Migmatites of the Ulten Zone (NE Italy), a Record of Melt Transfer in Deep Crust

The Ulten zone is a pre-Alpine unit in the central-eastern Alps (Italy), formed of high-grade gneisses a,ld migmatites, intercalated metabasites, ecologites and spinel +/- garnet peridotites. These rocks suffered a tec toctono-metamorphic event at pressures >1.5 GPa and high temperature, followed by a decompressional uplift: and finally percolation by trondhjemitic melts which formed pockets and dykes. Melanocratic rocks associated with the leucocratic pockets have been dated at similar to 330 Ma through a Rb-Sr whale-rock errochron. The leucocratic pockets have very high Sr contents (1184-1358 ppm) and ((87)S/(86)Sr)(330 Ma) ratios around 0.705-0.706, whearas the dykes have lower SI contents (345-751 ppm) and higher ((87)Sr/(86)Sr)(330 Ma) ratios between 0.706 and 0.709. Melanocratic portions associated with the leucrocratic pockets have ((87)Sr/(86)Sr)(330 Ma), ratios around 0.707, suggesting that both leucocratic and melanocratic portions originated through melting induced by magmas derived from relatively non-radiogenic sources deeper in the Ulten crust. These melts interacted very weakly with surrounding gneisses, which have ((87)Sr/(86)Sr)(330 Ma) ratios of 0.713-0.718, or the metabasites [((87)Sr/(86)Sr)(330 Ma) = 0.710-0.714]. The epsilon(Nd 330 Ma) of the leucocratic rocks is strongly negative (-9 to -13), suggesting a crustal origin for these melts.