The Palaeozoic volcanics of the Rhenish Massif were altered by diagenetic processes as well as by hydrothermal metamorphosis (Hentschel 1951, 1970, Herrmann and Wedepohl 1970, Wedepohl et al. 1983, Schmincke and Sunkel 1987, Flick et al. 1990). Additionally, a very low grade metamorphism respectively a low temperature-middle pressure metamorphism in the Phyllite Zone of the Southern Taunus altered their mineralogy and geochemistry (Meisl et al. 1982, Massonne and Schreyer 1983). This influenced the contents of the main constituents Na2O and K2O, in basaltic rocks also SiO2, in that way that the TAS classification (LeMaitre 2002) cannot be applied as proposed by the International Union of Geological Sciences (2000). The mobile trace elements Ba, Pb, Rb, Sr, and Cs are also not suitable for magma genetic and geotectonic discrimination as well. Consequently, only certain immobile elements allow a geochemical interpretation. These include Nb, Y, Zr, Hf, Ta, Th, and the Rare Earth Elements (REE) and constraint Ti, P, and U. Of the compatible trace elements, Ni and Cr are particularly suitable. The feldspar-rich rocks of trachyte and rhyolite can be distinguished by their silica content without sharp division (Table 1–3, see electronic supplement).
A useful classification of the Palaeozoic volcanics of the Rhenish Massif can be applied by comparing these to the primitive mantle in a spider diagram (Fig. 5, 6, 9) as provided by MacDonald and Sun (1995). Thereby, mobile elements are better omitted because of the alteration of the rocks. The great number of analyses enhance the statistic assignment except for those of the ash layers from the Middle and Upper Devonian of which rather few could be sampled. Nevertheless, the basic pattern is depicted sufficiently well in the relevant diagrams.
The evaluation of the spider diagrams allows to differentiate between the subduction-related and the intraplate volcanics whereby the pattern of Nb and Ta is most significant (Fig. 5, 6). The volcanics of the Southern Taunus, Northern Taunus, the Lenne area and the ash-tuff layers from the other regions of the Rhenish Massif exhibit a strong negative Nb-Ta anomaly (Fig. 5) whereas this is slightly positive for those of the Lahn-Dill and Kellerwald areas (Fig. 6). The difference becomes even more apparent when the rather uniform subduction-related rhyolites and rhyodacites (Fig. 5) are compared with the alkali-trachytes and alkali-rhyolites of the intraplate volcanics directly. The incompatible elements are extremely enriched in the latter (Fig. 6c). E.g. contents of Zr amounted to 1802 ppm, of Nb to 335 ppm, Y to 216 ppm, Ga to 50 ppm, La to 471 ppm, and Ta to 20 ppm (Table 2, see electronic supplement). The contents of Fe2O3 are quite high and amounted to 9 wt.-%. Excess iron was precipitated as magnetite on contraction joints locally (Table 2, sample 31663, see electronic supplement). In contrast, the absolute contents in the subduction-related rhyolites and rhyodacites for the indicative elements Nb with 33 ppm and Ta with 2.0 ppm only reach 10% of these values (Table 1, see electronic supplement). The contents of Fe2O3 are generally between 2 and 4 wt.-%. Both magma types display a distinct negative Eu anomaly.
A differentiation series is visible in the spider diagram regarding the intraplate volcanics of the Givetian–Frasnian-Phase of the Middle–Upper Devonian volcanic cycle. The contents of the incompatible elements are increasing from alkali basalts and basanites via trachybasalts and trachyandesites to alkali trachytes and alkali rhyolites (Fig. 6a-c). The ɛ Nd-values of > + 3,5 of two alkali trachyte samples point to an origin of the earth’s mantle (pers. comm. Christian Pin, Département de Géologie, Université Clermont, France).
Assimilation of mantle xenoliths by alkali basalts and basanites of the Famennian volcanic phase is visible in the alteration of the melts by steepening the curves (Fig. 6d). Differentiation was less pronounced as shown by higher contents in Ni and Cr (Table 1, see electronic supplement).
