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
KARSTEN M. HAASE*,
BJÖRN GOLDSCHMIDT and
C.-DIETER GARBE-SCHÖNBERG
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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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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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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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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Fig. 1. (a) Map of the area of Tertiary to Quaternary volcanic activity between the Eifel and Siebengebirge (SG) in the west, the Westerwald in the centre, and the Vogelsberg and Hessian Depression in the east. Also shown is the Rhine Graben and the Lower Rhine Embayment in the north. (b) Enlarged map of the study area indicating the sample locations; •, Westerwald region;  , 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.
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The Westerwald volcanic field covers about 800 km
2 and consists
of (1) a larger part (
500 km
2) in the NE that comprises several
mafic lava flows and (2) a smaller part in the SW (
280 km
2)
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.
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SAMPLING AND ANALYTICAL METHODS
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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.
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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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 An
64 to An
87. 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 An
26 to An
45. 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.
Two trends are observed in
Fig. 2e for the more evolved rocks
with more than 45 wt % SiO
2: (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). Na
2O and K
2O concentrations increase with
increasing SiO
2 up to
63 wt % SiO
2; 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.
The Westerwald and Vogelsberg lavas show a broad negative correlation
between Ce/Pb and SiO
2 content (
Fig. 3b). The majority of the
basalts have Ce/Pb ratios between 25 and 35; however, three
primitive Vogelsberg margin samples with SiO
2 <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).
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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Fig. 5. (a) Neodymium vs Sr isotope ratios for the Westerwald, Vogelsberg, Eifel, Siebengebirge and Hessian Depression lavas (Wörner et al., 1986; Wittenbecher, 1992; Wedepohl et al., 1994; Jung & Masberg, 1998; Wedepohl & Baumann, 1999; Bogaard & Wörner, 2003) compared with the field for Eifel granulites (Stosch & Lugmair, 1984; Loock et al., 1990; Rudnick & Goldstein, 1990; Stosch et al., 1991). Two lava groups can be defined based on the Sr–Nd isotope trends: (1) the Eifel group contains lavas from the Eifel, Westerwald and Siebengebirge; (2) the Vogelsberg group contains Vogelsberg and Hessian Depression lavas. Eifel granulites have very heterogeneous compositions and some samples plot outside the diagram. The bold lines show the EC-AFC model (Spera & Bohrson, 2001) discussed in the text and the tick marks show increments of the crystallized and assimilated mass. (b) Diagram of 207Pb/204Pb vs 206Pb/204Pb, showing that Westerwald, Eifel and Siebengebirge lavas generally have higher 206Pb/204Pb than Vogelsberg and Hessian Depression lavas. (c) Diagram of 208Pb/204Pb vs 206Pb/204Pb, showing the various volcanic groups and the field for Eifel granulites. The MORB field is for North Atlantic MORB between 53 and 39°N and 35 and 5°N, i.e. outside the Azores hotspot (Dupré & Allègre, 1980; Ito et al., 1987; Shirey et al., 1987; Sun & McDonough, 1989; Dosso et al., 1991, 1999; Schilling et al., 1994; Yu et al., 1997).
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Fig. 6. Diagram of 87Sr/86Sr isotopes (a) and 143Nd/144Nd (b) vs 206Pb/204Pb for the Westerwald, Vogelsberg/Hessian Depression, and Eifel/Siebengebirge lavas. Also shown are EC-AFC lines for mantle melts mE (Eifel group end-member) and mV (Vogelsberg group end-member) assimilating the lower-crustal granulites S35 and S32 from Table 4, which are different for the Eifel and Vogelsberg lavas. Data sources as in Fig. 6.
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The lavas with SiO
2 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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Fig. 7. (a) 143Nd/144Nd vs SiO2 for lavas from the Westerwald and Vogelsberg compared with the field of Eifel granulite xenoliths. It should be noted that lavas with more than about 48 wt % SiO2 have relatively low 143Nd/144Nd overlapping with the isotopic compositions of the granulites. (b) 87Sr/86Sr vs Sr contents for the same samples showing EC-AFC model curves discussed in the text. Tick marks on lines show increments of fractional crystallization and assimilation. Data sources as in previous figure.
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DISCUSSION
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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.
In conclusion, those lavas with high SiO
2 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
% SiO
2 implies that there are no primary SiO
2-rich basaltic
(
tholeiitic) magmas which would reflect shallow and high-degree
partial melts of the mantle. The only lavas with high Ce/Pb
and SiO
2 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.
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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Fig. 10. K/La vs La for Westerwald and Vogelsberg lavas with Ce/Pb >20 from this study and Bogaard & Wörner (2003). The same model and parameters as in Fig. 9 have been used. The tick marks with numbers indicate percentages of the degree of melting.
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Fig. 11. Solidi for dry mantle (McKenzie & Bickle, 1988) and CO2-saturated mantle (Falloon & Green, 1990) peridotite as well as adiabats for various mantle potential temperatures. The average potential temperature of the upper mantle is estimated at about 1300°C. The stability fields of spinel and garnet lherzolite for dry and CO2-saturated mantle, as well as that of amphibole, are shown as grey lines (Falloon & Green, 1990; Foley, 1991; Robinson & Wood, 1998). Experimental results for generation of basanitic and nephelinitic melts are also shown.  , 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). Crustal thickness for the Westerwald region is from Prodehl et al. (1992) and the lithospheric thickness from Babuska & Plomerova (1992). The dashed rectangles show the approximate melting regions of the plume and TBL magmas as discussed in the text, and the dashed line marks a possible adiabat for TBL material melting in the amphibole stability field.
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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
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ABSTRACT
INTRODUCTION
