Journal of Petrology

Journal of Petrology Advance Access originally published online on April 27, 2006
Journal of Petrology 2006 47(8):1637-1671; doi:10.1093/petrology/egl023

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Petrogenesis of Tertiary Mafic Alkaline Magmas in the Hocheifel, Germany

CAROLINE JUNG¹, STEFAN JUNG^1,2,*, EDGAR HOFFER¹ and JASPER BERNDT³

¹ INSTITUT FÜR MINERALOGIE, PETROLOGIE UND KRISTALLOGRAPHIE, FACHBEREICH GEOWISSENSCHAFTEN, PHILIPPS UNIVERSITÄT MARBURG LAHNBERGE/HANS-MEERWEIN-STRASSE, 35032 MARBURG, GERMANY
² MAX-PLANCK-INSTITUT FÜR CHEMIE, ABT. GEOCHEMIE POSTFACH 3060, 55020 MAINZ, GERMANY
³ INSTITUT FÜR MINERALOGIE, UNIVERSITÄT MÜNSTER CORRENSSTR. 24, 48149 MÜNSTER, GERMANY

RECEIVED JANUARY 11, 2005; ACCEPTED MARCH 24, 2006

	ABSTRACT

TOP ABSTRACT INTRODUCTION GEOLOGICAL SETTING ANALYTICAL TECHNIQUES PETROGRAPHY GEOCHEMISTRY DISCUSSION CONCLUSIONS REFERENCES

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

KEY WORDS: alkali basalts; continental volcanism; crustal contamination; partial melting; Eifel, Germany

	INTRODUCTION

TOP ABSTRACT INTRODUCTION GEOLOGICAL SETTING ANALYTICAL TECHNIQUES PETROGRAPHY GEOCHEMISTRY DISCUSSION CONCLUSIONS REFERENCES

 
The geochemistry of primitive, alkaline mafic volcanic rocks together with geochemical evidence from mantle-derived xenoliths can potentially yield valuable information about the nature of the inaccessible parts of the Earth's upper mantle. One problem usually addressed in studies on volcanic rocks erupted in continental areas is the identification of the source region of the alkaline magmas—either the subcontinental lithospheric mantle (SCLM) or a sublithospheric source, e.g. a mantle plume. In contrast to the suboceanic mantle, the subcontinental lithospheric mantle is likely to have remained isolated from the convecting upper mantle after initial crustal extraction. This upper mantle can have had a complex geological history, involving ancient depletion events, followed by later re-enrichments, most probably from metasomatizing fluids or melts (e.g. Hawkesworth et al., 1990). The relatively large volumes of alkaline mafic magmas (nephelinites, basanites, alkali basalts) that are generated during continental rifting are often considered to be produced predominantly by partial melting of asthenospheric mantle (McKenzie & Bickle, 1988; White & McKenzie, 1989; Wilson & Downes, 1991; Arndt & Christensen, 1992). On the other hand, there is growing evidence that, at least in some volcanic provinces, magmas are generated by partial melting of metasomatically enriched SCLM (Hawkesworth et al., 1990; Gallagher & Hawkesworth, 1992; Bradshaw et al., 1993). In this respect, the SCLM can contribute in several ways to the composition of continental basalts including: (1) mixing of small-degree melt fractions from the lithosphere with magmas from the asthenosphere (Ellam & Cox, 1991); (2) direct melting of the lithosphere (Bradshaw et al., 1993); (3) partial melting of detached fragments of SCLM recycled into the asthenosphere during earlier tectonic episodes (Zindler & Hart, 1986).

In central Germany, several thousand km³ of mafic magma were generated during the Tertiary and constitute part of the Central European Volcanic Province (CEVP, Fig. 1). These volumes are probably too large to have been produced solely within the lithosphere (Wedepohl, 1985; Jung & Hoernes, 1998; Jung & Masberg, 2000; Bogaard & Wörner, 2003). Consequently, it has been suggested that the individual volcanic fields of the CEVP (Massif Central, Bohemian Massif, Eifel, Siebengebirge, Westerwald, Hessian Depression, Rhön, Vogelsberg, Urach, etc.) were fed from asthenospheric partial melts generated within upwelling upper mantle plumes or diapirs (Granet et al., 1995; Wilson & Patterson, 2001). The trace element and Sr–Nd–Pb isotope geochemistry of the most primitive alkaline mafic lavas from the CEPV suggest derivation from mantle sources similar to those of ocean island basalts (Wörner et al., 1986; Blusztajn & Hart, 1989; Wilson & Downes, 1991; Hegner et al., 1995; Wilson et al., 1995; Jung & Masberg, 1998; Wedepohl & Baumann, 1999; Jung & Hoernes, 2000). Moreover, with the advent of high-resolution mantle seismic tomography, showing slow velocity domains in the mantle at various depths, an upper mantle origin for the European volcanism linked to a series of diapiric upwellings has been suggested (Granet et al., 1995; Hoernle et al., 1995; Goes et al., 1999; Ritter et al., 2001; Wilson & Patterson, 2001; Keyser et al., 2002). For the Rhenish Massif, a columnar low P-wave velocity anomaly was detected beneath the Eifel (Ritter et al., 2001). This 100 km wide structure extends up to 400 km depth and could be interpreted to be equivalent to an excess mantle temperature of 150–200°C in the absence of volatiles or partial melts. The geochemical diversity of lithospheric mantle xenoliths entrained within the mafic magmas of the CEPV indicates that, locally, subduction zone processes during the Hercynian orogeny may have induced substantial trace element and isotopic enrichment of the lithospheric mantle beneath central Europe (e.g. Witt-Eickschen & Kramm, 1997). Partial melting of such zones of metasomatized lithospheric mantle, combined with interaction between asthenospheric melts and lithospheric melts, has been proposed to explain the geochemical characteristics of the most primitive mafic alkaline rocks of the CEPV (Wilson & Downes, 1991; Granet et al., 1995; Wilson & Patterson, 2001). In addition, crustal contamination of the mantle-derived magmas has been widely documented within the CEVP (Massif Central: Wilson et al., 1995; Vogelsberg: Jung & Masberg, 1998; Bogaard & Wörner, 2003; Rhön: Jung & Hoernes, 2000; Jung et al., 2006; Urach–Hegau: Blusztajn & Hegner, 2002; Westerwald: Haase et al., 2004). Elucidation of the details of interaction of asthenosphere-derived melts with the lithosphere (both crust and mantle) is often difficult because both crust and ancient subcontinental mantle can have similar geochemical and Sr–Nd–Pb isotope characteristics.

View larger version (55K): [in this window] [in a new window]   Fig. 1 (a) Distribution of Cenozoic volcanic rocks (black shaded fields) in Central Europe (modified from Wedepohl et al., 1994). Numbers denote K–Ar or Ar–Ar ages compiled from Lippolt (1982) and Wilson & Downes (2006). (b) Location of the volcanic outcrops within the Hocheifel (•) with sample sites (). Inset shows the location of the Hocheifel volcanic field relative to the east (Osteifel) and west Eifel volcanic fields.

 
Given the well-characterized nature of the crust and mantle lithosphere in the Eifel (Stosch & Lugmair, 1984, 1986; Loock et al., 1990; Rudnick & Goldstein, 1990; Stosch et al., 1992; Witt-Eickschen & Kramm, 1998; Witt-Eickschen et al., 1998, 2003), this area provides an ideal setting in which to attempt to identify the sources of this specific type of intra-plate volcanism. However, neither comprehensive whole-rock geochemistry nor Sr–Nd–Pb isotopic data have been published for the Tertiary Hocheifel area. Such data are essential to constrain the role of fractional crystallization, crustal contamination, and mantle source heterogeneities in the petrogenesis of the magmas. In this study, major and trace element data and Sr–Nd–Pb isotope data are reported for primitive alkaline mafic magmas from the Hocheifel area; these data are used to constrain the mantle source region of these basalts. Major and trace element and Sr–Nd–Pb isotope data for more differentiated lavas from the same area are used to highlight the effects of fractional crystallization and crustal contamination in the genesis of these alkaline lavas.