The Lower Carboniferous volcanic cycle displays an evolution of tholeiitic via transitional to alkali basalts which thereby contrasts to the Middle–Upper Devonian volcanic cycle yielding just alkali volcanics. This is especially obvious in the samples of the Kellerwald area (Fig. 6e). Primitive alkali basalts which brought big spinel-lherzolitic xenoliths to the surface mark the end of volcanic activities in the Lahn-Dill area. The contents of the Pt element group and of Au confirm the origin of the xenoliths from the mantle (Nesbor 2004). Assimilation of the xenoliths during the rise to the surface led to a steepening of the curve (Fig. 6f) comparable to the primitive volcanics of the Famennian. Additionally, a parallel shifting of the curve to the bottom is obvious due to the great amount of the mantle xenoliths being assimilated (Fig. 6d, f). The volcanics of the Lower Carboniferous cycle are only slightly differentiated, with correspondingly higher Ni and Cr contents. The picrites resulting from cumulation inside the tholeiitic melts exhibit typical high contents in Ni and Cr of 1335 ppm and of 1469 ppm respectively (Table 2, see electronic supplement).
The geochemical patterns of discrimination as outlined above are supported by applying the discrimination diagram Zr/TiO2 versus Nb/Y by Winchester and Floyd (1977). All samples of areas yielding subduction-related volcanics plot in the fields of rhyodacites (dacites) and rhyolites (Fig. 7a, c-f). The meta-andesites plot in the field of andesite (Fig. 7b) with slight scattering due to the influence of stronger alteration.
The intraplate volcanics of the Givetian–Frasnian-Phase of the Middle–Upper Devonian cycle display a pronounced differentiation series that shows an enrichment of Nb and Zr from basanites and alkali basalts via trachybasalts and trachyandesites to alkali trachytes and alkali rhyolites (pantellerites) finally (Fig. 8a-c). The latter both can be separated by their content of SiO2 which is about 10 wt.-% higher in the alkali rhyolites (Table 2, see electronic supplement). An extreme enrichment of incompatible elements is valid for both. This holds true especially in the alkali rhyolites where excess F had been precipitated as violet fluorite occasionally (Flick and Nesbor 1988). The volcanics of the Famennian Phase plot into the fields of alkali basalts and basanites volcanics (Fig. 8d) respectively as to be expected. The basalts of the Lower Carboniferous cycle of the Lahn-Dill and Kellerwald areas display a distinct development from tholeiites via transitional basalts to alkali basalts (Fig. 8e, f) as in the spider diagram. This is clearly visible in their geochemistry but not in their mineralogical composition.
The geochemistry of the Gießen nappe samples (mid-ocean ridge basalts) does not display similarities to the volcanics of the autochthonous Rhenish Massif. The contents of the immobile elements are very low generally and are not even 10-fold of the contents of the primitive mantle (Fig. 9a, Table 3, see electronic supplement). The curves proceed almost horizontal and show a slightly negative trend for the light rare earth elements (LREE). Ta displays a negative anomaly. The Zr/TiO2 versus Nb/Y diagram confirms with corresponding low element ratios (Fig. 9b). Just one sample was available from the Steinhorn nappe which is assigned to andesite by its mineralogy. This is confirmed by the spider diagram (Fig. 9c). The pronounced anomaly of Nb-Ta confirms the subduction relation of the rock. Negative anomalies of Zr and Y can be explained by secondary changes that resulted from the strong alteration of the volcanics. That is obvious in the Zr/TiO2 versus Nb/Y diagram as well. This causes the sample to plots in the field of sub-alkalic basalts (Fig. 9d).
Discussion and conclusions
In the Southern Taunus, a small relic of a magmatic arc has been preserved within the Phyllite Zone (Fig. 1), which emerged in the Silurian on the southern margin of Avalonia above the northward subducting Rheic Ocean (Franke 1995, 2000, Franke and Dulce 2017, Franke et al. 2017, 2019, Franke 2023, Oncken 1997). The meta-rhyolites and -andesites exposed there exhibit the element characteristics of subduction zones (Fig. 5a, b, 7a, b). The Southern Taunus unit therefore differs fundamentally from the units of the Phyllite Zone in the southwest (Hunsrück) and northeast (South Harz) which are occupied by sedimentary rocks and are significantly younger or older. Consequently, the three units originated separately and experienced a Barrow-type low temperature-middle pressure metamorphism together during the Variscan Orogeny (Massonne and Schreyer 1983, Massonne in Anderle et al. 1990, Massonne 1995). In that this differs from the very low-grade metamorphism within the further rock sequences of the Rhenohercynian. A normal stratigraphic sequence by covering the magmatic arc by Baltic erosion products from the rising Caledonian Orogen during the Lower Devonian as proposed by Franke (2023) and Linnemann et al. (2023) respectively is therefore to be excluded. Both units would then have undergone the same metamorphism during the Variscan Orogeny - which can be ruled out.
