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 abundantCambro-Ordovician calc-alkaline to peraluminous metagranitesand metavolcanic rocks with two notable features: first, theywere apparently produced with no connection to any major tectonicor metamorphic event; second, they have an unusually high zirconinheritance. U–Pb dating combined with cathodoluminescenceimaging reveals that about 70–80%, in some samples nearer100%, of the zircon grains contain inherited pre-magmatic cores,despite the temperature reached by the magmas (about 900°C,calculated using the Ti-in-zircon thermometer) being high enoughto dissolve all the available zircon (from the rock's zirconsaturation temperature, 770–860°C). The fact thatthe dissolution of zircon was so incomplete can only be attributedto the kinetics of heat transfer to and from the magmas. Three-dimensionalmodeling of zircon dissolution behavior in melts with a compositionsimilar to the Iberian Cambro-Ordovician magmas indicates thatthe survival of zircons from the suggested late Pan-Africanprotolith would be possible only if melt production was rapid,specifically less than 10⁴ years, and probably about 2 x 10³years, from the beginning of melting (700°C) to the thermalpeak (900°C). Melt production was followed by fast magmatransfer to upper crustal levels resulting either in surfaceeruption or in the emplacement of small (< 400 m thick) sillsor laccoliths. We suggest that these elevated rates of crustalmelting could only have been caused by intrusion of mantle-derivedmafic magmas, most probably at the base of the crust. This scenariois consistent with a rifting regime in which crust and mantlewere mechanically decoupled; this would explain the scarcityof contemporaneous crustal deformation. Furthermore, fast meltingrates in the lower crust followed by fast melt transportationto the upper crust could also explain the lack of contemporaneousmetamorphism. The speed of the partial melting process resultedin the production of felsic magmas that inherited the geochemicalcharacteristics of their granitoid crustal protolith. This explainsthe apparent contradiction between the calc-alkaline to peraluminousgeochemical characteristics of the magmas and the inferred extensional(i.e. rift-related) tectonic setting. Our model is compatiblewith the hypothesis of fragmentation and dispersal of terranesfrom the northern margin of Gondwana that led to the openingof the Rheic and Galicia–South Brittany oceans and, ultimately,caused the detachment of the Iberian microplate from Armoricaand Gondwana during the early Paleozoic. KEY WORDS: igneous petrology; migmatite; granite; geochemistry; crustal contamination; ICP-MS; laser ablation