DEPOSITIONAL AGES OF METASEDIMENTS FROM TINOS ISLAND, GREECE: NEW INSIGHTS FROM DETRITAL ZIRCON GEOCHRONOLOGY

The Attic-Cycladic crystalline belt (ACCB) in the central Aegean region represents a major tectonostratigraphic unit of the Hellenides. The ACCB records a complex structural and metamorphic evolution which documents various stages in the closure of the Neotethys ocean and the collapse of the resulting orogen. The general geological framework is well-documented, but key information concerning the pre-metamorphic evolution is only poorly constrained, especially for the northern part of the Cyclades archipelago (Tinos, Syros, Andros). An unresolved question concerns the depositional age of clastic metasediments which represent one of the principal rock types of the schist-marble sequences of the study area. This issue is here addressed in an ionprobe (SHRIMP) U-Pb zircon study focusing on siliciclastic metasediments from Tinos Island. On Tinos, a representative crustal segment of the ACCB is exposed in at least three structural subunits. Most parts of the island belong to the lower tectonic unit which has experienced eclogite- to epidote blueschist facies metamorphism (Cretaceous to Eocene) and a greenschist facies overprint (Oligocene/Miocene). The volcano-sedimentary succession of the lower unit also includes block-matrix associations with meta-igneous blocks enclosed in a metasedimentary matrix. The depositional age of the metasediments forming the mélange matrix is unknown, but have previously been linked to Triassic to Jurassic sedimentation. Zircon grains were separated from three siliciclastic rock samples. The U-Pb zircon ages document heterogeneous populations of mixed provenance, that include detritus from Precambrian, Paleozoic and Mesozoic sources. The most important finding of this study is the documentation of a Cretaceous age group with a distinct cluster at ca. 80 Ma in two samples collected a few meters below and above a block-bearing horizon. This age group suggests a considerably younger depositional age than previously assumed. The Cretaceous ages perfectly correlate with U-Pb zircon ages of meta-igneous blocks, lending support to interpretations suggesting an olistostromatic origin for the Cycladic mélange sequences.