	GEOLOGICAL SETTING

TOP ABSTRACT INTRODUCTION GEOLOGICAL SETTING ANALYTICAL TECHNIQUES PETROGRAPHY GEOCHEMISTRY DISCUSSION CONCLUSIONS REFERENCES

 
The Hocheifel volcanic field has an areal extent of 1400 km² and consists mainly of deeply eroded volcanic plugs and necks. Locally, remants of lava flows appear. The Hocheifel volcanic field forms part of an east–west-trending belt of Tertiary–Quaternary volcanic fields in central Germany; these include the Eifel, Westerwald, Vogelsberg, Hessian Depression, Rhön, Heldburg and Oberpfalz (Fig. 1a). The trend of these volcanic fields is perpendicular to the main NNE–SSW-trending Rhine graben rift system of Central Europe, which has been interpreted as the result of Alpine tectonism further south (e.g. Ziegler, 1992). In Germany and elsewhere in Central Europe, Tertiary (mainly Miocene to Pliocene) basin development provides evidence for lithospheric extension, although the huge volumes of basaltic rocks in the Vogelsberg area (500 km³) and the Cantal (Massif Central, France) are unlikely to be attributed to continental extension alone. As noted by Wilson & Downes (1991), most of the major volcanic fields sit on uplifted Variscan basement massifs. However, basement uplift is not coeval with rift development, typically starting some 20–40 Myr after the beginning of rifting (Ziegler, 1992). Within Central Germany, although some of the Cenozoic volcanic fields are located mainly on Hercynian fault blocks within the Rhenish Massif (e.g. Eifel, Westerwald, Heldburg), Tertiary volcanic activity in the Rhön, the Hessian Depression and the Vogelsberg occurs within graben-like structures that transect the Rhenish Massif. In the Rhön area and Hessian Depression magmatism is not obviously associated with basement uplift, and dextral strike-slip movement of the lithosphere probably caused passive asthenospheric upwelling (Schreiber & Rotsch, 1998).

Geophysical data indicate that the Cenozoic rifts of the CEVP are associated with a marked uplift of the Moho discontinuity. The maximum crustal thinning coincides with the trace of the northern Rhine graben, although this area has been shown to be largely non-magmatic (Wilson & Patterson, 2001). Crustal thickness beneath the Eifel is estimated to be between 28 and 32 km (Mengel et al., 1991; Prodehl et al., 1992). Babuska & Plomerová (1988) estimated a lithosphere thickness of 100–140 km prior to the Cenozoic rifting and suggested a present-day depth of less than 60 km for the asthenosphere–lithosphere boundary beneath the Rhenish Massif.

Volcanism within the CEVP spans the entire Cenozoic period (Wilson & Downes, 1991); in the Hocheifel area it appears to have ranged from middle Eocene to late Oligocene (45 Ma–24 Ma; Lippolt, 1982), although new Ar–Ar data suggest a smaller age range and two distinct periods of activity (44–40 Ma and 38–34 Ma; Fekiacova et al. 2003). The magmatic rocks are mainly basanites, nephelinites and alkali basalts plus rare hawaiites, mugearites, benmoreites and trachytes. Volcanism in the neighbouring East and West Eifel produced about 300 small-volume monogenetic centres between 700 and 10·8 ka BP (Schmincke et al., 1983; Mertes & Schminke, 1985; Wörner et al., 1985). Two geochemically, spatially and temporally distinct groups of sodic–potassic alkaline volcanic rocks were erupted in the East Eifel. In the NW nephelinites, leucitites and more differentiated rocks were erupted >400 kyr ago whereas in the SE basanites and more differentiated rocks erupted between 400 and 10 ka BP. The west Eifel volcanic field consists of leucitites, basanites and nephelinites, which cover an area of 600 km² and erupted between 700 and 10 ka BP (Mertes & Schmincke, 1985, and references therein). Wilson & Downes (1991) suggested that the most primitive mafic alkaline volcanic rocks have major and trace element and Nd–Sr–Pb–O isotope systematics that suggest the involvement of both lithospheric and asthenospheric mantle source components in their petrogenesis. The sodic magma types (melilitites, nephelinites, basanites, alkali olivine basalts) originated by partial melting of a common asthenospheric mantle source, termed the EAR (European Asthenospheric Reservoir), whereas the potassic lavas (leucitites, leucite basanites) were derived from locally enriched portions of the mantle lithosphere. The Hercynian basement through which the magmas erupted consists mainly of greenschist- to amphibolite-facies metapelites, metabasites and orthogneisses of the Mid German Crystalline Rise and is overlain by Palaeozoic (Lower to Upper Devonian) limestones and sandstones and Cenozoic (Triassic) sandstones, carbonates and clays (Mengel et al., 1991, and references therein).

	ANALYTICAL TECHNIQUES

TOP ABSTRACT INTRODUCTION GEOLOGICAL SETTING ANALYTICAL TECHNIQUES PETROGRAPHY GEOCHEMISTRY DISCUSSION CONCLUSIONS REFERENCES

 
Sixty-five samples were taken from the Hocheifel volcanic field according to the distribution of deeply eroded volcanic edifices given by Huckenholz & Büchel (1988). Based on the petrographic descriptions and average chemical compositions given by Huckenholz & Büchel (1988), a significant number of accessible sample sites were re-investigated to cover the entire range of lava compositions (Fig. 1b and Table 1). All samples were taken from remnants of lava flows, plugs and necks that cover the entire volcanic field. Whole-rock samples were prepared by crushing in an agate shatterbox to obtain 250 g of the macroscopically freshest material. Aliquots were analysed for major and trace elements in fused lithium tetraborate glass beads using standard X-ray fluorescence (XRF) techniques (Vogel & Kuipers, 1987) at the Mineralogical–Petrological Department at the University of Bonn. Rare earth elements (REE) were analysed by inductively coupled plasma atomic emission spectrometry (ICP-AES) following separation of the matrix elements by ion exchange (Heinrichs & Herrmann, 1990) at the Department of Mineralogy, Petrology and Crystallography at the University of Marburg. Loss on ignition (LOI) was determined gravimetrically at 1050°C (Lechler & Desilets, 1987) and FeO was measured titrimetrically using standard techniques. Accuracy was monitored by repeated measurements of international and in-house standards; the results are in good agreement with the recommended values for the international rock standard JB 2 given by Govindaraju (1994) (Table 2).

View this table: [in this window] [in a new window]   Table 1: Sample localities and petrographic characteristics

 

View this table: [in this window] [in a new window]   Table 2: Representative clinopyroxene, amphibole, plagioclase and olivine composition from Hocheifel basalts

 
Pb, Sr and Nd isotope analyses were carried out at the Max-Planck-Institut für Chemie at Mainz by thermal ionization mass spectrometry using a Finnigan MAT 261 multiple sample, multicollector mass spectrometer operating in the static mode. Whole-rock chips were leached in 6N HCl for at least 2 h on a hotplate. Subsequently, the samples were washed three times with ultrapure H₂O. After this treatment, the samples were dissolved in concentrated HF and after evaporation redissolved in 2·5N HCl and 0·6N HBr and loaded on Teflon^® columns filled with DOWEX^® AG 1x8 anion exchange resin (100–200 mesh) in chloride form (Mattinson, 1986). The Pb was extracted using conventional HBr–HCl techniques and was loaded on Re single filaments following the H₃PO₄–silica gel method (Cameron et al., 1969). Strontium and REE were separated by using standard cation exchange columns with a DOWEX^® AG 50 W-X 12 resin using 2·5N HCl for Sr and 6N HCl for the REE. Nd was separated from the other REE by using HDEHP-coated Teflon^® columns and 0·12N HCl. Neodymium isotopes were normalized to ¹⁴⁶Nd/¹⁴⁴Nd = 0·7219. Repeated measurements of the La Jolla Nd standard gave ¹⁴³Nd/¹⁴⁴Nd = 0·511848 ± 0·000021 (2; n = 28). The reproducibilitiy of the Sr standard (NBS 987) is ⁸⁷Sr/⁸⁶Sr = 0·710224 ± 0·000024 (2; n = 14) and the fractionation was corrected to ⁸⁶Sr/⁸⁸Sr = 0·1194. Lead analyses were corrected for mass fractionation by a factor of 0·11% per a.m.u. The reproducibility of the standard NBS 982 was estimated to be 0·068%, 0·064% and 0·071% for the ²⁰⁶Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb ratio, respectively. The total procedure blank is <60 pg Pb during this study and is therefore considered negligible.

