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

Abstract

The Ulten zone is a pre-Alpine unit in the central–eastern Alps (Italy) formed of high-grade gneisses and migmatites, intercalated metabasites, eclogites and spinel ± garnet peridotites. These rocks suffered a tectono-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 ~330 Ma through a Rb–Sr whole-rock errorchron. The leucocratic pockets have very high Sr contents (1184–1358 ppm) and (87Sr/86Sr)330 Ma ratios around 0.705–0.706, whereas the dykes have lower Sr contents (345–751 ppm) and higher (87Sr/86Sr)330 Ma ratios between 0.706 and 0.709. Melanocratic portions associated with the leucocratic pockets have (87Sr/86Sr)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 (87Sr/86Sr)330 Ma ratios of 0.713–0.718, or the metabasites [(87Sr/86Sr)330 Ma = 0.710–0.714]. The εNd 330 Ma of the leucocratic rocks is strongly negative (−9 to −13), suggesting a crustal origin for these melts.