Polymetamorphic evolution of pre-Alpidic basement relics in the external Hellenides, Greece

摘要

Pre-Alpidic basement rocks in Eastern Crete and the Dodecanese islands of Lipsos, Leros, Kalymnos and Telendos reveal compelling similarities concerning their lithologies and their pre-Alpidic metamorphic evolution. The basement rocks in both areas can be subdivided in two subunits. The lower Myrsini Unit on Crete and Panormos Unit in the Dodecanese experienced an amphibolite facies overprint at 570-630 deg C and 5.5-8.0 kbar. The upper Chamezi Unit on Crete and Emporios Unit in the Dodecanese were metamorphosed under upper greenschist facies conditions at 475-550 deg C and 5.5-6.5 kbar. For both units, a clockwise prograde P-T path can be deduced. New K-Ar ages of 200-320 Ma on amphiboles and white micas from the basement of the Dodecanese indicate a main metamorphic overprint in the Variscan and a partial resetting by the Alpidic overprint. Almost identical ages are known for the crystalline rocks in Eastern Crete. These petrologic and geochronological similarities classify both basement sequences as part of the Apulian microplate. The Alpidic history of the two occurrences, however, differs distinctly. Whereas in Eastern Crete a low grade blueschist facies overprint (250-330 deg C at 4.5-8 kbar) was followed by a nearly isothermal uplift, the Dodecanese basement achieved its Alpidic overprint under lower greenschist facies conditions (300-370 deg C at 1-3 kbar). The main difference in the Alpidic metamorphic evolution is the HP/LT-overprint during the Miocene subduction of the Phyllite-Quartzite Unit in Crete. Such an oveiprint was not observed in the Dodecanese, where the transport of the basement with the Lycian nappe pile occurred under lower greenschist facies conditions in Alpidic time. Consequently, the two basement units were probably separated in Mesozoic time. A possible scenario for this separation could be a lineament between the Anatolian and the Apulian microplate, along which a transfer of basement rocks may have occurred.