Evolution of the petrological and seismic Moho‐implications for the continental crust‐mantle boundary

Abstract

ABSTRACTThe chemical and petrological composition of mafic rocks from the lower continental crust are discussed by comparing mafic granulites and meta‐gabbroic rocks from the Ivrea Zone and the Northern Hessian Depression (NHD) xenolith suite. Both regions contain contrasting types of meta‐mafic lithologies (i) former basaltic rocks with trace element patterns ranging from MORB‐Iike to subduction‐related or intra‐plate tholeütes and (ü) Ca‐and Al‐enriched, plagiodase‐dominated gabbroic rocks showing positive Eu‐anomalies generated by complex deep crustal magmatic processes such as fractionation, accumulation of plagiodase and pyroxene, and crustal contamination. The absence of typical garnet‐omphadte parageneses in these rocks indicates that the eclogite stability field was not reached during Palaeozoic orogenic processes.A compilation of experimentally determined P‐wave velocities and densities for mafic granulites, gabbroic rocks, eclogites and peridotites is used to evaluate key physical properties of lower crustal mafic rocks during crystal thickening caused by continent‐continent collision. In a step‐by‐step scenario it is demonstrated that the position of the seismic Moho (defined as a first‐order velocity discontinuity) and the petrological Moho (defined as the boundary between non‐peridotitic crustal rocks and olivine‐dominated rocks) is not identical for the case that mafic rocks are transformed into edogites at the base of orogenically thickened crust. P‐wave velocities of eclogites largely overlap with those of peridotites, although their densities are significantly higher than common upper mantle rocks.As a consequence, refraction seismic field studies may not detect edogites as crustal rocks. This means that the seismic Moho detected by refraction seismic field studies appears at the upper boundary between edogites and overlying crustal units. Since edogites generally have higher densities than peridotites, they might be recycled into the deeper lithosphere thereby transferring excess Eu into the upper mantle. This process could be a due for understanding the negative Euanomaly in the upper continental crust which is apparently not balanced quantitatively by the abundance of common mafic crustal rocks.