	PETROGRAPHY

TOP ABSTRACT INTRODUCTION GEOLOGICAL SETTING ANALYTICAL TECHNIQUES PETROGRAPHY GEOCHEMISTRY DISCUSSION CONCLUSIONS REFERENCES

 
All samples are porphyritic and contain partly altered olivine and clinopyroxene phenocrysts. Usually, the majority of the olivine and clinopyroxene phenocrysts have grain sizes of 1–2 mm and 0·5–5 mm, respectively, and the samples containing such phenocrysts do not appear to be accumulative. Rare olivines in some basanites and nephelinites have a mosaic texture and incipient undulose extinction. They probably represent entrained material from disintegrated peridotite xenoliths. Chemically, these olivines appear to be enriched in MgO and depleted in FeO relative to the dominant euhedral olivines (Table 2). The dominant olivines within the nephelinites and basanites are euhedral phenocrysts with sharply defined crystal edges. Sometimes, the olivines are more skeletal with evidence of marginal resorption, re-entrants and internal cavities. These olivines have slightly lower MgO but higher FeO than the olivines, with incipient undulose extinction (Table 2). Both types of olivine show a narrow rim with depletion of MgO and enrichment of FeO.

Zoned clinopyroxenes are generally composed of a subhedral to anhedral colourless to pale brown core and a darker brown, slightly pleochroic mantle. The core of these clinopyroxenes is usually more MgO-poor and FeO-rich than the rim (Table 2). The evolution of clinopyroxene in the sequence basanite–mugearite–tephrite shows increasing MgO but decreasing FeO, suggesting a complex pre-eruption history (Fig. 2a). In some samples, clinopyroxenes with an olive green to light green core, a colourless to pale brown mantle and a dark brown rim occur in addition to the clinopyroxenes described above. The green core of these clinopyroxene is enriched in FeO and Al₂O₃ and depleted in MgO. Whereas the core is unzoned, the pale brown mantle shows increasing MgO starting at a much higher MgO content. This evolution is followed by a decrease in MgO and a narrow zone of MgO enrichment when approaching the rim. In this sequence, FeO, Al₂O₃ and TiO first decrease and then increase through the mantle. Towards the outermost rim, the composition of the clinopyroxene shows a complex evolution of generally increasing Al₂O₃, FeO and TiO₂, starting at lower values than the mantle values, whereas MgO decreases towards the outermost rim (Fig. 2a).

View larger version (71K): [in this window] [in a new window]   Fig. 2 (a) Representative clinopyroxene compositional profiles from samples HEJ 21 (basanite), HEJ 10 (mugearite), HEJ 12 (nephelinite) and HEJ 1 (tephrite); (b) representative amphibole compositional profiles from samples HEJ 21, HEJ 10 and HEJ 1, and a plagioclase compositional profile from sample HEJ 10.

 
The mostly euhedral form of some of the green cores suggests that they crystallized from a melt. This melt must have been more differentiated than the host basanite because of the lower MgO but higher Al₂O₃, FeO and TiO₂ abundances. On the other hand, the chemical characteristics of the mantles suggest that they crystallized from a more mafic magma (higher MgO, lower Al₂O₃, FeO and TiO₂). The outermost rims of the green-core cpx vary according to normal low-pressure fractionation trends (e.g. increasing Ti, Al and Fe, and decreasing Mg). These features are characteristics of clinopyroxene crystallizing from an alkali basaltic magma (e.g. Duda & Schmincke, 1985). The development of the more primitive mantles around the more evolved cores can be attributed to mixtures of the host basanitic melt with evolved (?tephritic) melts stored in the mantle, which resulted from earlier episodes of magma generation and fractionation. Mixing between evolved and primitive melts, believed to be associated with magma replenishment (e.g. Huppert & Sparks, 1988), can account for the progression from evolved cores towards mafic rims of the clinopyroxene phenocrysts. Therefore, the crystallization scenario shown by the different clinopyroxene types suggests discrete storage zones for the alkali basaltic magmas.

Some basanites and some of the more differentiated rocks contain optically homogeneous, unzoned brown amphibole (kaersutite) phenocrysts with a grain size between 1 mm and 2 cm (Table 2 and Fig. 2b). Plagioclase is common only in some alkali basalts and in the more differentiated rocks and is generally unzoned (Table 2). Rare large plagioclase crystals show the development of K-feldspar-rich rims (Fig. 2b).

	GEOCHEMISTRY

TOP ABSTRACT INTRODUCTION GEOLOGICAL SETTING ANALYTICAL TECHNIQUES PETROGRAPHY GEOCHEMISTRY DISCUSSION CONCLUSIONS REFERENCES

 
Chemical composition of the Hocheifel lavas
The mafic magmatic rocks from the Hocheifel area are mostly relatively primitive basanites, nephelinites and alkali basalts with rare hawaiites, mugearites and benmoreites, according to the total-alkali vs SiO₂ classification sheme of Le Bas et al. (1986) (Table 3; Fig. 3a). According to the classification proposed by Huckenholz & Büchel (1986) and Le Bas (1989), samples with >10% normative nepheline, CaO + Na₂O + K₂O < 18 at SiO₂ + Al₂O₃ < 55 and <41 wt % SiO₂ are classified as nephelinites. For the nephelinites and basanites, TiO₂, MgO, CaO and FeO_(total) decrease and K₂O and Al₂O₃ increase with increasing SiO₂ (Fig. 3). Na₂O (not shown) shows considerable scatter among the nephelinites and basanites but generally increases with increasing SiO₂. For the evolved rocks (hawaiites, mugearites and benmoreites), TiO₂, MgO, CaO and FeO decrease with increasing SiO₂, whereas K₂O, Al₂O₃ and Na₂O increase (Fig. 3). P₂O₅ decreases from nephelinites to alkali basalts but increases with increasing SiO₂ in the more differentiated rocks. Basanites and nephelinites have high CaO/Al₂O₃ ratios and this ratio decreases in the more differentiated rocks with increasing SiO₂ (Fig. 3b).

View larger version (21K): [in this window] [in a new window]   Fig. 3 (a) Total alkali–silica diagram (Le Bas et al., 1986), (b) CaO/Al2O3 vs SiO2, (c) K2O vs SiO2, (d) FeO (total) vs SiO2, (e) TiO2 vs SiO2, (f) CaO vs SiO2, (g) MgO vs SiO2 and (h) Al2O3 vs SiO2 for the Hocheifel lavas.

 

View this table: [in this window] [in a new window]   Table 3: Chemical composition of Hocheifel lavas

 
Trace element data are reported in Table 3 and Figs 4 and 5. Most nephelinites, basanites and the alkali basalts have Ni, Cr and Co contents that approach the values commonly assumed for primary magmas (e.g. Frey et al., 1978). Scandium contents range from 10 to 40 ppm for the nephelinites, basanites and the alkali basalts (Fig. 4d). The most interesting feature is the strong overlap in compatible trace element abundances for nephelinites, basanites and alkali basalts. Similarly, incompatible trace element abundances (Nb, Ba, Zr, La) show strong overlap in nephelinites, basanites and the alkali basalts (Fig. 5), although the nephelinites tend to have higher La abundances than the basanites and alkali basalts. Consequently, ratios of incompatible trace elements (Zr/Y, La/Nb, Zr/Nb; Fig. 6) also show some overlap for nephelinites, basanites and alkali basalts. On the other hand, ratios of Ba/La, Ba/Nb and K/Nb tend to increase with increasing SiO₂ from nephelinites to basanites and alkali basalts (Fig. 6). Most of the differentiated rocks have higher Ba/La, Ba/Nb, K/Nb, Rb/Nb and Zr/Nb ratios than the nephelinites, basanites and alkali basalts, and Ba/La and Ba/Nb decrease with increasing SiO₂ within this group (Fig. 6). The Rb/Nb and K/Nb ratios are positively correlated and the nephelinites tend to have lower Rb/Nb and K/Nb ratios than the basanites and alkali basalts. The differentiated rocks have the highest Rb/Nb and K/Nb ratios. La/Nb ratios are remarkable constant among the nephelinites, basanites and alkali basalts. Nephelinites, basanites and alkali basalts have light REE (LREE)-enriched REE patterns similar to those of many ocean island basalts (OIB) and alkaline volcanic rocks from continental settings (Fig. 7). The differentiated rocks have similar REE patterns; two samples have a pronounced depletion in middle REE (MREE) (Fig. 7). Nephelinites, basanites and alkali basalts show strong enrichment of highly incompatible and moderately incompatible trace elements (Fig. 8). K and Rb are strongly depleted relative to elements with similar incompatibility (Fig. 8) whereas in some nephelinites, the basanites and alkali basalts additional slight depletions of P and Ti can be observed. Some nephelinites are enriched in P. Apart from these anomalies, primitive mantle-normalized concentrations increase with increasing incompatibility and show typical OIB or intracontinental basalt patterns (Fig. 8).

