Constraining deformation phases in the Aar Massif and the Gotthard Nappe (Switzerland) using Th Pb crystallization ages of fissure monazite-(Ce)

Fissure monazite-(Ce) (hereafter called monazite) commonly crystallizes during deformation at low metamorphic grade and offers the possibility to date protracted deformation over several millions of years and to identify distinct deformation phases. We performed Th-Pb geochronology at the microscale on 10 samples of fissure monazite from two Alpine crystalline massifs (Aar Massif and Gotthard Nappe). Ion microprobe ages show that the earliest stage of crystallization recorded by fissure monazite domain ages occurred around 15.9 Ma in the Gotthard Nappe and about 1 My later in the Aar Massif, with the latest crystallization event recorded at 6 Ma. Protracted monazite crystallization in fissures indicates that deformation datable with fissure monazite lasted about 10 Ma. Comparison of Th-Pb crystallization ages of fissure monazite to existing thermochronological data shows that early monazite crystallization coincide with zircon fission track ages, whereas the youngest monazite crystallization overlaps with apatite fission track ages. Monazite also grows contemporaneous with muscovite/illite crystallization (K-Ar ages) in fault gouges. As monazite can grow during dissolution/precipitation cycles induced by tectonic activity, their chronology allows to further constrain in the Aar Massif the final Handegg phase at 12–11.5 Ma, and the coeval activity of the Pfaffenchopf (11.5–9 Ma), and the Oberaar and Rhone-Simplon phases (11.5 and 6 Ma). In the Gotthard Nappe, monazite crystallization constrains a major portion of the Chièra backfolding phase at 14–13 Ma, and confirms that the south-western termination of the nappe was affected by the Rhone-Simplon phase.