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U–Pb zircon geochronology of volcanic deposits from the Permian basin of the Orobic Alps (Southern Alps, Lombardy): chronostratigraphic and geological implications

Published online by Cambridge University Press:  15 September 2014

FABRIZIO BERRA*
Affiliation:
Università degli Studi di Milano, Dipartimento di Scienze della Terra A. Desio, Via Mangiagalli 34, 20133 Milan, Italy
MASSIMO TIEPOLO
Affiliation:
C.N.R. – Istituto di Geoscienze e Georisorse, U.O.S. di Pavia, Via Ferrata 1, 27100 Pavia, Italy
VALERIA CAIRONI
Affiliation:
Università degli Studi di Milano, Dipartimento di Scienze della Terra A. Desio, Via Mangiagalli 34, 20133 Milan, Italy
GIAN BARTOLOMEO SILETTO
Affiliation:
Regione Piemonte
*
Author for correspondence: fabrizio.berra@unimi.it

Abstract

U–Pb zircon ages from volcanic rocks of Early Permian age (Southern Alps, Lombardy), associated with fault-controlled transtensional continental basins, were determined with the laser ablation (LA)-ICP-MS technique. Four samples were collected at the base and at the top of the up to 1000 m thick volcaniclastic unit of the Cabianca Volcanite. This unit pre-dates the development of a sedimentary succession that still contains, at different stratigraphic levels, volcanic intercalations. Age results from a tuff in the basal part of the unit constrain the onset of the volcanic activity to 280 ± 2.5 Ma. Ignimbritic samples from the upper part of the unit show a large scatter in the age distribution. This is interpreted as the occurrence of antecrystic and autocrystic zircons. The youngest autocrystic zircons (c. 270 Ma) are thus interpreted as better constraining the eruption age, constraining the duration of the volcanic activity in the Orobic Basin to about 10 Ma. The new geochronological results compared with those of other Early Permian basins of the Southern Alps reveal important differences that may reflect (1) a real time-transgressive beginning and end of the volcanic activity or (2) the complex mixing of antecrystic and autocrystic zircon populations in the analysed samples.

Type
Original Articles
Copyright
Copyright © Cambridge University Press 2014 

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