Geochemistry and isotope systematics of calc-alkaline volcanic rocks from the Saar-Nahe basin (SW Germany) - implications for Late-Variscan orogenic development

Late Carboniferous (300-290 Ma) calc-alkaline basalts, andesites, and rhyol ites typical of volcanic are settings occur in the intermontane Saar-Nahe b asin (SW Germany) within the Variscan orogenic belt. The volcanic rock suit e was emplaced under a regime of tensional tectonics during orogenic collap se and its origin has been explained by melting of mantle and crust in the course of limited lithospheric rifting. We report major, trace and rare-ear th-element data (REE), and Nd-Pb-Sr-O isotope ratios for a representative s ample suite, which are fully consistent with an origin closely related to p late subduction. Major and trace element data define continuous melt differ entiation trends from a precursor basaltic magma involving fractional cryst allization of olivine, pyroxene, plagioclase, and magnetite typical of magm a evolution in a volcanic are. This finding precludes an origin of the ande sitic compositions by mixing of mafic and felsic melts as can be expected i n anorogenic settings. The mafic samples have high ME numbers (Mg# = 65-73) , and high Cr (up to 330 ppm) and Ni (up to 200 ppm) contents indicating de rivation from a primitive parental melt that was formed in equilibrium with mantle peridotite. We interpret the geochemical characteristics of the nea r-primary basalts as reflecting their mantle source. The volcanic rocks are characterized by enrichment in the large ion lithophile elements (LILE), n egative Nb and Ti, and positive Pb anomalies relative to the neighboring RE E, suggesting melting of a subduction-modified mantle. Initial epsilon(Nd) values of -0.7 to -4.6, Pb, and Sr-87/Sr-86((t)) isotope ratios for mafic a nd felsic volcanics are similar and indicate partial melting of an isotopic ally heterogeneous and enriched mantle reservoir. The enrichment in incompa tible trace elements and radiogenic isotopes of a precursor depleted mantle may be attributed to addition of an old sedimentary component. The geochem ical characteristics of the Saar-Nahe volcanic rocks are distinct from typi cal post-collisional rock suites and they may be interpreted as geochemical evidence for ongoing plate subduction at the margin of the Variscan orogen ic belt not obvious from the regional geologic context.