The Silurian subduction volcanism which has not been dated more precisely yet (Sommermann et al. 1992, 1994) continued intermittendly throughout the Lower Devonian and beyond (Heyckendorf 1985, Kirnbauer 1991, Geologischer Dienst Nordrhein-Westfalen [Ed.] 2017). These volcanic products are widely exposed in the Rhenish Massif (Fig. 1) and are derived from rhyolitic to rhyodacitic magma (Fig. 7c, d). The volcanic deposits of subduction-related volcanism become more distal from the Middle Devonian onwards and are less easily detectable in the thick siliciclastic sequence of sedimentary rocks (Kubanek and Zimmerle 1986). Element contents of all 67 analyzed samples show the typical subduction-related pattern with the pronounced negative Nb-Ta anomaly and the moderate contents of HREE (Fig. 5). In general, the rocks are rhyolites, rhyodacites and andesites which are typical of magmatic arcs over subduction zones (Fig. 7).
The intraplate volcanism that began in the Lower Givetian is also widespread over a large area of the Rhenish Massif (Fig. 2). Similarities of the Middle–Upper Devonian cycle in its magma development and in the temporal sequence of volcanic activities to many young intraplate volcanic regions are obvious (Ehrenberg et al. 1992, 1994, Schmincke 1994).
The pantelleritic trachytes and pantellerites extremely enriched in incompatible elements (Figs. 6c, 8c) are of special importance as such rocks are typical of areas with pronounced rifting (MacDonald et al. 1987, MacDonald and Baginski 2009, Marshall et al. 2009, White et al. 2009, 2012). Excess iron was precipitated as magnetite and excess fluorine as deep purple fluorite on contractional fractures in some alkali rhyolites (Flick and Nesbor 1988) comparable to the East African Rift Zone (Marshall et al. 1998, MacDonald et al. 2008). These volcanic rocks demonstrate pronounced back-arc rifting of an increasingly thinned shelf of the southern margin of Avalonia during the Middle Devonian (Fig. 10).
The volcanic products of the Lower Carboniferous cycle are comparatively less complex in composition. The exclusively basaltic volcanics show a development from tholeiitic basalts through transitional basalt to alkali basalts (Fig. 6e, f, 8e, f). From that it can be concluded that the initially relatively high degree of melting in the mantle was increasingly lowered. Finally, primitive alkaline basaltic magma rose directly from the mantle, carrying numerous and sometimes very large spinel-lherzolithic mantle xenoliths (Nesbor 2004). This development reflects the increasing narrowing of the Rhenohercynian back-arc basin before its final closure in the Viséan.
The intraplate volcanism in the Rhenish Massif during the Devonian and the Lower Carboniferous was not limited to the Lahn-Dill, the Kellerwald and the Waldeck areas but extended far beyond the recent borders into the Taunus and the Sauerland (Fig. 2). Numerous volcanic dykes that cut through the Lower Devonian sedimentary rocks prove their originally widespread distribution (Fuchs and Leppla 1930, Mittmeyer 1978, Wedepohl et al. 1983, Flick and Nesbor 1988, Anderle 2010, Militzer et al. 2018). These former feeder dykes were very dynamic systems in which large quantities of melt could be transported in a short period of time. As a result, large submarine volcanic structures must have developed above the dyke systems in the Taunus and Sauerland during the Middle to Upper Devonian and Lower Carboniferous, comparable to the volcanic complexes that still cover the Lahn-Dill, the Kellerwald and the Waldeck areas. Today, these have been eroded in the anticlinal structures mentioned by now. A relic of such volcanic structures is evidenced by alkaline basaltic pillow fragment breccias ("Schalstein") of the Givetian–Frasnian Phase in an Eocene maar structure in the Taunus as mentioned above.
