Vestiges of lost tectonic units in conglomerate pebbles? A test in Permian sequences of the Southalpine Orobic Alps

Abstract

AbstractMicrostructural analysis andP–Testimates of metamorphic pebbles in Permian conglomerates of the Central Southern Alps, representing the erosion product of the collapsing Variscan chain, are the discriminating tools for determining the metamorphic sequences representing potential sources of the conglomerates. In the selected case, basement units are precisely outlined on the basis of qualityP–T–d–tpaths that allow reconstruction of their metamorphic evolutions (tectonometamorphic units); this facilitates individuation of the basement sources with much better confidence. The lower Permian volcaniclastic sequence of the Eastern Orobic Basin, which overlies the Variscan Val Vedello basement, comprises the Aga and Vedello conglomerates, which are the oldest deposits containing a considerable amount of up to metre-sized metamorphic pebbles. Microstructural and mineral chemical data on metamorphic pebbles of the Aga and Vedello conglomerates were used to infer quantitative pre-PermianP–Tevolutions, which are compared with those of the tectonometamorphic units constituting the surrounding Southalpine metamorphic basement. Two types ofP–Tevolution are recorded in the metamorphic pebbles of Aga and Vedello conglomerates: Type 1 is characterized by an amphibolite-facies imprint, followed by greenschist retrogression; Type 2 is characterized by three successive greenschist-facies re-equilibrations. The Type 1P–Tevolution of metamorphic pebbles matches with that of the adjacent tectonometamorphic unit of the Val Vedello basement. Type 2 is similar to those recorded in units B and C of the North East Orobic basement, and it differs from that of the adjacent Val Vedello basement. This suggests that the Aga and Vedello conglomerates were fed by two different basement sources: one consisting of the present day Val Vedello basement, and the other compatible with units B and C of the North East Orobic basement. According to theP/Tratios of theTmax–PTmaximprints, both basement sources recorded the Variscan collision but at a different crustal level. The age (c.278 Ma) of the Aga and Vedello conglomerates constrains the minimum exhumation age for their basement sources.