In the pennidic metamorphic formation M. Forno Amphibolite pillow structures have been discovered near the Passo del Muretto on the Italian-Swiss border. The cores of the pillows, completely homogeneous because of the metamorphism, are made up by an actinolitic hornblende and by plagioclase (32 % An). In the variolitic borders, consisting of actinolitic hornblende, plagioclase (28 % An) and epidote, textural and mineralogical gradations, both of primary and metamorphic origin, are evident. The interpillow matrix consists chiefly of an actinolitic hornblende, and of some plagioclase (55 % An). Relics of the primary magmatic assemblage are: corroded augites within the interpillow matrix and in a small transition shell in between core and variolitic border; some clouded plagioclase, and, probably, sphene. The main phase of the alpine metamorphism developed statically under amphibolite facies conditions. A very slight retrogressive metamorphism under zeolite/greenschist facies conditions could also be detected. The occurrence of pillow structures in this area confirms the original mafic volcanic nature of the M. Forno Amphibolite and relates this formation to other ophiolitic associations of the Pennidic belt of the Alps.
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Con il contributo del Ministero per i Beni e le Attività Culturali