Mg-metasomatism of metagranitoids from the Alps: genesis and possible tectonic scenarios.

The genetic process and the possible tectonic scenarios responsible for the origin of continental Mg-metasomatic rocks in the Alps are proposed by integration of field, petrological, geochemical (bulk-rock chemistry and stable isotope compositions) and fluid inclusion data previously reported for Mg-metasomatic rocks derived from post-Variscan granitoids (e.g., whiteschists, leucophyllites). Despite the heterogeneous data, the large range of peak metamorphic conditions (from the lowermost greenschists-facies to the ultrahigh-pressure medium-temperature eclogite facies) and the distinct time of metasomatism (from rifting to exhumation), some common features have been recognized in all of the continental Mg-metasomatic rocks belonging to the palaeogeographic realms of both Europe and Adria: i) they occur along shear zones within the metagranitoids; ii) they display a simple MgO–Al2O3–SiO3–H2O±K2O-silicate mineralogy; iii) based on major- and trace-element compositions, they can be grouped in four homogeneous Classes, representative for a progressive increase in Mg-metasomatism from the host rock to the centre of the shear zone; iv) from Class 0 (protolith) to Class 3 (Mg-richest rocks) the lithologies show an increase in Mg, Ni, H2O, Fe, Cr, and a decrease in Na, Ca, Sr, P, K, Rb, Si; v) fluid inclusion data, coupled with bulk-rock chemical compositions, indicate that the metasomatic fluid was a Ni-Mg-rich brine, containing also Fe and Cr, and possibly undersaturated in Si; vi) δ18O and δD data are indicative for seawater, locally mixed with meteoric water, as source of the metasomatic fluid. All the data indicate that the genetic process responsible for the Mg-metasomatic rocks is most likely the same all along the entire range of the Alps. The proposed process assumes highly channelized fluids (derived from ultramafics that has previously interacted with seawater) that infiltrated the continental crust along high strain zones and produced chromatographic fractionation of major and trace elements. Distinct petrological data and timing of metasomatism from each locality suggest at least three possible tectonic scenarios for this genetic process: rift-related ocean-continent transition, oceanic-continental subduction, and continent-continent collision.