H2O content of deep-seated orogenic continental crust: the Ulten Zone, Italian Alps

To estimate the amount of H2O stored at lower crustal levels after burial, we considered the pile of migmatitic paragneisses in the Variscan Ulten Zone as a case study area. We constructed a pseudosection in the system K2O-Na2O-CaO-FeO-MnO-MgO-Al2O3-SiO2-TiO2-H2O for an average paragneiss, a relevant prograde PT path (8.5 kbar, 600◦C; 11.5 kbar, 750◦C; 14.0 kbar, 1000◦C) and H2O contents between 0 and 10 wt.%. Based on an assemblage of garnet + biotite + white mica + kyanite + 20–30 vol.% former melt (now represented mainly by leucosomes composed of plagioclase+quartz), a bulk H2O content of 3.2 ± 1.1 wt.% was estimated for a peak temperature ranging between 770 and 800◦C. Before melting, somewhat less than 1.8 wt.% H2O was stored in minerals. Thus, a considerable amount of H2O must have either resided in pore spaces along grain boundaries or, much less likely, infiltrated the paragneisses from below. Evidently, significant quantities of H2O as a free phase may be stored in buried sialic crust, resulting in considerable melting of deep-seated rocks during continent–continent collision.