CARBONIFEROUS TO TRIASSIC FELSIC MAGMATISM IN THE EXTERNAL HELLENIDES OF CRETE AND ITS IMPLICATIONS ON THE PRE-ALPINE OROGENY AND PALEOGEOGRAPHY IN THE EASTERN MEDITERRANEAN

In late Paleozoic/early Mesozoic times the Paleotethys was forming a highly mobile oceanic ream that was pinching into the supercontinent Pangea separating Gondwana from Eurasia. Progressive subduction of oceanic lithosphere and collision of intervening microplates led to the closure of the Paleotethys, whereas rifting caused the opening of the Neotethys. The role of the External Hellenides inside this collision vs. rifting scenario is currently highly debatable. Evidence for Carboniferous and Permian crustal stacking and related Barrovian-type metamorphism is given by U-Th-Pb ages of metamorphic monazites, separated from pre-Alpine basement exposed on Crete (ca. 330 and ca. 260 Ma). We present new U-Pb TIMS zircon ages of felsic gneisses which yield additional constraints on the timing of orogenic activity in the External Hellenides.

A metaryolithe of the Rogdia beds of the Talea Ori of central Crete yielded a concordant age at 300.3 ±0.9 Ma interpreted as emplacement age. Inherited zircons yielded concordant ages at ca. 305 and ca. 336 Ma.

A rhyolite pebble in a metaconglomerate of the lowermost Achlada beds of the Talea Ori yielded a concordant age at 242 ±1.6 Ma.

A felsic gneiss pebble from a metaconglomerate of the Tyros Unit of eastern Crete (Chamezi beds) yielded a concordant age at 290.7 ±2.0.

A rhyolite layer inside Carnian sediments of the Tyros unit (Toplou beds), yielded a concordant age at 228 ±1.7 Ma which is interpreted as the emplacement age. Inherited zircons yielded concordant ages at 307.4 ±1.2 and 361.2 ±1.2 Ma.

The new ages determined from felsic gneisses of Crete, together with published emplacement ages obtained from metagranitoids of Kithira, suggest continuous magmatic activity from late Devonian to middle Triassic times. Triassic orogenic activity and related exhumation of deep-seated basement is also indicated by (1) Proterozoic (meta)granitoid pebbles in the Tyros unit, (2) Middle Triassic 39Ar-40Ar cooling ages of white mica of basement gneisses, and (3) a Middle Triassic gap in sedimentation of the Tyros unit.

The new emplacement ages as well as the inherited Upper Devonian age are similar to emplacement and inherited ages determined from felsic gneisses of the Pelagonian unit suggesting that both units have a close relation in late Paleozoic/early Mesozoic times.