The base of the Cycladic blueschist unit on Tinos Island (Greece) re-visited: Field relationships, phengite chemistry and Rb-Sr geochronology

The Cyclades archipelago in the Aegean Sea is an important study area for subduction-related metamorphism and the exhumation of high-pressure/low-temperature rocks. Of special importance for interpretation of the general tectonic development in the central Aegean region are tectonic windows that expose the rock sequences below the Cycladic Blueschist Unit (CBU). Previous work suggested that the lowermost dolomite-phyllite-quartzite sequence on Tinos Island represents such a tectonic subunit with a metamorphic and deformational history that is different to the overlying blueschist- and greenschist-facies rocks. The tectonic contact was interpreted as a thrust fault. A re-evaluation of the arguments used to support this interpretation suggests that this conclusion is questionable. Previous studies inferred that the basal sequences only underwent greenschist-facies metamorphism and were not affected by a high-pressure event. However, mineral assemblage and phengite composition in the basal phyllites cannot be distinguished from those of overlying rock sequences, which undoubtedly have experienced high-pressure metamorphism and a pervasive greenschist-facies overprint. Rb-Sr geochronology of phyllites and quartzites (phengite - whole rock pairs), previously interpreted to belong to the lower plate, yielded dates that are indistinguishable from values obtained for strongly overprinted rocks collected at higher lithostratigraphic levels. It can also be shown that sedimentary structures are preserved in many places within the CBU. The presumed absence of such features was originally interpreted as a major contrast to the fossil-bearing basal sequences, indicative for different deformational styles. We postulate that the para-autochthonous basal unit beneath the CBU is not exposed on Tinos Island. Field observations, petrological and geochronological data of the Panormos area are fully compatible with the interpretation that the dolomite-phyllite-quartzite succession is an integral part of the CBU, as originally suggested by MELIDONIS (1980).