Protolith ages of meta-igneous and metatuffaceous rocks from the Cycladic blueschist unit, Greece:: Results of a reconnaissance U-Pb zircon study

The Cycladic blueschist unit (CBU) in the central Aegean Sea comprises a pre-Alpidic crystalline basement that is overlain by thrust sheets of a metamorphosed volcanosedimentary sequence. Melanges are widespread and enclose a wide variety of meta-igneous rock fragments in a serpentinitic and/or metasedimentary matrix. Previous geochronology has documented the general temporal framework for the Alpidic metamorphic history but did not produce well-constrained protolith ages for the main volcanosedimentary succession and for melange blocks. This issue is addressed here in an ion probe U-Pb zircon study focusing on the islands of Andros, Sifnos, and Ios. The new results indicate a consistent regional pattern of Triassic ages (ca. 237-245 Ma) for the magmatic precursors of metatuffaceous and metavolcanic rocks, which occur as intercalations within metasediments. This narrow range of igneous crystallization ages records an important period of volcanic activity in the larger study area. On Andros, such ages were also determined for a disrupted tabular body of meta-acidic gneisses ( several hundred meters in length) that has been considered an olistolith within a meta-olistostrome. Field observations are compatible with the interpretation that this fragment represents a layer of the main succession that has been disrupted by large-scale boudinage. A metagabbro and a meta-acidic gneiss from a high-pressure melange on Andros yielded Jurassic Pb-206/U-238 ages of and 156.2 +/- 2.3 160.0 +/- 2.0 Ma, respectively, that correspond very well to ages reported for ophiolites from mainland Greece, the Balkan region, and Crete. The new results and literature data document that remnants of both Jurassic and Cretaceous meta-ophiolites are included in the melanges of the CBU. It is currently not known whether both age groups occur within the same melange or at different lithostratigraphic and/or tectonic levels.