A rifted tectonic setting for hercynian high-thermal gradient metamorphism in the pyrenees

Abstract

Hercynian regional metamorphic terrains in the Pyrenees contain evidence of very high-temperature gradients within the crust during metamorphism, with temperatures as high as 700°C attained at 10–12 km below the surface. Stable isotope studies demonstrate that the crust was simultaneously flushed by marine fluids to at least this depth. The absence of any evidence for crustal collision, and the Upper Palaeozoic stratigraphic record for the area, suggest that the tectonic setting for the metamorphism was a zone of continental rifting associated with strike-slip movement. In this zone anatexis occurred at two distinct levels: Cambro-Ordovician pelites at the base of the Palaeozoic sedimentary pile melted to produce per-aluminous magmas, while in the lower Hercynian crust, very large-scale melting generated voluminous granodioritic magmas which then invaded high-structural levels. The thermal structure of the Hercynian crust was profoundly influenced by both convective and advective heat transfer, due to movement of surface derived aqueous fluids, and intrusion of magmas.