In a detailed geological mapping at the scale of 1:1000, a metamorphic profile section in the transition zone from amphibolite to granulite facies in Val Strona di Omegna (province Novara, Northern Italy) was petrographically, structurally and geochemically analysed. The Val Strona offers an almost complete traverse through the IVZ, which is interpreted as a tilted section through attenuated lower continental crust.
Starting from the base of the sequence, the lithologies of the area of study area consist predominantly of alternating granulite- to amphibolite facies metapelites and metabasites with minor insertions of ultrabasic rocks, calcsilicates and pegmatoids. Macroscopic and microscopic evidence was used to classify the different lithologies. The main focus of petrographic investigation was set on the amphibolites and granulite facies metabasites.
Mineral paragenesis and accompanying metamorphic reactions were determined by thin section observations. Together with macroscopic evidence, the microscopic results were used in order to expose the most important reactions and processes in metapelites and metabasites of the upper amphibolite facies and transition zone. The analysed series of rocks shows clear evidence for a polymetamorphic development. Relicts from an early high pressure stadium can be found in kelyphitperidotites intercalated in a shear zone located at the border between transitional to granulite facies rocks. The regional amphibolite to granulite facies metamorphism took place under medium pressure conditions, and is usually well preserved in almost any lithological unit. A multi-step retrograde overprint down to greenschist facies conditions and lower may especially be located along the shear zones.
Evidence for at least five different deformation phases can be found by microstructural analysis and macroscopic observation. The oldest deformation phases (D1 and D2) coincide with the regional amphibolite- to granulite facies metamorphism. The main activity along the conjugate network of high temperature shear zones started during the D3-deformation phase, which also indicates the beginning of regional decompression and cooling associated with permian crustal thinning. This tectonic regime continues throughout the D4-deformation phase which involves a continuous development down to low-grade metamorphic conditions. D5 summarizes all phases of brittle faulting, which indicate an alpine origin.
Geochemical analysis of amphibolites and granulite facies metabasites provides clear evidence for mainly N-MORB-characteristics of the protoliths. Reference samples taken from outside the area of study show to some extend alkali basaltic affinities. However, element mobilisation due to high grade metamorphism has to be taken into account, especially for the LILE. Still, the orthogenetic character and their classification as former oceanic basalts is out of doubt.