View larger version (18K): [in this window] [in a new window]   Fig. 4 (a) Cr (in ppm) vs SiO2 (wt %), (b) Ni (in ppm) vs SiO2, (c) Co (in ppm) vs SiO2 and (d) Sc (in ppm) vs SiO2 for the Hocheifel lavas.

 

View larger version (17K): [in this window] [in a new window]   Fig. 5 (a) Nb (in ppm) vs SiO2 (wt %), (b) Ba (in ppm) vs SiO2, (c) Zr (in ppm) vs SiO2 and (d) La (in ppm) vs SiO2 for the Hocheifel lavas.

 

View larger version (23K): [in this window] [in a new window]   Fig. 6 (a) Zr/Y vs SiO2, (b) La/Nb vs SiO2, (c) Zr/Nb vs SiO2, (d) Ba/La vs SiO2, (e) Ba/Nb vs SiO2 and (f) K/Nb vs SiO2 for the Hocheifel lavas.

 

View larger version (17K): [in this window] [in a new window]   Fig. 7 Rare earth element abundances of (a) nephelinites, (b) basanites, (c) alkali basalts and (d) more differentiated rocks from the Hocheifel area. Normalization values are from Boynton (1984).

 

View larger version (19K): [in this window] [in a new window]   Fig. 8 Primitive mantle-normalized incompatible element patterns of (a) nephelinites, (b) basanites, (c) alkali basalts and (d) more differentiated rocks from the Hocheifel area. Normalization values are from Sun & McDonough (1989).

 
Sr–Nd–Pb isotope chemistry
Sr–Nd–Pb isotope data are reported in Table 4. The ⁸⁷Sr/⁸⁶Sr ratios of the Hocheifel basalts are low and the ¹⁴³Nd/¹⁴⁴Nd ratios are high; thus, the samples plot in the ‘depleted field’ relative to Bulk Earth in the Sr–Nd isotope diagram (Fig. 9). Sr and Nd isotope data for the primitive nephelinites, basanites and alkali basalts form an elongated trend from more depleted compositions similar to the European Asthenospheric Reservoir (EAR; Cebriá & Wilson, 1995) towards Bulk Earth values. Generally, this trend is broadly similar to the trends defined by Cenozoic mafic alkaline rocks from elsewhere in Germany (Wörner et al. 1986; Wedepohl et al., 1994; Hegner et al., 1995; Jung & Masberg, 1998; Jung & Hoernes, 2000; Bogaard & Wörner 2003; Haase et al., 2004) and also to other CEVP provinces; for example, the Massif Central, Poland, and the Pannonian basin (Alibert et al., 1987; Blusztajn & Hart, 1989; Downes, 1984; Wilson & Downes, 1991, 2006; Embey-Isztin et al., 1993; Harangi, 1994; Downes et al., 1995). It is noteworthy that the trend defined by the Hocheifel lavas is similar to that of the neighbouring East Eifel and West Eifel volcanic fields (Wörner et al., 1985), which is, however, displaced to slightly higher ⁸⁷Sr/⁸⁶Sr ratios at a given ¹⁴³Nd/¹⁴⁴Nd ratio. The more differentiated samples tend to have more radiogenic ⁸⁷Sr/⁸⁶Sr and less radiogenic ¹⁴³Nd/¹⁴⁴Nd than the mafic alkaline lavas and overlap with the compositional fields of Eifel mantle xenoliths and lower crustal granulite xenoliths (Fig. 9).

View larger version (30K): [in this window] [in a new window]   Fig. 9 143Nd/144Nd vs 87Sr/86Sr for the Hocheifel mafic alkaline lavas and more differentiated rocks. Stippled area represents data for the Quaternary Eifel volcanic field (Wörner et al., 1986). Dark grey area represents Eifel peridotite xenolith data from Stosch & Lugmair (1986) and Witt-Eickschen et al. (1998, 2003). Light grey field represents Eifel lower crustal xenolith data (Stosch &Lugmair, 1984; Loock et al., 1990). EAR, European Asthenospheric Reservoir (Cebriá & Wilson, 1995).

 

View this table: [in this window] [in a new window]   Table 4: Sr, Nd, and Pb isotope compositions of Hocheifel lavas

 

View this table: [in this window] [in a new window]   Table 5: Compositions and parameters used for the EC-AFC model calculations (Spera & Bohrson, 2001)

 
The Pb isotope compositions of the nephelinites, basanites and alkali basalts overlap and are variable, defining a linear array subparallel to the Northern Hemisphere Reference Line (NHRL). This trend ranges from high ²⁰⁶Pb/²⁰⁴Pb ratios (20) similar to the EAR to more unradiogenic values (19) similar to other volcanic provinces from the CEVP (Fig. 10). A subset of basanites has distinctly lower ²⁰⁷Pb/²⁰⁴Pb ratios than the other samples (Fig. 10). Published Pb isotope data for East Eifel and West Eifel volcanic fields have slightly higher ²⁰⁷Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb ratios at a given ²⁰⁶Pb/²⁰⁴Pb ratio (Wörner et al., 1986).

View larger version (27K): [in this window] [in a new window]   Fig. 10 (a) 207Pb/204Pb and (b) 208Pb/204Pb vs 206Pb/204Pb for Hocheifel mafic alkaline lavas. NHRL, Northern Hemisphere Reference Line (Hart, 1984). Locations of HIMU, EM I and EM II are from Zindler & Hart (1986). Other fields are as in Fig. 8.

 

	DISCUSSION

TOP ABSTRACT INTRODUCTION GEOLOGICAL SETTING ANALYTICAL TECHNIQUES PETROGRAPHY GEOCHEMISTRY DISCUSSION CONCLUSIONS REFERENCES

 
Fractional crystallization
Most of the nephelinites, basanites and alkali basalts from the Hocheifel volcanic field have MgO, Cr and Ni contents high enough for these rocks to represent near-primary magmas (e.g. Hart & Davies, 1978). Some samples have lower concentrations of MgO, Ni and Cr, and for these samples fractionation of olivine and clinopyroxene is likely. For these samples, decreasing CaO and increasing Al₂O₃ leds to decreasing CaO/Al₂O₃ ratios with increasing SiO₂, which is also consistent with clinopyroxene fractionation. Increasing Al₂O₃ contents and the lack of negative Eu anomalies indicate that plagioclase was not a major fractionating mineral phase at this stage, implying that fractionation took place at pressures >5 kbar, equivalent to depths >15 km within the lower crust. The more differentiated rocks have the lowest Ni, Cr and V abundances, indicating that olivine, clinopyroxene and Fe–Ti oxides were important fractionating mineral phases in the petrogenesis of the more evolved Hocheifel magmas. In accordance with previous studies on the evolution of alkaline magma series, it is suggested that the alkali basalts represent the parental magmas from which the more differentiated rocks originated by fractional crystallization (Wilson et al., 1995). The three most differentiated samples with SiO₂ > 50 wt % (HEJ 10, HEJ 11, HEJ 29) have the lowest Sr/Nd ratio and small negative Eu anomalies, indicating removal of Sr by plagioclase fractionation (Fig. 7). Another fractionated sample (HEJ 37; Fig. 7) shows a deficency of the MREE relative to the LREE and heavy REE (HREE), which can be explained by significant fractionation of amphibole and/or titanite. Some nephelinites and basanites, and most of the alkali basalts, contain green-core clinopyroxenes; in accordance with previous studies (e.g. Duda & Schmincke, 1985; Jung & Hoernes, 2000; Haase et al., 2004) the appearence of green-core clinopyroxenes (with cores enriched in Al, Fe and Na and with low Ti/Al ratios) is evidence for high-pressure or, at least, polybaric fractionation of the host magmas. Polybaric fractionation at deep crustal levels may also be associated with crustal contamination, which will be evaluated below.

