Dating metamorphism and tectonic juxtaposition on Andros Island (Cyclades, Greece): results of a Rb-Sr study

This paper reports new geochronological data from the island of Andros, one of the less-studied islands of the Cycladic blueschist belt in the central Aegean Sea. On Andros, two tectonic units can be distinguished, the Makrotantalon unit and the Lower unit, which are separated by a low-angle normal fault, related to large-scale regional extension. Mineral assemblages indicate greenschist-facies P-T conditions for the last metamorphic overprint of both units. In contrast to the structurally lower unit, unambiguous indications for an earlier high-pressure stage were not recognized in rocks collected above the tectonic contact. Owing to a polyphase metamorphic evolution and incomplete resetting of the Rb-Sr isotope system during overprinting, phengite geochronology indicates a wide range in dates between c. 104 and 21 Ma for the Makrotantalon unit, as observed in rocks of similar structural position elsewhere in the Cyclades. The new Rb-Sr data support the interpretation, but are not conclusive evidence, that tectonic slices within the hanging wall were affected by two periods of Cretaceous metamorphism (c. 100-90 Ma and c. 80-70 Ma) and a Miocene event (c. 21 Ma). Tectonic juxtaposition was accomplished around e. 21 Ma. The Lower unit is correlative with the Cycladic high-pressure occurrences. Rb-Sr phengite dating yielded the same range in ages as determined elsewhere in the region for white mica of high-pressure rocks (c. 50-40 Ma) and their overprinted, greenschist-facies derivatives (c. 23-21 Ma). An age gradient towards the tectonic contact with the overlying Makrotantalon unit is not developed. The new results fit well into the previously established chronological framework for the larger study area. Indications for regional differences in the timing of the HP stage and/or the greenschist-facies overprint have not yet been found.