The orthogneiss-derived migmatites of the Variscan Ulten Zone, Northeastern Italy: limited magma movement through the lower crust

Anatectic melt (or magma) created in migmatites, is potentially mobile (Sawyer 2008). The Variscan Ulten Zone, on the contrary, provides an example of limited melt transfer within a km-sized lower crust. In the southern side of the Maddalene, the mountain range that straddles the Ulten and Non Valleys of Northeastern Italy, the high-grade basement of the Ulten Zone is dominated by metapelite-derived stromatic to nebulitic migmatites with intercalated minor granitic orthogneisses. In this area we have successfully recognized different stages of partial melting within the orthogneisses. The source rock of the orthogneiss-derived migmatites is a leucocratic gneiss made of alkali-feldspar + quartz + plagioclase and white mica (up to 12 vol%), and accessory biotite, garnet and ilmenite. The migmatites are characterised by the occurrence of patch or layers of felsic and isotropic leucosome. In the migmatites, the residuum contains skeletal grains of white mica, suggesting that the breakdown of white mica controlled the partial melting process. Thermodynamic modelling indicates that the complete consumption of white mica produces a melt fraction of 5%. The major-element chemical composition of the orthogneiss-derived migmatites is similar to that of the orthogneiss source rock, thus supporting the hypothesis that the partial melting of the Ulten orthogneiss occurred without, or limited, removal of material. Correspondingly, the stromatic to nebulitic migmatites produced by 20-30% partial melting of the Ulten metapelites (Braga, Massonne in press) have a composition consistent with their source rocks. The observations from the Variscan Ulten Zone suggest that the melt fraction in a deep crust is not always capable of migrating away from the source region. The occurrence of a melt-rich lower crust during orogenic evolution decreases the strength of the thickened crust thus favouring the gravitational collapse of mountain belts (Vanderhaeghe, Teyssier 2001). References: BRAGA R., MASSONNE H.-J., in press: H2O content of deep-seated orogenic continental crust: the Ulten Zone, Italian Alps. Int. Geol. Rev. SAWYER E.W., 2008: Working with migmatites: nomenclature for the constituent parts, Mineralogical Association of Canada Short Course, 38: 1-28. VANDERHAEGHE O., TEYSSIER C., 2001: Partial melting and flow of orogens. Tectonophysics, 342: 451-472.