Crustal contamination
Major element and compatible trace element variations in the alkali basalts and the more differentiated rocks of the Hocheifel indicate that fractional crystallization processes affected the magmas during ascent. Fluid-inclusion barometric studies have shown that similar alkaline magmas from the Quaternary Eifel volcanic field stagnated in the lower crust at pressures of about 0·65 GPa, equivalent to 20 km depth (Duda & Schmincke, 1985; Sachs & Hansteen, 2000). In view of the occurrence of petrographically similar, green-core, clinopyroxenes in most of the alkali basalts, basanites and nephelinites (Table 1) from the Hocheifel, it is suggested that these magmas also stagnated in the lower crust at comparable depths.

The lower crust beneath the Eifel is composed of mafic and felsic granulites, in which mafic granulites, interpreted as basaltic cumulates, predominate over felsic granulites (Mengel et al., 1991; Sachs & Hansteen, 2000). The upper crust consists of Palaeozoic sedimentary and volcanic rocks as well as rare Mesozoic and Cenozoic sedimentary rocks. Rare metasedimentary granulites also occur. Some granulites show evidence of metasomatism and partial melting (formation of secondary hydrous phases, presence of glass) and it has been shown that this metasomatic event is most probably related to the Quaternary–Tertiary magmatism (Sachs & Hansteen, 2000). Lower crustal xenoliths from the Eifel have been extensively studied (Stosch & Lugmair, 1984; Stosch et al., 1986, 1992; Loock et al., 1990; Rudnick & Goldstein, 1990) and, therefore, major and trace element and Sr–Nd–Pb isotope data are available. These granulites have Sr–Nd isotope compositions that extend from Bulk Earth values towards more unradiogenic ¹⁴³Nd/¹⁴⁴Nd but more radiogenic ⁸⁷Sr/⁸⁶Sr isotope compositions (Fig. 9). Felsic granulites tend to have more radiogenic ⁸⁷Sr/⁸⁶Sr isotope compositions, although some mafic granulites are also fairly radiogenic. The nephelinites, basanites and alkali basalts have higher ¹⁴³Nd/¹⁴⁴Nd and lower ⁸⁷Sr/⁸⁶Sr than the lower crustal xenoliths, and only a few of the more differentiated rocks overlap with the Sr–Nd isotope composition of the xenoliths (Fig. 9). The Pb isotope compositions of the xenoliths plot above the NHRL in ²⁰⁷Pb/²⁰⁴Pb vs ²⁰⁶Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb vs ²⁰⁶Pb/²⁰⁴Pb space (Fig. 10). Metasedimentary granulitic xenoliths have higher ⁸⁷Sr/⁸⁶Sr and lower ¹⁴³Nd/¹⁴⁴Nd than the mafic and felsic granulites, but similar Pb isotope compositions to them.

The primitive nephelinites, basanites and alkali basalts have a considerable spread in K/Nb that ranges from 52 to 209 despite their limited variation in SiO₂ (Fig. 6). For the CEVP as a whole the range in K/Nb ratios has been explained as a result of mixing of partial melts of two different mantle end-members (Wilson & Downes, 1991). On the other hand, the positive correlation between K/Nb and SiO₂ (Fig. 6), and the highest K/Nb ratios in the most evolved samples may also reflect crustal contamination processes, as all crustal components (lower crust, bulk crust, upper crust) have high K/Nb ratios (>500; Taylor & McLennan, 1985).

Primitive alkaline volcanic rocks with OIB affinities commonly have low Zr/Nb ratios ranging from 2 to 4 (Weaver, 1991), whereas the continental crust has higher and more variable Zr/Nb ratios ranging from 8 to 14 (Taylor & McLennan, 1985; Rudnick & Fountain, 1995). The higher Zr/Nb ratios in most of the differentiated lavas and the observed correlations of Zr/Nb and Zr, Zr/Nb and K/Nb, Zr/Nb and ⁸⁷Sr/⁸⁶Sr, and Zr/Nb and ¹⁴³Nd/¹⁴⁴Nd suggest assimilation of lower crustal rocks with a composition similar to that of lower crustal xenoliths from the Eifel (Fig. 11). Assimilation of lower crustal rocks and fractional crystallization would have occurred simultaneously. However, thermal considerations suggest that bulk assimilation of lower crustal rocks is unlikely and that contamination of the fractionating alkali mafic magma with a partial melt of the lower crustal wall-rocks is more appropriate. The heat required for partial melting is released by the fractional crystallization process. Recent models indicate that this process is an energy-constrained assimilation–fractional crystallization process (EC-AFC; Spera & Bohrson, 2001). The Spera & Bohrson (2001) model was used to test the influence of concurrent crustal assimilation and fractional crystallization upon the composition of the differentiated lavas from the Hocheifel using the parameters given in Table 4. In contrast to the model parameters given by Spera & Bohrson (2001), we used a higher initial temperature for the lower crust of 900°C. This higher temperature is in agreement with recent estimates of lower crustal temperatures from the Eifel (>800°C, Sachs & Hansteen, 2000), and probably mirrors more closely the effects of rifting, uplift of the asthenosphere–lithosphere boundary and continuing magmatism in Tertiary–Quaternary times. Moreover, at this high inferred temperature, high rates of assimilation relative to fractional crystallization are likely (Reiners et al., 1995). In our model, we used sample HEJ 53 as the parental melt; this is one of the most unfractionated alkali basalts based on its moderately high Ni and Cr abundances and low ⁸⁷Sr/⁸⁶Sr and high ¹⁴³Nd/¹⁴⁴Nd isotope ratios (Tables 2 and 3). As the assimilant we used sample S 32 (Stosch & Lugmair, 1984; Loock et al., 1990), which is a mafic granulite xenolith with fairly high ⁸⁷Sr/⁸⁶Sr and low ¹⁴³Nd/¹⁴⁴Nd. Zr and Nb concentrations are not available for this granulite xenolith and for modelling purposes we used 70 ppm Zr and 6 ppm Nb (average of lower crust; Taylor & McLennan, 1985) for our lower crustal contaminant. It can be seen that the range in Sr and Nd isotope composition, Zr/Nb and K/Nb ratios and Zr concentrations of the differentiated lavas can be reproduced by an EC-AFC model (Fig. 11), implying that energy-constrained assimilation–fractional crystallization processes played an important role in the evolution of the differentiated lavas from the Hocheifel. Based on this model, the trace element and isotope composition of some of the differentiated lavas can be explained by 40–70% fractional crystallization and 10–50% assimilation of a granulite-facies lower crust with a trace element and isotope composition similar to S 32 (Figs 11 and 12). The degrees of assimilation are rather high and probably unrealistic; however, the composition of the lower crust beneath the Rhenish Massif is somewhat unconstrained with respect to its trace element and isotope composition and more suitable end-members may exist.

View larger version (21K): [in this window] [in a new window]   Fig. 11 (a) Zr/Nb vs Zr, (b) Zr/Nb vs K/Nb, (c) Zr/Nb vs 143Nd/144Nd and (d) Zr/Nb vs 87Sr/86Sr for mafic alkaline lavas from the Hocheifel. Lines show the results of EC-AFC calculations with model parameters and end-member compositions from Table 4. Grey lines with regular-font numbers denote mass crystallized and black lines with italic numbers denote mass assimilated, both as wt %.

 

View larger version (18K): [in this window] [in a new window]   Fig. 12 (a) 87Sr/86Sr vs 143Nd/144Nd, (b) 87Sr/86Sr vs 206Pb/204Pb and (c) 143Nd/144Nd vs 206Pb/204Pb for mafic alkaline lavas from the Hocheifel. Lines show the results of EC-AFC calculations with model parameters and end-member compositions from Table 4. Grey lines with regular-font numbers denote mass crystallized and black lines with italic numbers denote mass assimilated both as wt %.

 
Magma generation and partial melting processes
It is generally accepted that the upper mantle is composed predominantly of peridotite with minor amounts of pyroxenite, both with or without garnet. Primitive alkaline, silica-undersaturated melts such as nephelinites and basanites can form at high pressure from garnet peridotite sources (Kushiro, 1996), but not from garnet pyroxenite (Rapp et al., 1991). Therefore, the most likely magma source for the primitive nephelinites and basanites from the Hocheifel is a garnet peridotite. The strong fractionation of HREE, with Dy/Yb >2 (Fig. 13), further suggests that the primitive lavas from the Hocheifel represent partial melts of garnet peridotite. Seismic models indicate that the lithosphere–asthenosphere boundary beneath the Rhenish Massif is strongly elevated and is located at 50–60 km (Babuska & Plomerová, 1992; Goes et al., 2000), whereas the transition from garnet to spinel peridotite is estimated at 2·5–3·0 GPa, equivalent to 75–90 km depth (McKenzie & Bickle, 1988; Robinson & Wood, 1998). Previous xenolith-based studies on the composition of the upper mantle beneath the Rhenish Massif indicate that the upper mantle consists of metasomatized spinel peridotite with amphibole and phlogopite (Witt-Eickschen & Kramm, 1998; Witt-Eickschen et al., 1998, 2003).These xenoliths are interpreted to represent fragments of the lithospheric mantle, and it is reasonable to assume that partial melting must have occurred at depths in excess of 60 km (i.e. below the base of the lithosphere).

