ZIRCON INHERITANCE REVEALS EXCEPTIONALLY FAST CRUSTAL MAGMA GENERATION PROCESSES IN CENTRAL IBERIA DURING THE CAMBRO-ORDOVICIAN

Abstract

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°C, calculated using the Ti-in-zircon thermometer) being high enough to dissolve all the available zircon (from the rock's zircon saturation temperature, 770–860°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 104 years, and probably about 2 × 103 years, from the beginning of melting (700°C) to the thermal peak (900°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.