Post-collisional Variscan lamprophyres (Black Forest, Germany): Ar-40/Ar-39 phlogopite dating, Nd, Pb, Sr isotope, and trace element characteristics

Subsequent to high-pressure and temperature metamorphism of the axial zone of the Variscan foldbelt in central Europe at ca, 340 to 330 Ma, formation of lamprophyre dikes during transtensional tectonics may be viewed as the b eginning of the post-collisional stage of the orogeny. We report the result s of Ar-40/Ar-39 mica dating, major and trace element data, and isotope com positions for lamprophyre and rhyodacite samples from the southern Black Fo rest, The chemical compositions of these rocks shed light on the upper mant le and crust at the end of the Variscan orogeny. Ar-40/Ar-39 plateau ages f or four phlogopite-biotite separates from lamprophyres indicate emplacement at 332 to 314 Ma. This event coincides with melting of the crust as indica ted by Ar-40/Ar-39 biotite plateau ages of ca. 332 Ma for rhyodacite dikes which are probably related to coeval undeformed granites, Incompatible trac e element patterns of the lamprophyre samples reveal the characteristics of evolved continental crust and are interpreted as evidence for melting of s ediment in a subduction-modified mantle. Nd, Sr, and Pb isotope composition s indicate an enriched mantle source with epsilon(Nd)-values of -1.5 to -6. 8 which is similar to Variscan crust, Significant contamination of the lamp rophyric melts by Variscan crust can be ruled out as mantle-derived phlogop ite phenocrysts have similar epsilon(Nd)-values as in the whole-rock sample s. We propose that the isotope compositions and incompatible trace element characteristics of the lamprophyres were predominantly inherited from melte d sediment. The isotope compositions of Variscan lamprophyres from western Europe suggest that enriched upper mantle was only partly delaminated when ascending hot mantle triggered melting of the lower crust, as has been invo ked for the origin of post-collisional granites. The isotope compositions o f Tertiary basalts and mantle xenoliths indicate a depleted upper mantle un der western Europe, implying that the enriched Variscan material was effici ently removed and mixed into the convecting mantle. (C) 1998 Elsevier Scien ce B.V. All rights reserved.