View larger version (15K): [in this window] [in a new window]   Fig. 13 La/Yb vs Dy/Yb covariation for the Hocheifel basalts. Partial melting curves were calculated using a non-modal, fractional melting model (Shaw, 1970). Sources are grt–amph peridotite (cpx 0·07, opx 0·19, ol 0·55, grt 0·08, amph 0·11), which melts in the proportions cpx 0·25, opx 0·15, ol 0·05, grt 0·3, amph 0·25, and sp–amph peridotite (cpx 0·08, opx 0·25, ol 0·554, sp 0·033, amph 0·083), which melts in the proportions cpx 0·27, opx 0·25, ol 0·08, sp 0·13, amph 0·27. Source composition (La 2·1 ppm, Yb 0·17 ppm, Dy 0·31 ppm) represents average of 36 peridotite xenoliths from the Hessian Depression (Hartmann & Wedepohl, 1990). Mineral–melt distribution coefficients are taken from McKenzie & O'Nions (1991), Hart & Dunn (1993), Kelemen et al. (1993), Johnston (1994) and LaTourette et al. (1995). Numbers on model curves indicate the per cent melting. Points at 80grt/20sp, 60grt/40sp and 40grt/60sp indicate mixing proportions of melts from garnet peridotite (i.e. 80%) with melts from spinel peridotite (i.e. 20%).

 
A useful approach to model partial melting of common upper mantle sources is based on REE systematics (e.g. a plot of La/Yb vs Dy/Yb, Fig. 13; Thirlwall et al., 1994; Baker et al., 1997); such plots can easily distinguish between melting in the garnet peridotite stability field and melting in the spinel peridotite stability field because of the strong fractionation of HREE by garnet. Additionally, mixing of melts from garnet and spinel peridotite sources should produce linear arrays in such a diagram. The nephelinites and most basanites and alkali basalts appear to form a coherent group with higher La/Yb ratios than the rest of the basanites and one alkali basalt, which form another group with higher Dy/Yb ratios (Fig. 13). Considering each group individually, it appears that the samples plot on mixing lines between melts from garnet peridotite and melts from spinel peridotite. Moreover, this diagram suggests that simple partial melting exclusively in the garnet peridotite stability field or spinel peridotite stability field cannot account for the spread of data. Partial melting of spinel peridotite should also produce a positive correlation between Ce/Yb and Yb abundances. Nephelinites, basanites and alkali basalts appear to show a series of negative correlations between Ce/Yb and Yb abundances (Fig. 14a). Additionally, nephelinites, basanites and alkali basalts show a positive correlation between Ce/Yb and Ce abundance (Fig. 14b). These features indicate that: (1) nephelinites, basanites and alkali basalts originate from sources with similar LREE enrichment; (2) nephelinites represent smaller melt fractions than most basanites and alkali basalts; (3) at least the nephelinites originate from a garnet-bearing source in which, during partial melting, garnet was progressively eliminated from the source. High CaO/Al₂O₃ ratios in the nephelinites and decreasing CaO/Al₂O₃ with increasing SiO₂ in the sequence nephelinite–basanite–alkali basalt are also consistent with increasing degrees of partial melting of a garnet-bearing source. Therefore, the most plausible model that can account for the REE variation involves initial partial melting in the garnet stability field, followed by mixing of melts from garnet peridotite with melts from spinel peridotite, both containing amphibole. Most of these samples cluster at Dy/Yb ratios between 2 and 3; this is typical for the upper mantle beneath the CEVP; the mantle xenoliths have flat to slightly LREE-enriched REE patterns (Stosch & Lugmair, 1986; Witt-Eickschen & Kramm, 1998).

View larger version (15K): [in this window] [in a new window]   Fig. 14 (a) Chondrite-normalized Ce/Yb vs Yb and (b) chondrite-normalized Ce/Yb(n) vs Ce for mafic alkaline lavas from the Hocheifel. Normalization values are from Boynton (1984).

 
It is noteworthy that the nephelinites, basanites and alkali basalts have a large range in K/La ratios and that the nephelinites have the highest Ce/Yb_(norm.) and La concentrations and the lowest K/La ratios (Fig. 15). The model curves shown in Fig. 15 imply that the primitive Hocheifel lavas could result from 1–2% partial melting in the garnet peridotite stability field, compatible with experimental results that indicate melting degrees in excess of 1% to generate basanites from peridotite sources (Kushiro, 1996). Similar low degrees of melting have been inferred from a number of volcanic provinces of the CEVP (Wilson & Downes, 2006). In the Hocheifel, small-degree partial melts from garnet-bearing peridotite were mixed with melts produced by a similar degree of melting of a spinel peridotite source (Fig. 13), suggesting that partial melting and mixing of melts occurred close to the spinel–garnet transition zone. The transition from garnet peridotite to spinel peridotite occurs at 2·5–2·7 GPa (Robinson & Wood, 1998), indicating that the Hocheifel lavas formed at 80 km depth. The nephelinites, basanites and alkali basalts display a negative correlation between K/La and Ce/Yb_(norm.) (Fig. 15a) implying that partial melting of garnet or spinel peridotite alone is not likely because low-melt fractions from such sources have both high Ce/Yb_(norm.) and K/La ratios (Haase et al., 2004). It is, therefore, very likely that a residual mineral phase that fractionates K from La was present. This mineral phase was probably amphibole rather than phlogopite, because phlogopite fractionates K/La even more efficiently than amphibole and also fractionates Ba/La. Additionally, Ba concentrations are high in the primitive Hocheifel lavas, which argues against significant amounts of phlogopite in the melt residue. Because the nephelinites with high Ce/Yb_(norm.) and low K/La ratios formed in the stability field of garnet peridotite in the presence of amphibole, it is suggested that the melting region is located at 2·5–3·0 GPa and 1250–1300°C (Fig. 16). The amphibole may have formed by mantle metasomatism caused by migration of small-degree melts from an upwelling plume beneath the Rhenish Massif.

View larger version (15K): [in this window] [in a new window]   Fig. 15 (a) K/La vs chondrite-normalized Ce/Yb and (b) K/La vs La for mafic alkaline lavas from the Hocheifel. Partial melting curves were calculated using a non-modal, fractional melting model (Shaw, 1970) with the sources and melting modes given in Fig. 12. Source composition has 592 ppm K, 2·1 ppm La, 4·5 ppm Ce and 0·17 ppm Yb, which is the average of 36 peridotite xenoliths from the Hessian Depression (Hartmann & Wedepohl, 1990). Mineral–melt distribution coefficients are from McKenzie & O'Nions (1991), Hart & Dunn (1993), Kelemen et al. (1993), Johnston (1994) and LaTourette et al. (1995).

 

View larger version (19K): [in this window] [in a new window]   Fig. 16 Pressure–temperature diagram to illustrate the potential source region for mafic alkaline lavas from the Hocheifel. Solidi for dry mantle and CO2-saturated mantle are from McKenzie & Bickle (1988) and Falloon & Green (1990), respectively. Also shown are adiabats for various mantle potential temperatures. Stability fields for spinel and garnet peridotite and amphibole in upper mantle rocks are from Falloon & Green (1990), Foley (1991) and Robinson & Wood (1998). , experimental results of Huckenholz & Gilbert (1980) for the stability of amphibole as a phenocryst phase in basanite and nephelinite from the Hocheifel as discussed in the text. Crustal thickness (Moho) is adopted from Prodehl et al. (1992) and the lithosphere–asthenosphere boundary is taken from Babuska & Plomerová (1992). , experimental results of phlogopite–garnet peridotite melting from Mengel & Green (1986) and Thibault et al. (1992). The black diamond approximates the inferred melting region of the Hocheifel lavas.

