Pre-Variscan tectonic events and Late Ordovician magmatism in the Central Pyrenees: U–Pb age and Hf in zircon isotopic signature from subvolcanic sills in the Pallaresa massif

Abstract

Late Ordovician magmatism is well-represented in all massifs of the Eastern Pyrenees and the Catalan Coastal Ranges but has not been reported in the central parts of the Pyrenean Axial Zone. This work shows the first geochronological evidence for Late Ordovician magmatism in the Pallaresa massif. This massif is a large E–W trending antiformal structure cored by Paleozoic rocks with a Variscan tectonothermal overprint. The lower Paleozoic sequence shows an Upper Ordovician succession lying unconformably on older Cambrian–Ordovician beds. Despite Variscan overprint, the Cambrian–Ordovician rocks record evidence of a pre-Variscan penetrative fabric which is not present in the rest of the overlying Paleozoic succession. In the eastern part of the Pallaresa massif, rhyodacitic to dacitic subvolcanic rock forming sills intercalated within the Cambrian–Ordovician succession close to the base of the Upper Ordovician rocks have been identified and sampled. These volcanic rocks have a Variscan penetrative foliation and lack the additional pre-Variscan deformation. U–Pb zircon dating of one these subvolcanic levels by U–Pb CA-ID-TIMS, has provided an age of 453.6 ± 1.5 Ma (Sandbian) and the zircon has given ɛHf450 values between − 2.4 and − 5.9, suggesting a mixed source. These new data strengthen the Paleozoic correlation of the Central and Eastern Pyrenees indicating that the Late Ordovician magmatism in the Pyrenees was associated with a pre-Variscan event responsible for the Upper Ordovician angular unconformity and the presence of a penetrative pre-Variscan deformation affecting the underlying Cambrian–Ordovician sequence. This new data, coupled with the occurrence of Zn–Pb SEDEX o Mississippi Valley type deposits associated with Upper Ordovician rocks, suggest the evidence of extensional tectonic activity during Late Ordovician times.