The Quaternary Volcanic Fields of the East and West Eifel (Germany)

Abstract

The two Quaternary volcanic fields in the Eifel region of Germany (West Eifel Volcanic Field - WEVF; East Eifel Volcanic Field - EEVF) resemble each other in their temporal, spatial, structural and compositional evolution but also differ significantly in several parameters. Most volcanoes in both fields erupted foiditic potassic (K₂O/Na₂O >1) lavas with phenocrystic phlogopite and microlitic leucite being mineralogically most diagnostic as are the corresponding major and trace element characteristics. Volcanoes are dominantly scoria cones, of which about half erupted lava flows, and maars, their formation being partly governed by magma-water interaction. Phreatomagmatic eruptive activity reflecting variable degrees of magma/water mixing occurred during the growth of many scoria cones especially during the initial growth stage.

Volcanic activity in the WEVF started slowly less than 700 ka ago after the Rhenish shield had begun an accelerated phase of uplift with highly silica-undersaturated foiditic magmas near Ormont at the border with Belgium in the NW and peaked in the central part of the field between ca. 600 and 450 ka. Following a subsequent lull in activity, volcanism migrated to the SE, the frequency of volcano formation increasing during the past <100 ka, the youngest eruption having occurred at 11 ka. Most lavas are mafic with rare intermediate and local small highly evolved centers in the eastern central part of the field. Magma fractionation at high pressure, such as near the crust/mantle boundary, is reflected in common green-core clinopyroxene phenocrysts in many types of lavas - in both fields - and high temperature overprinting, partial melting and metasomatism of lower/middle crustal granulites. Very mafic and much less silica-undersaturated sodic olivine nephelinites and relatively LILE-poor sodic basanites with groundmass plagioclase, both being distinctly less isotopically enriched than the foidites, erupted in the southeastern WEVF during the past <50 ka side-by-side with foidites.

Distinct suites of ultramafic xenoliths, each with many variants, are recognized: (1) depleted and enriched peridotites (lherzolites, dunites, harzburgites and wehrlites) comprising several groups (highly deformed porphyroclastic xenoliths in the periphery and high-temperature recrystallized anhydrous types and metasomatized types near the center) and (2) cumulate- textured hornblendites, glimmerites and pyroxenites. The fact that clastic maar deposits are especially rich in peridotite and other ultramafic xenoliths is explained by xenolith-rich mafic volatile-rich magmas rising from greater depth coupled with high expulsion speeds during phreatomagmatic explosions. The near-absence of peridotite xenoliths and the abundance of clinopyroxene-, phlogopite- and amphibole-rich ultramafic cumulates containing remnants of peridotites in maar deposits in the southeastern part of the WEVF is probably due to more efficient filtering and dissolution of mantle peridotite fragments in subcrustal to lower crustal magma reservoirs in which the cumulates formed.

Volcanic activity in the EEVF started about 460 ka ago or slightly earlier. As in the WEVF, early activity in the western part of the EEVF is dominated by mafic foiditic lava compositions. A prominent phonolitic complex in its center (Rieden, ca. 430-360 ka) is represented by intrusions, domes, ignimbrites, and widespread fallout tephra. A younger eastern subfield beginning with the partly trachytic highly evolved phonolitic Wehr crater complex at about 215 ka ago was followed soon by the emplacement of potassic basanitic to tephritic scoria cones chiefly in the Neuwied tectonic basin. Volcanism extended east as far as Rhine River and south to close to the Moselle. Most EEVF basanite volcanoes formed ca. 215-190 ka ago. These lavas differ from the young sodic basanites in the WEVF by higher concentrations of Al, K, Ba, Rb and lower concentrations of Fe, Na, P, Sr, LREE, Zr/Nb ratios exceeding 3 for a given Mg#. The EEVF basanites are also less mafic and commonly evolved to early-erupted tephrite, volcanic edifices being generally larger than those in the WEVF. Volcanic activity was minor until 12,900 years BP when the phonolitic Laacher See Volcano (LSV) erupted >6 km³ magma, mostly during a few days, resulting in a Plinian fallout tephra layer recognized from southern Sweden to northern Italy. This is the most important very late Pleistocene stratigraphic marker bed in Central Europe. The partially evacuated strongly zoned reservoir was located ca. 5–8 km beneath the surface. The eruption, like many scoria cones in both fields, started phreatomagmatically. Rhine River was dammed during the eruption by massive tephra accumulation, forming a 20 m deep lake. The uncontrolled rupture of the tephra dam generated flood waves recognizable in deposits at least as far north as Bonn. The sulfur-rich LSV magma coupled with eruption columns at least 25 km high probably impacted climate significantly in the northern hemisphere.

