LA-ICPMS U-Pb dating of polymetamorphic garnet and its inclusions from the Adula nappe, Central Alps

The Adula nappe, Lepontine Alps, Switzerland and Italy, was shaped in a south-dipping subduction zone during the Alpine orogenic cycle. It comprises crustal gneisses, metasediments, and mica schists, as well as eclogites, amphibolites and peridotites. Rocks from the Adula nappe record a gradual increase in pressure and temperature from north to south with high-pressure (HP) metamorphism followed by low-pressure (LP)/high‒temperature (HT) conditions. HP metamorphism and the subsequent LP-HT overprint are considered to reflect a single P‒T evolution. However, geochronological data from the Adula nappe indicate Variscan and Alpine metamorphism. The P‒T conditions recorded by the Adula nappe are thus attributed to Variscan-Alpine polymetamorphism and a polyphase Alpine deformation history, which obscured the mineral equilibria developed during each high-grade event.

In this contribution, we investigate the U-Pb systematics of polymetamorphic garnet from the Adula Nappe by LA-ICPMS. We furthermore explore the U-Pb dates of U-rich inclusions in garnet to constrain their entrapment and formation history and thereby complementing the age information obtained from garnet. The location of the laser spots analyses is guided by BSE imaging, which resolves different garnet growth zones.

Our preliminary results indicate a plethora of internal garnet textures, that document the fragmentation of Variscan garnet and subsequent Alpine overgrowth, amongst others. Moreover, U-Pb dating of the accessory phases in garnet reveals that Variscan garnet also contains Alpine-aged inclusions.