Underplating of H2O-rich magmas in post-collisional tectonic settings

The Southern Alps expose the Permian Mafic Complex, an 8 km thick gabbro-diorite intrusion formed during the post-collisional phase of the Variscan orogeny. The lower sector of the Mafic Complex mainly consists of amphibole-rich gabbros, thereby suggesting the emplacement of H2O-rich magmas in deep levels of the extending continental crust. To ascertain the nature of these magmas and the processes driving to their chemical differentiation, we have carried out a petrological-geochemical study of a 2.5 km thick section from the Lower Mafic Complex. The amphibole gabbros display a widespread foliation developed under hyper-solidus conditions and locally include concordant layers consisting of: (i) amphibole-free norites enclosing granulite-facies metasedimentary lenses, and (ii) amphibole-rich ultramafic rocks. The amphibole gabbros display Mg# [molar Mg/(Mg + Fe2+tot) × 100] ranging from 48 to 68, associated with nearly homogeneous incompatible trace element signature and Nd-Sr-O isotopic compositions. The Mg# variations therefore record a magmatic differentiation process with no substantial assimilation of crustal material. In detail, the amphibole gabbros have an enriched Nd-Sr isotopic signature (i.e., initial εNd ca. −3.3, initial87Sr/86Sr ca. 0.7071) that is coupled with a moderate O isotopic enrichment (δ18OWR = +6.3 ‰ to +6.5 ‰, δ18OPx = +6.4 ‰ to +6.9 ‰). The norites typically have an evolved chemical signature and slightly enriched Nd-Sr-O isotopic compositions compared to the amphibole gabbros, indicating that the two rock-types were cogenetically related through a process of crustal contamination. Assimilation of crustal material was driven by anhydrous melts released from basement granulite-facies metasediments and was promoted by extensional deformation. We estimated that 15 % crustal component inhibited the crystallization of Ca-rich mafic silicates (i.e., amphibole and clinopyroxene) and triggered the orthopyroxene saturation in the melt. The ultramafic rocks interlayered with the amphibole gabbros provide evidence for involvement of chemically distinct magma pulses and record reaction processes with melts derived from the adjacent crystallizing gabbros. The primary magmas forming the studied section of the Lower Mafic Complex were most likely generated from a H2O-rich mantle source of subcontinental lithospheric origin undergoing different extents of partial melting. The building of the whole Mafic Complex involved primary magmas derived from mantle sources having markedly heterogeneous compositions.