Journal of Petrology, Volume 40, Issue 12: December 1999.

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

A. DEL MORO¹, S. MARTIN² AND G. PROSSER³

¹ISTITUTO DI GEOCRONOLOGIA E GEOCHIMICA ISOTOPICA, CNR, VIA CARDINALE MAFFI 36, 56100 PISA, ITALY
²DIPARTIMENTO DI GEOLOGIA PALEONTOLOGIA E GEOFISICA-CNR CENTRO DI GEODINAMICA ALPINA, VIA GIOTTO 1, 35100 PADOVA, ITALY
³CENTRO DI GEODINAMICA, UNIVERSITÀ DELLA BASILICATA, VIA ANZIO, 85100 POTENZA, ITALY

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 [epsilon]_{Nd 330 Ma} of the leucocratic rocks is strongly negative (-9 to -13), suggesting a crustal origin for these melts.

Keywords: geochemistry;Italy;leucosome; Sr-Nd isotopes; trondhjemite

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