Old lead components in the young Bergell Massif, south-east Swiss Alps

Abstract

The time of emplacement and origin of the Bergell Massif is considered in the light of recent isotopic investigations, particularly U + Pb in zircons. Zircons from the three main types of granitic rocks (Bergell granodiorite and tonalite, and Novate granite) all yield data which plot to the right of the Concordia curve; this would suggest an old lead component, with the youngest intrusive phase (Novate granite) giving the oldest apparent age (Pb²⁰⁶/U²³⁸ age is 65 m.y.). A loss of Rn gas in the U²³⁸ to Pb²⁰⁶ chain might provide an alternative mechanism to account for the displacement of some of the data to the right of the Concordia curve. The time of crystallization of the Massif is 30.3 m.y. based on 1. a concordant monazite, 2. a chord joining monazite and zircons and 3. sphenes. This age post-dates the time of formation of the nappes and main Alpine regional metamorphisms in the south-east Swiss Alps. In conjunction with other data, the ages indicate that the Bergell was not formed by recrystallization or remelting of pre-existing country rock in situ. The isotopic data of zircons from certain boulders in the molasse of the Po-plain indicate that they were definitely derived from the Bergell Massif.