The Variscan lower continental crust: evidence for crustal delamination from geochemical and petrophysical investigations

Abstract

Abstract Seismic observations along the European Geotraverse (EGT) Central Segment indicate an overall felsic composition of the Variscan crust, including 15 vol. % of mafic lower-crustal (MLC) rocks. As the Variscan continental crust is largely post-Archaean, its bulk composition should be basaltic andesitic rather than felsic (tonalitic), because primitive post-Archaean crust was formed by mantle extracted basaltic andesite magmas. The observed proportions of the Variscan continental crustal layers do not match the requirement of a mafic overall crust. Combining laboratory-derived, in situ seismic data ( V P ) with the refraction seismic data and the evidence from geological and geochemical investigations suggests that large amounts of MLC rocks are missing in the present-day lower crust. A mass balance calculation of the basaltic andesite bulk crust, considering a refined compilation of the Variscan MLC, gives a volume proportion of 37 vol. % for the felsic and 63 vol. % for the mafic components, for primitive crust prior to collision tectonics and orogenic root delamination. If large parts of this MLC have undergone eclogite-facies metamorphism at the orogenic root, large amounts of former basaltic-gabbroic and MLC cumulates could have been subtracted from the crust by its subsequent delamination processes which results in the currently observed proportion of the continental crust of the Variscan Orogen with a predominance of felsic rocks.