 52.
Wörner, G., Zindler, A., Staudigel, H. & Schmicke, H.-U. (1986). Sr, Nd, and Pb isotope geochemistry of Tertiary and Quaternary alkaline volcanics from West Germany. Earth and Planetary Science Letters 79, 107–119.[CrossRef][Web of Science]
Yu, D., Fontignie, D. & Schilling, J.-G. (1997). Mantle plume–ridge interactions in the Central North Atlantic: a Nd isotope study of mid-ocean ridge basalts from 30°N to 50°N. Earth and Planetary Science Letters 146, 259–272.[CrossRef][Web of Science]
Ziegler, P. A. (1992). European Cenozoic rift system. Tectonophysics 208, 91–111.[CrossRef][Web of Science]
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