Intracontinental extensional magmatism with a subduction fingerprint: the late Carboniferous Halle Volcanic Complex (Germany)

Abstract

The Halle Volcanic Complex (HVC) is part of the transtensional intracontinental Saale Basin, which formed on the Mid-German Crystalline Rise located at the southern margin of the late Carboniferous/early Permian volcanic province of central Europe. Magmatic activity ranged from early trachybasalts, trachyandesites, and trachydacites followed by calc-alkaline, mildly peraluminous low-Si rhyolites, the latter of which had intruded at a very shallow crustal level. Two groups of geochemically heterogeneous and isotopically distinct mafic–intermediate rocks have to be distinguished, which originated from enriched mantle (lower crustal) sources and experienced crustal contamination to various extents. These rocks preceded the emplacement of rhyolites that are remarkably uniform in major and trace element chemistry as well as Nd isotope composition. Distinctly negative ε_{Nd(T=300 Ma)} (–6.7 to –7.0) of the rhyolites implies significant involvement of crustal material. The Pb isotopic composition of K-feldspar and trace element content of the rhyolites are compatible with remelting of Saxothuringian rather than Rhenohercynian crustal domains of the Variscan orogen. Slightly differing REE abundances in the rhyolites are attributed to an inhomogeneous distribution of accessory minerals. In conflict with their generation in an extensional environment, the trace element signature of the HVC rocks indicates a magmatic arc or collisional setting rather than an intracontinental within-plate setting. The composition of rhyolites from extensional settings at Halle and the adjacent Northeast German Basin demonstrates that trace element composition and geodynamic environment may not be correlated. Furthermore, the geochemistry of these rocks implies that the same type of magmatism may take entirely different chemical expressions in dependence of the structural and chemical composition of the underlying lithospheric block, which might be used to map hidden destroyed terrain boundaries in ancient orogens.