Calcalkaline volcanics occur in a number of intra-montane basins within the Hercynian orogenic belt in Europe. The origin of these rock suites is enigmatic and various scenarios such as crustal melting, reactivation of a fossil subduction-modified mantle, and magma mixing during crustal extension have been proposed (Lorenz and Nicholls, 1984). In the Saar-Nahe-Basin about 295 Ma old mafic to felsic volcanics with typical subduction-related chemical characteristics are intercalated with clastic sediments deposited in a fluviatile milieu. Basalts, andesites and rhyolites occur as lava flows, sills, dykes, diatremes and subvolcanic intrusions. We report Sr-, Nd-, Po isotopic compositions for 35 representative samples, REE and other incompatible element concentrations for l7 samples. Basalts have Mg values of 61 to 69 and approach near-primary compositions. REE patterns in basalts and andesites are subparallel with highest concentrations in the andesites, indicating an origin of the latter by fractional crystallization. LREE are moderately fractionated with (La/Sm)n of 3 to 4. HREE distribution patterns are little fractionated with (Gd/Lu)n ~1.6. Rhyolites are more enriched in LREE than the basalt-andesite suite but show lower HREE concentrations, resulting in crossing patterns. Basalts and andesites show similar Eu/Eu* values (~ 0.85), interpreted as a source characteristic rather than resulting from fractional crystallization in the plagioclase stability field. A single rhyolite sample containing garnet shows a strong fractionated HREE pattern, indicating a different source than for the majority of rhyolites. Extended REE patterns of all samples have typical characteristics of subduction-related volcanics such as high LILE concentrations, negative Nb and positive Pb anomalies. Low Ce/Pb ratios of 3 to 6 and Nb/Th ratios of 0.6 to 1.4 are consistent with a subduction-related origin. Nb/Ta ratios of 16 to 18 and Zr/Hf ratios of 36 to 40 in basalts and andesites are similar to primitive mantle values, whereas rhyolites have lower ratios indistinguishable from crustal ratios.
Initial eNd values for basalts and andesites range from -0.7 to -2, those for rhyolites are slightly lower (-2.7 to -3.5) and all indicate a large proportion of an older crustal component. Nd isotopic data fall within the range of Hercynian basement with initial eNd values from -2 to -8 (Liew and Hofmann, 1988). Pb isotopic compositions plot on an upper crustal trend in agreement with Nd isotopes. In spite of a wide compositional range of the basalt-andesite suite (SiO2: 48 to 59 wt.%), their eNd values are similar, thus precluding significant assimilation of evolved crust during fractional crystallization. This finding suggests that Nd isotopic compositions of the basalt-andesite suite were inherited from a modified mantle source. The Nd isotopic homogeneity in the basalts is contrary to the effects expected of crustal magma contamination, that usually results in variable isotopic compositions between mantle and crustal endmembers.
As to the origin of the Saar-Nahe basalt-andesite suite, we suggest melting of a subduction-modified mantle during late Hercynian plate convergence at about 300 Ma (V. Bachtadse, pers. comm.). Rhyolites have been generated from crust as is indicated for the HREE-depleted sample. Fractional crystallization of the main rhyolite suite from andesites appears not likely because of their lower HREE concentrations and assuming bulk partition coefficients for the HREE < 1 during fractional crystallization. At present we cannot preclude assimilation and fractional crystallization processes as origin for the rhyolites. Important is the Sm-Nd isotopic evidence for garnet formation and high temperature conditions at about 300 Ma. This evidence supports our hypothesis for a crustal origin of all rhyolites. Differences in REE patterns can be explained with melting of heterogeneous lower crust.
Liew, T. C. & Hofmann, A. W., C.ontrib. Mineral. Petrol. 98,129-138 (1988).
Lorenz, V. & Nicholls, J. A., Tectonophysics 107, 25-56 (1984).