Geochemistry and geochronology of HP melanges from Tinos and Andros, cycladic blueschist belt, Greece

U-Pb zircon geochronology. Sr-Nd isotope and bulk-rock geochemistry have been applied to meta-igneous and meta-sedimentary rocks from high-pressure metamorphic melanges exposed on the Cycladic islands of Tinos. Syros and Andros Ion microprobe (SHRIMP) U-Pb zircon dating of 7 samples representing metaigneous blocks (Tinos), a blackwall zone (Tinos) and chlorite-talc schists from block-matrix contacts (Syros and Tinos) yielded Cretaceous ages of c 80 Ma. Many of the criteria commonly used to distinguish between magmatic or metamorphic zircon genesis (internal structure. Th/U ratio. REE characteristics. Thin zircon thermometry. enclosed mineral phases) do not provide unambiguous constraints for the mode of formation. However, a magmatic origin for Cretaceous zircon of meta-gabbros and eclogues is considered likely Supporting evidence for a previously suggested metamorphic origin for c 80 Ma zircon in eclogite has not been found. Zircon of the same age occurring in chlorite-talc schists is presumably related to non-magmatic processes. Well-defined Cretaceous age groups clustering at c 79 Ma also occur in the detrital zircon populations of 2 quartz mica schists representing the melange matrix on Tinos, and suggest a much later time for sediment accumulation than previously assumed The importance of c 57 Ma zircon ages remains unclear, but may record either HP metamorphic processes or a post-57 Ma depositional age The youngest age group in a third quartz mica schist from Tinos, collected outside the main melange occurrences, clusters arc. 226-238 Ma. In all clastic metasediments from Tinos, most data points plot along the concordia between C. 300 and 900 Ma, single data points indicate concordant ages of c. 25 Ga, 23 Ga and 1 Ga, respectively. The youngest Pb-206/U-238 age group that has been recognized in a felsic paragneiss from Andros indicates an age of 163.1 +/- 3 9 Ma, and mostly represents overgrowths around zircon with ages in the range from similar to 272 to similar to 289 Ma. Single data points of other inherited cores provided 206Pb/238U ages of c 630 and c 930 Ma. Metagabbros from Tinos show a large compositional variability and were found at 4 locations, each with distinct compositional characteristics, suggesting different crystallization histories, different sources and/or significant post-magmatic disturbance The geochemistry of melange blocks and the identical U-Pb zircon ages suggest that the block-matrix associations on Tinos and Syros can be grouped together On a broader regional scale, there seem to be similarities between some meta-igneous rocks from Tinos and Evvia Field relationships indicate that the m langes occurring in southern Andros and northern Tinos can be correlated, but supporting geochemical and/or geochronological evidence for this interpretation could not be established Previously published Jurassic ages for mafic and felsic melange blocks from Andros suggest a genetic relationship to the ophiolite occurrences exposed in the larger Balkan region A similar regional correlation is also considered likely for the Cretaceous meta-gabbros from Tinos and Syros. but cannot be documented with certainty (C) 2010 Elsevier B.V All rights reserved.