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Journal of Petrology | Volume 40 | Number 12 | Pages 1803-1826 | 1999
© Oxford University Press 1999

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

A. Del Moro1, S. Martin2,* and G. Prosser3

1 Istituto di Geocronologia e Geochimica Isotopica, CNR Via Cardinale Maffi 36, 56100 Pisa, Italy
2 Dipartimento di Geologia Paleontologia e Geofisica–CNR Centro di Geodinamica Alpina Via Giotto 1, 35100 Padova, Italy
3 Centro di Geodinamica, Università Della Basilicata Via Anzio, 85100 Potenza, Italy

Received December 14, 1998; Revised typescript accepted May 27, 1999


  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 {varepsilon}Nd 330 Ma of the leucocratic rocks is strongly negative (–9 to –13), suggesting a crustal origin for these melts.

KEY WORDS: geochemistry; Italy; leucosome; Sr–Nd isotopes; trondhjemite

* Corresponding author. Telephone: +39-049-8272054. Fax: +39-049-8272070. e-mail: silvana{at}ux1.unipd.it


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