In-situ garnet U-Pb dating of metamorphic rocks: Exploring geochronological differences between garnet and accessory minerals

Garnet U-Pb dating by laser ablation – multicollector – inductively coupled plasma – mass spectrometry (LA-MC-ICPMS) in metamorphic rocks allows constraining the main stages of tectonic thickening during orogenesis. Dates obtained from garnet generally record prograde growth or the baric peak, attained during the tectonic stacking. This is of great importance because accessory minerals, such as zircon, usually analysed in metamorphic rocks date thermal peaks (or retrograde cooling) rather than prograde metamorphism and commonly constrains exhumation rather than tectonic thickening. In addition, the exact significance of the geochronological information obtained from these accessory minerals, however, is difficult to determine. Within this communication, we will explore the chronological differences between garnet and zircon from several metamorphic rocks, such as UHP gneisses, eclogites or granulites. This technique has the potential of providing time constraints of the earliest stages of continental collisions and of the dynamics of arc systems. These findings have a considerable impact on unravelling the timing of orogenic tectonothermal evolutions and supercontinent assemblies.