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

Published online by Cambridge University Press:  27 July 2009

DAVIDE ZANONI ,
M. IOLE SPALLA  and
GUIDO GOSSO
DAVIDE ZANONI*
Affiliation:
Dipartimento di Scienze della Terra ‘A. Desio’, Università degli Studi di Milano, Via Mangiagalli 34, 20133 Milano, Italy Department of Geology, University of New Brunswick, 2 Bailey Drive, Fredericton, NB E3B 5A3, Canada
M. IOLE SPALLA
Affiliation:
Dipartimento di Scienze della Terra ‘A. Desio’, Università degli Studi di Milano, Via Mangiagalli 34, 20133 Milano, Italy C.N.R.–I.D.P.A., Via Mangiagalli 34, 20133 Milano, Italy
GUIDO GOSSO
Affiliation:
Dipartimento di Scienze della Terra ‘A. Desio’, Università degli Studi di Milano, Via Mangiagalli 34, 20133 Milano, Italy C.N.R.–I.D.P.A., Via Mangiagalli 34, 20133 Milano, Italy
*
†Author for correspondence: dzanoni@unb.ca
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Abstract

Microstructural analysis and P–T estimates 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 quality P–T–d–t paths 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-Permian P–T evolutions, which are compared with those of the tectonometamorphic units constituting the surrounding Southalpine metamorphic basement. Two types of P–T evolution 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 1 P–T evolution 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 the P/T ratios of the Tmax–PTmax imprints, 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.

Keywords

pebble P–T pathsSouthern Alpslower Permian conglomeratesVariscan tectonometamorphic units
Type
Original Article
Information
Geological Magazine , Volume 147 , Issue 1 , January 2010 , pp. 98 - 122
Copyright
Copyright © Cambridge University Press 2009

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