 
It is still a controversial issue whether the volcanism of the CEVP is related to adiabatic decompression melting caused by thinning of the lithosphere during rifting or due to raised mantle temperatures (up to 200°C, Ritter et al., 2001) as a consequence of mantle plume activity. Data from experimental investigations can be used to constrain the conditions of formation of the Hocheifel lavas. Basanites, and even more Si-undersaturated melts such as nephelinites, can be generated by melting of amphibole- and/or phlogopite-bearing garnet or spinel peridotite at pressures >2 GPa and temperatures >1360°C in the presence of CO₂ (Mysen & Kushiro, 1977; Hirose, 1997). However, similar experimental investigations have shown that the generation of basanites and nephelinites is also possible at much lower temperatures of 1200–1250°C and pressures of 2·8–3·0 GPa (Mengel & Green, 1986; Thibault et al., 1992). The formation of Si-undersaturated melts requires the presence of H₂O and CO₂ (Brey & Green, 1977; Mengel & Green, 1986; Thibault et al., 1992; Hirose, 1997). Magmas with slightly higher SiO₂ concentrations of 45 wt %, similar to the alkali basalts from the Hocheifel, can be generated from melting of dry mantle at 3·0 GPa (Jaques & Green, 1980; Kushiro, 1996). Results from experimental investigations (Fig. 16) and the REE modelling (Fig. 13) suggest that the primitive Hocheifel lavas (nephelinites, basanites) were generated by melting of garnet peridotite at a pressure of >2·6 GPa at a mantle potential temperature of 1200°C, which is lower than the inferred average mantle temperature of 1300°C (McKenzie & Bickle, 1988). Huckenholz & Gilbert (1984) and Huckenholz et al. (1988) investigated experimentally the stability of Ca-amphibole in a basanite from Alte Burg near Reiferscheid and a nephelinite from the Nürburg. In their experiments, Ca-amphibole was stable between 0·2 GPa/1090°C and 3·0 GPa/1260°C in the basanite and 0·2 GPa/1085°C and 3·0 GPa/1240°C in the nephelinite. Based on these results, Huckenholz & Gilbert (1984) concluded that Ca-amphibole similar in composition to the amphibole that occurs in the mafic alkaline lavas crystallized between 1225°C and 1250°C at pressures of 1·5–2·0 GPa. Assuming that garnet is an important residual phase in the mantle source of the mafic alkaline lavas from the Hocheifel, temperatures of 1250–1300°C at a pressure of 2·5 GPa seems a more reliable P–T estimate. The inferred presence of amphibole in the source of the Hocheifel magmas may, therefore, point to a relatively low mantle potential temperature of about 1200–1250°C. Consequently, the data do not support the idea of a very hot mantle source with a maximum excess temperature of 200°C (i.e. Ritter et al., 2001), in the formation of the Hocheifel magmas. This conclusion is also compatible with the view that partial melting in the upper mantle beneath Europe requires either an anomalously hot mantle (for which we see no evidence here) or a volatile-rich mantle source (Wilson & Downes, 2006) which may be represented by the inferred amphibole-bearing peridotite source of the mafic Hocheifel lavas.

Nature of the mantle sources for the Tertiary Hocheifel basalts
The nephelinites and most basanites and alkali basalts do not appear to have been contaminated by crustal material; hence, their Sr–Nd–Pb isotope compositions should reflect those of their mantle sources. The Sr–Nd–Pb isotope compositions of the primitive Hocheifel lavas exhibit a large range of variation, implying substantial isotopic heterogeneity of the mantle sources involved. The ²⁰⁶Pb/²⁰⁴Pb isotope ratios display a large variation ranging from 19 to 20, probably indicating mixing of melts from two sources. The Pb isotope values are the highest found to date in the volcanic fields of the Rhenish Massif and exceed the radiogenic Pb isotope compositions of the mafic lavas from the Urach–Hegau volcanic field (Hegner et al., 1995; Wilson et al., 1995), the West Eifel (Wörner et al., 1986), the Siebengebirge (Wedepohl et al., 1999) and the Westerwald (Haase et al., 2004).

The composition of the shallow lithospheric mantle beneath the Rhenish Massif, and in particular beneath the Eifel, is relatively well known as a result of numerous geochemical and isotopic studies of spinel peridotite xenoliths from the East Eifel and West Eifel volcanic fields (Stosch & Seck, 1980; Stosch & Lugmair, 1986; Witt-Eickschen & Kramm, 1998; Witt-Eickschen et al., 1998, 2003). Most of these peridotitic xenoliths are less radiogenic in Nd and more radiogenic in Sr isotope composition, indicating that such sources cannot represent the source of the nephelinites and basanites. In Pb–Pb isotope space, however, there is broad overlap between the lithospheric spinel peridotites from the East and West Eifel and the nephelinites and basanites from the Hocheifel. Conditions of equilibration of the lithospheric peridotite xenoliths from beneath the Eifel have been estimated to be mostly <2·0 GPa and <1100°C, corresponding to a depth of about 60 km (Witt-Eickschen & Kramm, 1998; Witt-Eickschen et al., 1998, 2003). Therefore, the source of the basanites and nephelinites must be deeper in the mantle; most probably in the asthenosphere. The Sr–Nd–Pb isotope composition of the nephelinites and basanites, as well as their similarity to OIB and other primitive alkaline mafic magmas from Central Europe, further suggests that the source of these mafic alkaline lavas is located in the asthenosphere (Wilson & Downes, 1991, 2006), although some workers (Goes et al., 1999; Wedepohl & Baumann, 1999) have suggested a deep mantle origin for the primitive alkaline lavas from Europe. A restricted group of basanites have lower ²⁰⁶Pb/²⁰⁴Pb and ²⁰⁷Pb/²⁰⁴Pb ratios (Fig. 10). These samples also have lower ¹⁴³Nd/¹⁴⁴Nd ratios of <0·51285, relative to the nephelinites and the rest of the basanites, implying a source with lower (U + Th)/Pb and lower ¹⁴⁷Sm/¹⁴³Nd. This source, with EM affinities, is probably located close to the base of the TBL (thermal boundary layer; McKenzie & Bickle, 1988; Wilson et al., 1995) in the lower lithosphere.

It has been suggested that the Cenozoic volcanism in Central Europe is related to two distinct mantle sources (Wilson & Downes, 1991, 2006; Wilson & Patterson, 2001). One common mantle source [the EAR; European Asthenospheric Reservoir of Cebriá & Wilson (1995) or LVC; Low Velocity Composition of Hoernle et al. (1995)] has HIMU-like isotope characteristics and is inferred to be the source of the most primitive Na₂O-rich basalts, whereas another, more enriched mantle source (EM I) contributed to the geochemistry of the more K₂O-rich basalts. The EAR component is probably transported from the deeper mantle to the asthenosphere in small-scale mantle plumelets (Granet et al., 1995), one of which seems to exist beneath the Eifel (Ritter et al., 2001; Keyser et al., 2002). Mixing of partial melts from these two mantle sources may explain the variable Sr–Nd–Pb isotope compositions of the mafic lavas from the CEVP. In addition to the mafic lavas from the Urach–Hegau volcanic field in southern Germany, high ²⁰⁶Pb/²⁰⁴Pb ratios (>19·5) have until now been found only in some Quaternary West Eifel lavas (Wörner et al., 1986) and in some mafic lavas from the Westerwald and Siebengebirge (Wedepohl & Baumann, 1999; Haase et al., 2004). Excluding the Quaternary West Eifel lavas, the high ²⁰⁶Pb/²⁰⁴Pb ratios of the Westerwald, Siebengebirge and Hocheifel lavas probably implies that these volcanic centres were fed from melts of the same (homogeneous?) mantle source during the Tertiary. Haase et al. (2004) also observed moderately high ²⁰⁶Pb/²⁰⁴Pb ratios up to 19·6 in some mafic lavas from the Westerwald and Siebengebirge, although it is not entirely clear whether these Pb isotope compositions can be interpreted as mixtures of melts of the EAR source (with ²⁰⁶Pb/²⁰⁴Pb of 20, Hoernle et al., 1995) and a more unradiogenic source. The new Hocheifel data presented here, with several mafic lavas having ²⁰⁶Pb/²⁰⁴Pb ranging from 19·6 to 20·0, indicate that the EAR component is an important constituent of the upper mantle beneath the Rhenish Massif.

