Zircon Geochronology of the Ollo de Sapo Formation and the Age of the Cambro-Ordovician Rifting in Iberia

Abstract

The 600-km-long Ollo de Sapo Formation was generated during the Cambro-Ordovician rifting that affected the pre-Variscan basement of Europe and represents the largest accumulation of pre-Variscan igneous rocks in Iberia. It consists of variably metamorphosed felsic volcanic rocks and granites characterized by an abnormally elevated zircon inheritance (70%–80% or more of the zircon grains contain premagmatic cores), the age of which has long been a matter of debate. This article presents the results of dating the northwestern area of the Ollo de Sapo Formation via U-Pb spot (ion-microprobe and laser ablation–inductively coupled plasma mass spectrometry) and Pb-Pb single-grain zircon analysis, complementing our previous results from the southeastern area of the formation, and the results of a survey of the major- and trace-element and Sr-Nd isotope composition of these rocks, which unequivocally indicate that they are crustal melts. The age span recorded in the northwestern Ollo de Sapo Formation is 492–486 Ma, with the only exception being the San Sebastián metagranite (470 ± 3 Ma), a small body with notably less zircon inheritance and a Sr-Nd isotope composition more primitive than the rest of the Ollo de Sapo rocks. When these results are considered jointly with other reliable zircon ages of similar rocks from all over Iberia, it may be deduced that in the Central Iberian Zone the Cambro-Ordovician rifting started at ∼495 Ma, reached a maximum between 492 and 483 Ma, and ceased at ∼470 Ma. The age pattern of inherited zircons of the Ollo de Sapo Formation reveals that the source of the Cambro-Ordovician magmas was mostly igneous rocks of Ediacaran age, with minor Orosirian and Archean components. A detailed study of these components might be key for revealing which part of the north was connected to Iberia before the early Paleozoic breakup of Gondwana.