WHERE DOES THE HEAT COME FROM? CONTINUOUS PALEOZOIC MAGMATIC ACTIVITY ALONG THE FORMER NORTHERN GONDWANA MARGIN RECORDED IN GNEISS DOMES OF THE CENTRAL PYRENEES (WESTERN EUROPEAN VARISCAN BELT)

Magmatic activity along the northern Gondwana margin spanning almost the complete Paleozoic Era is recorded in gneiss domes from the Axial Zone of the Central Pyrenees, part of the western European Variscan orogenic belt. Beginning with the latest Ediacaran and lasting until the early Permian, felsic magmatic intrusions persisted during the Cadomian orogeny in North Gondwana, Ordovician rifting along the northern Gondwana margin with the opening of the Rheic Ocean, the subsequent subduction of the Rheic Ocean and the collision of Laurussia with microplates originating from the northern Gondwana margin in the Carboniferous producing the Variscan orogenic belt. The Ordovician rifting and the Variscan magmatic arc resulted in the majority of igneous intrusions, the former in the granitic protoliths of the large orthogneisses (471-465 Ma) that comprise the core of the Aston and Hospitalet domes, the later in the Upper Carboniferous granite plutons (e.g., Bassiès). In addition, the geologic setting of the latest Ediacaran Laparan orthogneiss (ca. 545 Ma) and the Visean Soulcem granite (ca. 339 Ma) remain unclear or debated. Furthermore, LA-ICP-MS U-Pb zircon geochronology of late Carboniferous-early Permian peraluminous, alkaline granite dykes and small intrusions within the Aston and Hospitalet orthogneisses record concordant Ordovician (ca. 478-450 Ma), Devonian (ca. 415-402 Ma, 385-383 Ma), and early-middle Carboniferous ages (ca. 357-350 Ma, 336-325 Ma). In summary, igneous rocks within the Aston and Hospitalet gneiss domes record nearly continuous magmatic zircon ages (ca. 545-287 Ma) throughout most of the Paleozoic. Current pre- and early Variscan plate tectonic reconstructions of the northern Gondwana domain do not provide magma or heat sources that could explain the Devonian and early Carboniferous magmatic activities. Intracontinental subduction or a mantle plume (TUZO) are invoked.

A comparative Nd isotope study of Visean and Moscovian/Kasimovian granite plutons reveals no distinction, yielding _εNd_(t) values of -6 to -9, similar to other felsic Variscan intrusions throughout the Pyrenean Axial Zone, suggesting a significant crustal component as magma source. Planned Hf isotope studies are expected to broaden our understanding of the source and plate tectonic setting of the magma.