The results of the study show that the Rhenish Massif was characterized by widespread subduction-related volcanism which lasted intermittently from the Silurian to the Upper Devonian, and probably even into the Lower Carboniferous. Already Floyd (1982) and Ziegler (1988) interpreted the Rhenohercynian as a series of ensial back-arc basins behind a subduction zone dipping shallowly to the north. Following, the formation of the Variscan Orogen and its prehistory were controversially discussed. Various authors assumed a closure of the Rheic Ocean and a collision of Laurussia with Armorica in the late Silurian or in the early Lower Devonian at latest (e.g. Oncken 1997, Oncken et al. 1999, 2000, Franke 2000, Franke et al. 2017, Franke 2023, Linnemann et al. 2023). This should have been followed by the reopening of a narrow ocean in the same position during the Lower Devonian, called the Rhenohercynian or Gießen-Lizard Ocean which was subducted in a southerly direction until the Lower Carboniferous. Thereby, the southern margin of Avalonia is interpreted as a passive continental margin.
However, traces of a continent-continent collision are not documented in Silurian or Lower Devonian sedimentary rocks of the Rhenohercynian or Saxothuringian basins or the Barrandian (Klügel 1997). The suture between the two continents is also not detectable. Furthermore, neither tectonic nappes nor flysch from a Silurian-Lower Devonian collision can be found. Also, there exits no plausible mechanism for the opening of a Rhenohercynian Ocean relatively short after the closure of the Rheic Ocean. Rapid subduction of this narrow ocean, even including the mid-ocean ridge, without leaving any trace also raises questions. Such young, hot and therefore specifically light oceanic crust is difficult to subduct and should therefore have been widespread obducted. The absence of ophiolites in the context of the Variscan collision in the Carboniferous, on the other hand, can be explained without contradiction by the subduction of old, relatively cool and therefore correspondingly heavy oceanic crust, as is the case in a large ocean such as the Rheic Ocean. This also explains the presence of intra-oceanic island arcs, such as the Frankenstein pluton (Altherr et al. 1999), whose formation is favored by cooler ocean crust with lower buoyancy. A recent example to the Variscan Orogen could be the present-day western part of the Pacific Ocean, with its magmatic arcs, back-arc basins with continental or oceanic crust, and numerous island arcs.
Due to these contradictions, an alternative model suggests the closure of the Rheic Ocean until in the Lower Carboniferous (Floyd 1982, Oczlon 1994, Smith 1996, von Raumer and Stampfli 2008, Zeh and Gerdes 2010, Nance et al. 2012, Torsvik and Cocks 2013, Stampfli et al. 2013, Eckelmann et al. 2014, Mende et al. 2019, Nesbor 2021). The subduction of the ocean from the Silurian onwards in a northerly direction under the Avalonian crustal segment and from the Lower Devonian onwards additionally southwards under the northern margin of Armorica is suggested (Fig. 10). The Rheic Ocean was closed by the Lower Carboniferous at the latest. During the entire period from the Silurian to the Lower Carboniferous, subduction-related volcanism was active in the southern part of Avalonia. The largely identical composition of the melts produced over this long period of up to 100 Ma requires a long-lasting, constant process of magma formation which is consistent with the subduction of a large ocean. Recycling material from an older magmatic arc over such a long period of time is not possible from a magmatic point of view since such a system does not remain static. In addition, it seems questionable why with the same composition of the magma, on the one hand, subduction-related volcanism took place in the Silurian, but on the other hand, later only recycled melts from a former magmatic arc should have been continually produced.
Provenance analyzes of detrital zircons support the results from the petrological and geochemical studies which allow a clear separation into different source areas (Eckelmann et al. 2014, Mende et al. 2019). Famennian sandstone samples from the autochthonous areas of the Lahn-Dill and the Kellerwald show a typical Baltic distribution pattern of zircon ages, while in the immediate vicinity equally old greywackes from nappe units that have been pushed over long distances onto the autochthonous areas have an Armorican origin. Since there was no cross-contamination in any of the samples it can be concluded that there have been a separating element, an ocean area, between the two source areas. Faunal differences between the autochthonous Rhenish Massif and various nappe units that still exist in the Upper Devonian point to the same conclusion. Consequently, no faunal balancing took place as a result of the separation of the areas (U. Flick 1999, 2018, 2021).
The sediments which constitute the nappes were originally deposited on the southern margin of the Rheic Ocean in front of the magmatic arc (Mende et al. 2019). Andesite clasts in greywackes of the Emsian within the Steinhorn nappe in the Lahn-Dill area bear witness to such subduction-related volcanism (Fig. 9c, d). The basalts of mid-ocean ridges (MORB, Fig. 9a, b) exposed at the base of the Gießen nappe are assigned to the ocean floor of the Rheic Ocean in its southern part (Fig. 10).