The degree of melting based on CaO/Al₂O₃ ratios is lowest in the melilite nephelinites that abound in the WEVF but are rare in the EEVF (resembling EM 1) and highest for the basanites in both fields, possibly also reflected in their higher eruptive volume and more common differentiation to intermediate lavas. At least three compositionally distinct mantle domains can be distinguished from each other in the Eifel fields based on available radiogenic Sr-, Nd-, and Pb- ratios of mafic lavas and many incompatible element concentrations and ratios. The dominant foidites in both fields and especially the potassic EEVF basanites are the most radiogenic magmas compared to other Cenozoic volcanic fields in central Europe. These magmas may have been derived from the base of the metasomatized lithosphere. The spatial overlap of the highly alkalic Quaternary magmas, erupted during the early/main stages in both fields, with the southern part of the Eocene Hocheifel field suggests that the geologically young metasomatism that may have affected the base of the lithosphere could have largely resulted from Tertiary magmatic activity. The lack of indicators for metasomatism in the much more widespread Tertiary Eifel lavas is difficult to explain otherwise. The much less radiogenic young basanites and even less radiogenic olivine nephelinites of the WEVF fall close to the broad field of Tertiary lavas in central Europe and may have been derived from a similar possibly asthenospheric mantle source.

In the WEVF, a foiditic magma source was reactivated during the past ca. 100 ka or less. Simultaneously, magmas from a new compositional mantle domain supplied sodic basanites and olivine nephelinites to the surface during the past about 50 ka erupting in the southeastern part of the field side-by-side with foiditic lavas. The two compositionally distinct but spatially adjacent melting domains were probably stacked vertically. In the EEVF, the compositional mantle domain supplying foiditic magmas to the surface terminated between about 350 and 215 ka ago, after which time compositionally different less foiditic but more potassic and enriched basanites and minor tephrites erupted in the eastern subfield, locally evolving to voluminous phonolite. The youngest volcano in the Eifel, Ulmener Maar of extremely LILE-enriched intermediate composition, formed about 11,000 a BP, 2000 years after LSV erupted.

Mantle source regions beneath the fields are chemically distinct on different scales, larger domains differing in isotopic and smaller-scale domains in trace element ratios. Compositionally contrasting, but closely spaced, compositional domains in the mantle a few km across - representing heterogeneous compositions within, and/or differential rise of portions of, a mantle diapir - were activated successively with time or even released magma nearly simultaneously. A prominent example is the practically synchronous eruption of the ol-nephelinitic Mosenberg center followed immediately by the nearby melilite nephelinitic Meerfelder Maar, the largest in the Eifel. An example on a larger scale is the juxtaposition of the leucitite and plagioclase-free phonolitic Rieden and the adjacent Wehr- Laacher See basanite/plagioclase-bearing phonolite systems.

