Zircon inheritance reveals exceptionally fast crustal magma generation processes in central iberia during the cambro-ordovician

The Variscan basement of the Central Iberian Zone contains abundant Cambro-Ordovician calc-alkaline to peraluminous metagranites and metavolcanic rocks with two notable features: first, they were apparently produced with no connection to any major tectonic or metamorphic event; second, they have an unusually high zircon inheritance. U-Pb dating combined with cathodoluminescence imaging reveals that about 70-80, in some samples nearer 100%, of the zircon grains contain inherited pre-magmatic cores, despite the temperature reached by the magmas (about 900 degrees C, calculated using the Ti-in-zircon thermometer) being high enough to dissolve all the available zircon (from the rocks zircon saturation temperature, 770-860 degrees C). The fact that the dissolution of zircon was so incomplete can only be attributed to the kinetics of heat transfer to and from the magmas. Three-dimensional modeling of zircon dissolution behavior in melts with a composition similar to the Iberian Cambro-Ordovician magmas indicates that the survival of zircons from the suggested late Pan-African protolith would be possible only if melt production was rapid, specifically less than 10(4) years, and probably about 2 x 10(3) years, from the beginning of melting (700 degrees C) to the thermal peak (900 degrees C). Melt production was followed by fast magma transfer to upper crustal levels resulting either in surface eruption or in the emplacement of small (<400 m thick) sills or laccoliths. We suggest that these elevated rates of crustal melting could only have been caused by intrusion of mantle-derived mafic magmas, most probably at the base of the crust. This scenario is consistent with a rifting regime in which crust and mantle were mechanically decoupled; this would explain the scarcity of contemporaneous crustal deformation. Furthermore, fast melting rates in the lower crust followed by fast melt transportation to the upper crust could also explain the lack of contemporaneous metamorphism. The speed of the partial melting process resulted in the production of felsic magmas that inherited the geochemical characteristics of their granitoid crustal protolith. This explains the apparent contradiction between the calc-alkaline to peraluminous geochemical characteristics of the magmas and the inferred extensional (i.e. rift-related) tectonic setting. Our model is compatible with the hypothesis of fragmentation and dispersal of terranes from the northern margin of Gondwana that led to the opening of the Rheic and GaliciaSouth Brittany oceans and, ultimately, caused the detachment of the Iberian microplate from Armorica and Gondwana during the early Paleozoic.