H_2O content of deep-seated orogenic continental crust: the Ulten Zone, Italian Alps

摘要

To estimate the amount of H_2O stored at lower crustal levels after burial, we considered the pile of migmatitic paragncisses in the Variscan Ultcn Zone as a case study area. We constructed a pseudosection in the system K_2O-Na_2O-CaO-FeO-MnO-MgO-Al_2O_3-SiO_2-TiO_2-H_2O 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 H_2O 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 H_2O 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.% H_2O was stored in minerals. Thus, a considerable amount of H_2O must have either resided in pore spaces along grain boundaries or, much less likely, infiltrated the paragneisses from below. Evidently, significant quantities of H_2O as a free phase may be stored in buried sialic crust, resulting in considerable melting of deep-seated rocks during continent-continent collision.