Early Ordovician metabasites from the Spanish Central System: A remnant of intraplate HP rocks in the Central Iberian Zone

Abstract

The rocks at Tenzuela represent the only known outcrop of HP metabasites in the internal sectors of the Iberian Belt, in the vast Central Iberian Zone (CIZ). These rocks appear as lenses of meta-tholeiites of continental affinity intruded within the Neoproterozoic metasedimentary sequences. The mafic types have high Th/Yb and Ta/Yb ra- tios that suggest, combined with initial εNd values ranging from +4.4 to +6.0, derivation from a slightly enriched mantle source. SHRIMP zircon data indicate an Early Ordovician age (206Pb/238U mean age of 473 ± 2 Ma) for this magmatic event. However, the U–Pb data do not yield a constrained age for the high- pressure metamorphic event undergone by the metabasites, which nevertheless can be estimated at just before 335 Ma, during the Variscan collision. The HP metabasites rarely overpass peak pressure conditions of 15 kbar, and thus are remarkably lower than those of eclogites related to oceanic subduction in ophiolite- or allochtho- nous complexes from NW Iberia. On the other hand, these pressures are similar to those of the metabasites inter- calated in orthogneisses along the Coimbra–Córdoba Shear Zone, in the boundary with the Ossa-Morena Zone (OMZ). In this regard, the new geochronological data set together with already published ages shows that the subduction-related Iberian eclogites (allochthonous complexes) are older (Lower to Middle Devonian) than the HP metabasites from more intracontinental settings, mostly recrystallized at lower HP conditions during the Visean. Thus, the studied metabasites should form part of an inner, diachronous belt of HP rocks within the European Variscan orogen, far away from the main subduction front. Nevertheless, the suggested geodynamic scenario needs future confirmation by accurate geochronological data for the HP metamorphism.