Geochemistry and geochronology of HP mxe9;langes from Tinos and Andros, cycladic blueschist belt, Greece
文摘
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 mxe9;langes exposed on the Cycladic islands of Tinos, Syros and Andros. Ion microprobe (SHRIMP) U–Pb zircon dating of 7 samples representing meta-igneous 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, Ti-in 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 eclogites 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 mxe9;lange 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 mxe9;lange occurrences, clusters at c. 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. 2.5 Ga, 2.3 Ga and 1 Ga, respectively. The youngest 206Pb/238U 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 272 to 289 Ma. Single data points of other inherited cores provided 206Pb/238U ages of c. 630 and c. 930 Ma. Meta-gabbros 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 mxe9;lange 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 mxe9;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 mxe9;lange 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.