An example of continuous plutonism during orogenesis – The emplacement of the Bassiès pluton in the central Pyrenees (SW-France)

The Axial Zone of the Pyrenees represents a window into the Variscan evolution of the northern Gondwana margin. One enigma of this evolution is the origin of the many and large granitoid intrusions and gneiss domes as well as the timing of their emplacement and deformation.

New zircon U-Pb data using laser ablation ICP MS indicate that granitoid plutonism is an almost continuous process over the entire Variscan orogenesis between 360 and 285 Ma that can be statistically constrained and shown for a number of these intrusions and domes. The Bassiés pluton is a key example which shows the oldest Carboniferous intrusive units with ages of 350 - 340 Ma, followed by the main intrusion phase between 330 and 320 Ma including the magmatic peak of 321 Ma. Two further age clusters occur around 314 to 300 Ma and around 285 Ma in the early Permian.

The polyphase development and in-situ melt recycling of the granitoids can also be inferred from core-rim relationships of zircon grains. There are zircons with core and rim ages of 346 and 326 Ma or 330 and 305 Ma, respectively. According to these results, it is required to revisit the mechanisms of pluton emplacement over such a long time span as well as the definition of the emplacement age. The youngest age is clearly not the crystallization age of the pluton, but plutonism is a long-lasting process that affects the Variscan crust over tens of millions or years, i.e. during the entire orogeny.