Partial melting during the evolution of the amphibolite to granulite facies gneisses of the Ivrea-Zone, Northern Italy.

B. Schnetger: Geochemisches Institut, Universität Göttingen, Goldschmidtstrasse 1, D-3400 Göttingen; present address: Institut für Chemie und Biologie des Meeres (ICBM), Carl v. Ossietzky Universität, Postfach 2503, D-26111 Oldenburg, Federal Republic of Germany.

Abstract

68 large samples of the amphibolite and granulite facies gneisses from the Ivrea-Zone were analysed for major, minor and 32 trace elements. Selected samples were analysed for sulfur and oxygen isotope com-position.
Intrusion of mafic magma into metasediments of the medium to lower crust caused a high tempe-rature granulite facies event in the Ivrea-Zone.
The amphibolite facies gneisses consist of quartz, biotite, plagioclase, garnet, k-feldspar, sillimanite and were identified as metapelites, metagreywack-es and quarzites with minor carbonates (calcsilicates).
Using data from the literature and our own data, it is shown that the granulite facies rocks are depleted in Cs, Li, Bi, U, Rb, Tl, K, P, Pb, Na and enriched in Ti, Al, Fe, Mg, S, Sc, V, Cr, Co, Cu, Zn, Y, Zr, Sr and heavy REE, when compared with the amphibolite facies rocks. Ba, the light REE and Th do not differ significantly between the two metamorphic grades.
This element behavior can be explained by batch partial melting of the amphibolite facies metasediments and extraction of a 20-40 % felsic melt. Most trace element signatures of the calculated melt are conformable to S-Type granites, if amphibolite facies gneisses are parents and the granulite facies rocks are the respective restites. Accessory minerals like monazite and zircon in the parents and restites are shown to have a strong effect on the trace element composition of the melt.
The assumptions and limitations of the melt model are discussed.
 

This paper has been published in: Chemical Geology, 113, 71-101 (1994)

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