Compositional variations of the Hocheifel magmas with time
To constrain the temporal and compositional variations of the mafic Hocheifel magmas, high-precision Ar–Ar age determinations (Fekiacova, 2004) and geochemical and isotope data from this study were combined. Mafic alkaline volcanism in the Hocheifel started at about 44 Ma, roughly 18 Myr earlier than in the Westerwald and Siebengebirge (Haase et al., 2004). From Fig. 17 it becomes evident that the early mafic alkaline lavas from the Hocheifel have low CaO/Al₂O₃, low La/Yb and high K/La ratios, reflecting a contribution from a source with minor garnet but substantial amounts of a K-bearing mineral, which, in this case, is most probably amphibole. Moreover, these melts have unradiogenic Nd but radiogenic Sr and Pb isotopic compositions and were probably derived by melting of the thermal boundary layer at the base of the lithosphere. The late mafic alkaline lavas have higher CaO/Al₂O₃, higher La/Yb and lower K/La ratios and radiogenic Nd but unradiogenic Sr and Pb isotopic compositions reflecting a dominant contribution from the plume. Together with the compositional and temporal constraints provided by Haase et al. (2004), a pulsing of the mantle plume beneath the Rhenish Massif can be suggested. At about 39 Ma, lavas with relatively low ²⁰⁶Pb/²⁰⁴Pb ratios (ca. 19·0) were erupted. As noted above, these earlier may reflect a contribution from the thermal boundary layer. Between 37 Ma and 35 Ma, the ²⁰⁶Pb/²⁰⁴Pb ratios of the lavas increase to 19·6, suggesting a contribution from the advancing plume. High ²⁰⁶Pb/²⁰⁴Pb ratios between 19·3 and 19·7 are also characteristic for the alkaline volcanism of the Siebengebirge and Westerwald, which occurred between ca. 28 Ma and 20 Ma. Between ca. 20 Ma and 10 Ma mafic alkaline lavas from the Vogelsberg and the Hessian Depression display significantly lower ²⁰⁶Pb/²⁰⁴Pb ratios between 18·8 and 19·3 Ma, again suggesting a contribution from the thermal boundary layer. In Quaternary times, mafic alkaline magmas from the Eifel display again high ²⁰⁶Pb/²⁰⁴Pb ratios of 19·5, implying reactivation of the plume source. It should be noted that the composition of the younger Hocheifel magmas is similar to that of the Eifel magmas erupted in Quaternary times, implying similar sources for the Tertiary and Quaternary volcanism.

View larger version (15K): [in this window] [in a new window]   Fig. 17 Temporal variations of chemical and isotopic compositions of the Hocheifel lavas. Ar–Ar ages are from Fekiacova (2004) and include (with decreasing age) Alte Burg (39·0 Ma), Kastelberg (37·3 Ma), Scharfer Kopf (37·0 Ma), Hohe Acht (36·3 Ma), Kapp (35·9 Ma), Neuenahrer Burgberg (35·2 Ma) and Nürburg (35 Ma). (For sample localities see Table 1 and Fig. 1b.)

 

	CONCLUSIONS

TOP ABSTRACT INTRODUCTION GEOLOGICAL SETTING ANALYTICAL TECHNIQUES PETROGRAPHY GEOCHEMISTRY DISCUSSION CONCLUSIONS REFERENCES

 
Geochemical and isotopic studies of Tertiary Hocheifel nephelinites, basanites and alkali basalts provide the following constraints on the evolution of these rocks.

(1) Most of the investigated samples are relatively primitive alkaline rocks, mostly nephelinites and basanites with high Mg-number (>0·60) and high Cr and Ni contents. Some samples are more differentiated magmas that have undergone polybaric fractionation of olivine + clinopyroxene + amphibole ± plagioclase + Fe–Ti oxides.

(2) Incompatible trace element abundances are remarkably similar for nephelinites, basanites and alkali basalts, although alkali basalts tend to have higher Ba/Nb, K/Nb and Ba/La ratios than nephelinites. Sr–Nd–Pb isotope data are broadly similar to other mafic lavas from the CEVP and show overlap for most of the nephelinites, basanites and alkali basalts. One group of basanites has lower ²⁰⁶Pb/²⁰⁴Pb (19·13–19·22), ²⁰⁷Pb/²⁰⁴Pb (15·46–15·50) and ¹⁴³Nd/¹⁴⁴Nd ratios (0·51276–0·51285) than the rest of the basanites and nephelinites, which have more radiogenic Pb and Nd isotope compositions. These features imply the existence of at least two different mantle sources in the petrogenesis of the mafic Hocheifel lavas. One source is similar to an asthenospheric OIB-type source (EAR: Cebriá & Wilson, 1995; LVC: Hoernle et al., 1995), whereas the source of the basanites with unradiogenic Pb and Nd isotope compositions could be a part of the TBL, located at the base of the subcontinental lithospheric mantle. The more differentiated samples have probably assimilated material from the lower crust similar in composition to the granulite xenoliths described from the Eifel.

(3) Variations in REE abundances are compatible with mixing of melt fractions from garnet peridotite with melt fractions from spinel peridotite, both containing residual amphibole. The incompatible trace element enriched, but isotopically depleted nature of the basalts requires a recently enriched mantle source.

(4) The petrogenetic model for the Hocheifel basalts implies the existence of melts from sub-lithospheric sources and melts from the base of the lithospheric mantle. Old, depleted subcontinental lithospheric mantle is too dry to yield significant quantities of melt, even when heated by an upwelling mantle plume; consequently, infiltrating fluids and melts from such plumes are required to metasomatize the base of the subcontinental lithospheric mantle. During ascent, partial melts from the upwelling plume heat and partially melt the metasomatized subcontinental lithospheric mantle by dehydration melting (Gallagher & Hawkesworth, 1992). If the overlying mantle lithosphere and crust is not drastically weakened, the geochemistry and isotope composition of the first small-degree partial melts should reflect that of the subcontinental lithospheric mantle, whereas the composition of the later, larger-degree partial melts may carry the signature of the plume itself. Melt generation at the base of the subcontinental lithospheric mantle can, therefore, lead to structural weakening of the lithosphere and can also promote thermal erosion of the base of the lithosphere. Substantial amounts of melt can be generated within the lithosphere for ß factors (the ratio of unstretched to stretched lithosphere, McKenzie & Bickle, 1988) <1·2 (Gallagher & Hawkesworth, 1992). In regions with ß factors >1·3 for a 100 km thick lithosphere, the upwelling plume will melt to increasingly larger degrees and magmas derived from the plume will dominate over those derived from the SCLM. In the model of Hawkesworth & Gallagher (1993), a sequence of events is predicted. The first melts come from the lower part of the thermal boundary layer where low-degree partial melts from the advancing plume freeze and precondition the base of the lithosphere. This event is followed by the generation of melts with a plume signature. Such features are consistent with the temporal and isotopic constraints provided by the mafic lavas from the Hocheifel. The minimum thickness of the lithosphere prior to the Cenozoic volcanism is estimated to be 100 km (Babuska & Plomerová, 1988). Tomographic studies suggest uplift of the lithosphere–asthenosphere boundary to a minimum depth of 50 km beneath the Eifel area (Panza et al., 1980), suggesting a ß factor of 2·0. At these ratios of unstretched to stretched lithosphere, partial melting in the asthenosphere starts at an initial mantle temperature of 1300°C (Hawkesworth & Gallagher, 1993), which is compatible with the estimates from this study.

	ACKNOWLEDGEMENTS

 
S. Hoernes (Universität Bonn) kindly provided XRF analysis, and A. W. Hofmann (Max-Planck-Insitut für Chemie, Mainz) is thanked for hospitality and for giving access to the mass spectrometry facilities. Considerable thanks go to Iris Bambach (Mainz) for her patience during managing of the line drawings. We appreciate the help of D. Neuhäuser, U. Poller, W. Todt and P. Maissenbacher during the second author's stay in Mainz. Constructive and unbiased reviews were provided by J.-M. Cebriá, M. Wilson and G. Wörner and are highly appreciated. Finally, we would like to thank M. Wilson for improving the style of the manuscript and for providing us with a preprint of the Wilson & Downes (2006) paper about Quaternary to Tertiary magmatism in Europe. This study was largely funded through grants from the Philipps-Universität (Marburg) to C.J.

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^*Corresponding author. Fax: ++49-6421-2828919. E-mail: jungs{at}staff.uni-marburg.de

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