GEOLOGY AND GEOCHRONOLOGY OF MÉLANGES FROM THE CYCLADIC BLUESCHIST BELT, GREECE

The Cycladic blueschist unit (CBU) in the central Aegean region is an important study area for all aspects of subduction zone metamorphism and the exhumation of high-pressure/low-temperature rocks. The CBU consists of a pre-Alpidic crystalline basement and a metamorphosed volcano-sedimentary sequence that is mainly build up by clastic metasediments, calcschists, marbles and meta-igneous rocks of basic and acidic bulk-rock composition. This cover also includes mélanges with meta-igneous blocks and tectonic slabs (< 1 meter to several hundred meters) that are enclosed in an ultramafic or metasedimentary matrix. The HP mélange on the island of Syros has attracted much attention, due to its lithological variability (e.g. eclogites, glaucophanites, meta-gabbros, jadeitites), high block abundance and distinct metasomatic reaction zones at contacts between blocks and an ultramafic matrix. However, less spectacular block-matrix associations are widespread on a regional scale. The geological background of these occurrences is yet not fully explored. Field relations suggest that at least the mélanges on Andros, Tinos and Syros can be correlated. The nature and origin of the block-matrix associations is controversial and has either been related to sedimentary or tectonic processes. Recent studies favour a tectonic mode of formation in the context of deformational processes during an Eocene HP episode. Blocks and matrix shared together a common history at identical P-T conditions which includes an eclogite- to epidote blueschist-facies stage and, in some cases, a greenschist-facies overprint. Differences in the earlier P-T histories of individual blocks are yet not well established. Previous ionprobe U-Pb zircon geochronology focusing on blocks from Syros and Andros documented a rock association of mixed provenance and protolith ages (c. 240 Ma, c. 155-160 Ma, c. 80 Ma). New geochronological results for blocks from Tinos (meta-gabbro, eclogite, jadeitite, chloritic blackwall zone) and for sheared and altered ultramafic matrix rocks from Syros indicate Cretaceous ages (c. 80 Ma). Detrital zircon geochronology of clastic metasediments that form the mélange matrix on Tinos also provided an 80 Ma age group, suggesting that the depostional age of the host rocks is much younger than previously assumed.