Partial melting of metabasic rocks in Val Strona di Omegna, Ivrea Zone, northern Italy

Abstract

Field and petrographic observations combined with major and trace element bulk rock geochemistry show that metabasic rocks within Val Strona di Omegna in the central Ivrea Zone partially melted during granulite facies regional metamorphism. A transition from granoblastic amphibolite facies metabasic rocks at the lowest metamorphic grades to metatexitic and diatexitic migmatites in the granulite facies records the effects of in situ fluid-absent partial melting. Coarse-grained euhedral clinopyroxene porphyroblasts within leucosomes are consistent with anatexis via incongruent fluid-absent melting reactions consuming hornblende, plagioclase and quartz to form clinopyroxene and melt. Field observations are supported by bulk rock geochemistry, in which high-grade samples are generally depleted in mobile elements relative to unmigmatised mid amphibolite facies rocks that may approximate pre-melting protolith compositions. Many of the metabasic rocks at the highest-grade parts of Val Strona di Omegna probably belong to the Kinzigite Formation and are unlikely to be part of the younger Mafic Complex as previously proposed.