Permo–Triassic and Jurassic 40Ar–39Ar ages from Greek ophiolites and associated rocks

Abstract

PRECISE radiometric dating of ophiolites and associated rocks is usually very difficult as the rocks are often altered by ocean floor metamorphism or during tectonic emplacement onto the continental margin. Only age determinations on primary mineral phases can be considered to approximate their initial crystallisation and cooling ages. The K–Ar technique is generally used to date ophiolitic rocks, but even if primary mineral phases are analysed, there is always the uncertainty that the resulting age could have been partially reset by later thermal or tectonic events. The 40Ar–39Ar step heating method is able to detect and often to correct for any partial loss of argon, and thus overcome these problems. We have determined and report here 40Ar–39Ar age spectra for hornblende separates from two unaltered samples of igneous rocks that are interpreted as the initial igneous products of rifting in the Othris region of central Greece. A third hornblende has been analysed from amphibolites believed to have been formed during tectonic emplacement of the Pindos ophiolite (Fig. 1). The ages suggest that the initial rifting began in Othris in the Permo–Triassic (∼248 Myr), while tectonic emplacement of the Pindos ophiolite occurred in the middle Jurassic time (∼180 Myr).