Provenance, age, and tectonic evolution of Variscan flysch, southeastern France and northeastern Spain, based on zircon geochronology
Small basins filled with Early Carboniferous (Mississippian) flysch in the Variscan chain in southwestern Europe formed as ocean basins between Laurussia and Gondwana closed and deformation was transferred into Gondwanan crust. To identify flysch source areas and the spatial distribution and timing of deformation as continental collision progressed, sensitive high-resolution ion microprobe (SHRIMP) U-Pb ages were determined for (1) detrital zircon and zircon in orthogneiss cobbles from flysch sediments in basins along a 300 km transect from the Montagne Noire in southeastern France across the Catalonian Massif to Minorca in northeastern Spain; (2) monazite from a potential source area; and (3) undeformed granite that crosscuts flysch sediments. A remarkable feature of flysch conglomerates all along the transect south of the Montagne Noire is that they contain pebbles and cobbles of deformed leucocratic granite, gneiss, and pegmatite, felsic volcanic porphyritic rocks, schist, and slate that resemble rocks exposed in massifs adjacent to the flysch basins. Age distributions of detrital zircon populations from 13 basins show that most grains are Neoproterozoic (younger than 850 Ma), Cambrian–Ordovician, and Early Carboniferous (Mississippian). Fewer Devonian zircons are all post-Emsian, and most are Frasnian–Famennian (Late Devonian); i.e., between ca. 359 and 385 Ma. A younger zircon population is composed mostly of igneous grains that are Early Carboniferous, in the range 360–325 Ma. In several basins, the youngest detrital zircon age group is Tournaisian; in others, it is Visean (345–330 Ma), only a few million years older than the age of flysch deposition based on biostratigraphy. The youngest zircon age group in flysch from the Canoves Basin along the Catalonian Massif has an age of 327 ± 4 Ma; this age overlaps the biostratigraphic age of Canoves flysch and coincides with monazite ages determined from a high-grade pelitic gneiss exposed in the Guilleries massif ~30 km away along strike. An orthogneiss cobble from Canoves has a crystallization age of 489 ± 5 Ma, matching the age of bedrock orthogneiss from Guilleries. These data suggest that Variscan deformation of Gondwanan crust began during the late Visean and that individual flysch basins developed in front of a series of uplifted thrust wedges where midcrustal igneous and metamorphic rocks were rapidly exhumed and eroded. A slaty cleavage is developed in all of the flysch basins, and undeformed granites that intrude four flysch basins have ages that range between 305 and 295 Ma.