The age of ophiolitic rocks of the Hellenides (Vourinos, Pindos, Crete): first U-Pb ion microprobe (SHRIMP) zircon ages

Abstract

Ophiolites in mainland Greece (Vourinos, Pindos and Othrys) form a NNW-SSE trending belt within the orogen of the Hellenides, extending further north into the Dinarides. Smaller ophiolite outcrops are also found on Aegean islands. A minimum Mid- to Late-Jurassic age has been inferred for these ophiolites, on the basis of paleontological data, as well as radiometric data from the so-called metamorphic soles at the base of the ophiolites. The latter data mark the end of spreading and initiation of intra-oceanic thrusting. U-Pb ion microprobe (SHRIMP) dating of co-magmatic zircon domains from one gabbro and one plagiogranite (Vourinos) yielded weighted mean 206Pb/238U ages at 168.5±2.4 and 172.9±3.1 Ma, respectively, interpreted as the time of crystallization of the rocks. One gabbro from Pindos yielded a crystallization age of 171±3 Ma (1 σ). All three ages are identical to within analytical uncertainty. They are also within uncertainty of the radiometric ages reported previously from metamorphic soles. This implies (a) that the time difference between the igneous crystallization of the Vourinos and Pindos ophiolitic rocks dated here and the formation of the metamorphic soles is short, at least not resolvable within the uncertainties of the current geochronological techniques and (b) that the ocean ridge remained active while intra-oceanic thrusting commenced. Our geochronological results and their correlation with existing radiometric data from the metamorphic soles are obviously neither in favor nor against a supra-subduction setting for the formation of the above ophiolites. Zircons from a hornblendite (considered as constituent of the ophiolites) from the island of Crete, close to Kerames area yielded a weighted mean 206Pb/238U age of 162.7±2.8 Ma. This age is in agreement with the oldest of a wide range of K-Ar amphibole ages (166-149 Ma) previously reported for this rock-type. Zircon in the hornblendite is both of an igneous origin, and with recrystallized areas interpreted to have formed immediately after its formation under high-T, in the presence of abundant fluids, both events, or processes, having occurred within the analytical uncertainty time span. The 162.7±2.8 Ma age reflects and its uncertainty encompasses then both the time of igneous crystallization and high-T metamorphism in a high-T shear zone.