The ongoing subduction of the Rheic Ocean to the north explains problem-free the opening of the continental Rhenohercynian back-arc basin on the Avalonian crustal segment covered with Baltic debris (Fig. 10a), as already considered by Floyd (1982). Avalonia was thus divided into a northern and a southern part (Zeh and Gerdes 2010). This rifting triggered intense intraplate volcanism on northern Avalonia, which, in addition to basaltic melts, also produced highly differentiated pantelleritic trachytes and pantellerites typical of rift zones (e.g. Pantelleria Rift, East African Rift Zone, MacDonald et al. 1987, MacDonald and Baginski 2009, Marshall et al. 2009, White et al. 2009, 2012). To the west, the continental Rhenohercynian back-arc basin probably opened into the Lizard Ocean. Due to the retreat of the subduction zone (roll back), southern Avalonia migrated increasingly southward with simultaneous expansion of the back-arc basin (Fig. 10b, c).
At no time did oceanic crust with correspondingly great water depths develop in the Rhenohercynian region. The lower content of vesicles in the majority of Lower Carboniferous volcanic rocks is due to the lower fluid content of tholeiitic magma. Accordingly, the alkaline basaltic pillow fragment breccias that also occur in the Lower Carboniferous consist of highly vesicular particles comparable to those of the Middle Devonian. A shallow water depth of max. 200–300 m is also evident from the geological context. The Paleozoic volcanics of the Rhenish Massif are concordantly integrated into a thick sedimentary rock sequence or are underlain by clastic sedimentary rocks several thousand meters thick. In this context, the siliceous rocks present in the Frasnian and Lower Carboniferous are not to be equated with great water depths in an ocean and sedimentation below the CCD but rather with the high content of dissolved silica in seawater. This is due to the fluids and solutions released by volcanism, but especially to the Si, Fe and Ca ions released into the seawater as a result of the profound alteration of the volcanic rocks (Flick et al. 1990).
The opening of the Rhenohercynian back-arc basin which began in the Lower Devonian resulted in the division of Avalonia into North and South Avalonia (Zeh and Gerdes 2010, Eckelmann et al. 2014, Nesbor 2019, 2021). The latter included the northern Ruhla Kristallin, the Spessart, the Böllstein Odenwald and the magmatic arc in the north of the Rheic Ocean. Consequently, the sedimentary input into the resulting basin no longer only came from the north but also from the south (Stets and Schäfer 2002, 2011). Today, they form the Taunus quartzite and the Hermeskeil sandstone (Fig. 10a). This model is supported by new provenance analyzes of detrital zircons which prove a Baltic source area for the Lower Devonian quartzitic sandstones (Taunus quartzite) and the highly metamorphic Silurian quartzites in the northern Ruhla Kristallin and in the Spessart (Zeh and Gerdes, 2010, Kirchner and Albert 2020, Linnemann et al. 2023). The continued existence of this magmatic arc is confirmed by radiometric dating of orthogneisses of the Böllstein Odenwald with an Upper Devonian protolith age (Anthes and Reischmann 2001, Reischmann et al. 2001, 2015, Dörr et al. 2017).
In contrast to the Böllstein Odenwald which consists of Avalonian crust, the Bergsträßer Odenwald is generally considered to be Armorican (Altherr et al. 1999, Stein et al. 2022, Dörr and Stein 2019). Consequently, the Rheic Suture is located within the Mid-German Crystalline Zone. Tectonic underplating of large areas of the Mid-German Crystalline Zone by Avalonian crust (e.g. Oncken 1997, Oncken et al. 1999, 2000, Franke 2000, 2023), is not visible in the geochemical and isotopic composition of the granitoids of the Bergsträßer Odenwald (Altherr et al. 1999). Dörr et al. (2022) pointed to the good correlation with those by Linnemann et al. (2004, 2007, 2008, 2014) analyzed samples with West African provenance. What is striking, however, is that a few zircon ages occupy the Mesoproterozoic age gap (approx. 1%). However, this phenomenon does not only occur in the Bergsträßer Odenwald but can also be observed in nearly all sedimentary rocks with Armorican provenance of the Rhenish Massif and partly also of the Harz Mountains (Eckelmann et al. 2014, Mende et al. 2019, Linnemann et al. 2023). This indicates a small sediment input from a previously unknown source into the western part of Armorica.