Volcano field analysis shows that magma mass eruption rates increased toward the center of both fields, coupled with an increasing degree of differentiation. The central parts of the fields show the highest erupted volumes and the highest flux of magmatic gases. These and other parameters are interpreted to mirror the central part of one or more magma collection zones in the upper mantle/Moho at least 30 (EEVF) to 50 km (WEVF) in length resulting in magma focusing in the center of both fields. Fields are dominantly oriented NW-SE, reflecting lithosphere cracking in response to the present lithospheric SW-NE-oriented tensional stress field north of the Alps which however was probably strongly enhanced by the similarly-oriented Paleozoic stress field. Cracks acting as magma pathways thus formed most easily perpendicular to the minimum compressional principal stress (σ ₃) in a NW-SE direction with σ ₁ (the maximum compressional principal stress) being vertical. Magma collection zones underlying both fields probably extended significantly laterally beyond the surface area of the volcanic fields because the most mafic magmas were erupted in the periphery of the fields. Magmas generated beyond the surface fields may have only risen as far as the crust/mantle transition zone in view of the abundant evidence for high-pressure fractionation at and below the crust/mantle boundary as well as surface degassing extending beyond the fields. Lithosphere cracking extended to the southeast during a lull in activity in the WEVF (between ca. 450-100 ka) as reflected in a migration of melt supply and surface volcanic activity. Migration of surface volcanism in the EEVF from W to ESE also occurred during a pause in surface volcanism between 350 and 215 ka and was associated with activation of a compositionally distinct melting domain. Both fields developed on either side, and in the hinge zones, of the area of maximum Quaternary uplift, magmas in the WEVF and western EEVF rising in uplifted parts of the Rhenish Massif while the eastern EEVF lavas erupted in the downfaulted Neuwied basin, part of the Rhine Rift structure.

Major Paleozoic structural discontinuities in both fields such as the Eifel N-S graben zone in the western and the Siegen thrust in the eastern field, and Tertiary faults in the Neuwied Basin, appear to have caused deviations in dike orientations and regionally significant boundaries in magma composition and xenolith suites. This suggests that some upper crustal fractures (zones of weakness) extend significantly downward into the lithosphere.

The total mass of magma supplied to the base of the crust and crustal reservoirs (estimated to have been between 300 and 500 km³) — and possibly rates of magma risen from the melting anomalies — was probably higher in the EEVF than in the WEVF. This is indicated by the volume of parent magmas that have to be postulated to generate the relatively voluminous highly evolved phonolite centers and possibly also by the much higher CO₂-flux in the EEVF provided present flux rates are representative. Magma supply to the crust — and possibly magma production — was strongly focused beneath several centers in the EEVF contrasting with more diffuse magma-leaking in the WEVF, a more typical intraplate volcanic field. It is uncertain, however, whether magma focusing in the EEVF was entirely due to higher magma supply from the mantle — possibly resulting from higher degrees of partial melting — or to lower rates of lithosphere extension allowing for higher crack and dike coalescence (Takada 1994) and thus magma focusing. Volcanic activity in the Eifel is presently dormant but not extinct judging from the past temporal pattern of eruptions. Future volcanoes are likely to grow in the southeastern part of both Eifel fields.

The absence of a shear-wave anomaly between 170 and 240 km in the seismic low velocity anomaly in the mantle (Eifel Plume) may be due to separation of an upper diapir (“blob”) providing thermal energy and melt to the basal lithosphere. The upper part (30–140 km) of the seismic low velocity anomaly in the mantle has a diameter of more than 100 km and thus extends significantly beyond both volcanic fields. This upper part may correspond to the magma migration or collection zone culminating between about 37 and 30 km below the surface where the crust-mantle boundary is not sharply defined and may be the site of voluminous magma underplating. The shapes, sizes, directions and volcano concentrations of both fields do not mirror the subcircular shape of the anomaly. Provided the present mantle anomaly (plume) represents the deep mantle roots to the Quaternary volcanism, two smaller dimensions of spatially and compositionally distinct ascending “magma supply fingers” are evident. The smaller ones are a few km across and have life times on the order of several 100 ka. Two or more of these make up a volcanic field, a deep plume source (mantle diapir) spawning one or more surface field.