THE OPHIOLITIC MELANGE OF CENTRAL CRETE (GREECE): REMNANTS OF ALPINE ACCRETIONARY WEDGE

The ophiolite-bearing terranes of the central Crete form the uppermost tectonic nappe of the orogenic belt and are represented by broken formation and tectonic mélange associated with remnants of a well-developed accretionary wedge emplaced on top of the cretan continental margin. These units consist of a Cretaceous-Upper Eocene matrix, which includes blocks of continental-type rocks and ophiolites with remnants of their original Upper Jurassic-Cretaceous (?) pelagic cover. The innermost portion of the accretionary wedge is represented by a polymetamorphosed and polydeformed tectonic unit made up of prevalent micaschists and calcschists with levels of marbles and quartzites, with associated blocks of oceanic and continental type rocks. The ophiolitic rocks are mainly represented by peridotites, metabasalts and metagabbros that show HP/LT mineral assemblages (glaucophane, chlorite, actinolite, titanite, epidote, albite and quartz). The intermediate structural subunit consists of a sequence of shales, schists, calcschists containing beds of quartzites, metarenites and meta-calcarenites. The associated ophiolites occur extensively as metre to kilometre-sized lens-shaped blocks. They are mainly represented by serpentinized peridotites and metagabbros with associated rare metabasalts. In places, metabasalts show remnants of their original sedimentary cover made up of varicoloured siliciferous shales, metaradiolarites and marbles. Metabasalts consist of fine-grained massive or banded rocks showing a green-schist facies assemblage represented by albite, chlorite, actinolite, epidote, titanite, opaques and rare biotite. In some samples the occurrence of assemblages with riebeckite/crossite suggests an evolution towards the HP sector of the green-schist facies. The lowermost subunit, which represents the frontal portion of the accretionary wedge, is composed of a chaotic turbiditic sequence of green to brown shale and marls, calcarenites and lithoarenites and volcanic-derived arenites. This sequence contains lens-shaped blocks of serpentinites, gabbros, basalts and Upper Jurassic oceanic sedimentary cover. Stratigraphic data deriving from micropaleontological analyses carried out on several samples collected on the matrix of the intermediate and the lowermost structural horizons show nannofossil associations indicating ages ranging from the Cretaceous to the Late Eocene. Representative samples of metabasalts, metagabbroes, serpentinized peridotite and serpentinized peridotite breccias are deeply altered suggesting a ocean floor metamorphism. Continental type rocks are represented by paragneisses and orthogneisses, amphibolites and silicate-bearing marbles intruded by Upper Cretaceous granitoids. These rocks are included within both the uppermost and the intermediate subunits and form lens-shaped blocks that ranging in length from metres to kilometres are at times separated by lenses of serpentinized peridotite. The structural analysis revealed the occurrence of four groups of structures that developed at different structural levels during continent collision suggest a south-easterly emplacement of the entire wedge on the cretan continental margin.