Ofioliti, 2005, 30 (2), 75-84 75 GEOLOGICAL EVOLUTION OF THE KOZIAKAS OPHIOLITIC COMPLEX (WESTERN THESSALY, GREECE) Panagiotis Pomonis*, Basilios Tsikouras and Konstantin Hatzipanagiotou University of Patras, Department of Geology, Section of Earth Materials, Patras, GR-265 00, Greece. * Corresponding author, e-mail: [email protected]. Keywords: ophiolite, mélange, peridotite, serpentinite. Koziakas, Thessaly. ABSTRACT The Koziakas ophiolitic complex is located in Western Thessaly and is interpreted as a dismembered ophiolite sequence, which overthrust the Western Thessaly Unit (WTU). The Koziakas basal Unit consists of sedimentary rocks and occupies the stratigraphically lowermost members of the WTU. The Ophi- olitic Unit overthrust the WTU and consists, from bottom to top, of a well-developed ophiolitic mélange, a sub-ophiolitic metamorphic sole, serpentinites, ser- pentinized peridotites, peridotites (harzburgite, lherzolite, plagioclase lherzolite), massive dolerites, massive basalts and pillow lavas interlayered with Mn- rich radiolarites. The ophiolitic mélange comprises a chaotic multicoloured complex of various rock fragments. A sparse network of gabbroic (locally rodingi- tized), plagiogranitic and doleritic dykes intrudes the mantle tectonites along two main cross-cutting directions: the first strikes NE-SW and dips 50o-70o to the NW, the second strikes N-S to NNW-SSE with a dip of about 65o-80o to the west. The dominant tectonic fabrics such as shear zones, fold axes, foliation planes and mineral streching lineations, imprinted mainly on the Koziakas mantle tectonites trend NW to NE and are consistent with a NE-SW emplacement direction. The similar radiometric ages and the comparable geological characteristics of the Koziakas ophiolitic complex with its adjacent ophiolite suites of Pindos, Vourinos and Othris imply that they probably developed in the same marginal basin represented by the Pindos ocean basin. INTRODUCTION The Koziakas basal Unit The Koziakas mountain range runs in Western Thessaly The basal Unit of the Koziakas ophiolite is represented by along a NNW-SSE direction, marking the natural boundary sedimentary rocks of the WTU, which occur in the western between the Pindos chain to the NW and the extended Thes- and central parts of the Koziakas mountain range. The origi- saly basin to the east. Previously, Aubouin (1959) consid- nal palaeogeographic record of the WTU has been destroyed ered the Koziakas formations as the “Ultrapindic Zone”; lat- by intense deformation during Late Eocene - Oligocene times er on Papanikolaou and Sideris (1979) as the “Western (Savoyat and Lalechos, 1969; 1972; Scandone and Radoic´ic´, Thessaly Unit (WTU). The steep western and central parts 1974; Ferrière, 1974; 1982; Jaeger, 1980), that formed two of the Koziakas range, characterized by high altitudes (high- series (Aubouin, 1959; Papanikolaou and Sideris, 1979): a) est top: Astrapi, 1901 m) consist of a sedimentary sequence the Thymiama series, comprising the uppermost Jurassic- of Triassic to Late Cretaceous age, while an incomplete Eocene part of the stratigraphic sequence and b) the Koziakas ophiolitic complex of Jurassic age crops out at the eastern series comprising the Upper Triassic - Lower Cretaceous part smooth slopes, following a NNW-SSE alignment. of the stratigraphic unit. The Koziakas subunit overthrust the The Koziakas ophiolite is one of several Mesozoic maf- Thymiama subunit, while both subunits thrust to the west ic-ultramafic complexes that occupy the westernmost part of over the Palaeocene-Eocene flysch of the WTU, which is the sub-parallel NNW-trending tectono-stratigraphic belts transitional to the Pindos flysch (Aubouin, 1959; Papaniko- along the Hellenides of Central Greece. It is neighboured by laou and Sideris, 1979; Lekkas, 1991). The Thymiama series the well-studied Northern Pindos ophiolite to the NW and includes, from bottom to top: a flysch formation known as the Othris ophiolite to the SE (Fig. 1). The Koziakas ophio- “Beotian flysch” of Late Jurassic - Early Cretaceous age, lite thrust to the west over the Western Thessaly Unit, while pelagic limestones of Late Cretaceous age and a clastic for- it is covered by the Quaternary deposits of the Thessalian mation with red limestones of Palaeocene - Early Eocene age. basin or by the Oligocene-Miocene molassic sediments of The Koziakas series consists from bottom to top of: a clastic the Meso-Hellenic trough to the east (Brunn, 1956). The ge- formation of Late Triassic - Middle Jurassic age, oolithic otectonic setting of the Koziakas ophiolitic complex is of limestones of Middle - Late Jurassic age and brecciated lime- particular significance, being the link between the internal stones with ophiolitic fragments of Early Cretaceous age. and the external Hellenides. The Koziakas ophiolite tectonically overlies the clastic The purpose of this paper is to present new structural da- formation of the Koziakas series (Fig. 2), which consists of ta from our detailed mapping of the Koziakas ophiolite, to radiolarites, shales, siltstones and oolithic limestones of describe its geological structure and to compare it with the Late Triassic - Middle Jurassic age (Aubouin, 1959; Jaeger, adjacent ophiolites of the Central Hellenides. 1979; 1980) with Triassic trachyandesitic rocks. The Trias- sic trachyandesites occur in the NW part of the Koziakas mountain range, near the Glykomilia village (Fig. 1). They GEOLOGICAL SETTING form small lava flows, about 50 m in thickness, concordant- ly associated with Norian conodonts-bearing red limestones Extensive fieldwork in the Koziakas ophiolitic complex (Portaikos Limestones) and radiolarites (Magganas et al., focused mainly on the distribution and mode of development 1997; Pomonis et al., 2004). The pelagic to shallow marine of the ophiolitic rocks, as well as their relation to the adjacent sediments of the Koziakas series display sometimes cyclic formations, and resulted in a detailed geological map (Fig. 1). alternations and turbiditic structures, suggesting accumula- 76 Fig. 1 - Geological map of the Koziakas region (Pomonis, 2003). 77 tion in a continental slope. The maximum exposed thickness sandstone is less common. All matrix-forming rocks are of of the clastic formation has been estimated to 500 m. variegated colours, with red, brown, grey, green, purple and brown dominant. Rock fragments of variable size and lithology are irregu- The Ophiolitic Unit larly distributed throughout the matrix. The proportion of The Koziakas Ophiolitic Unit occupies the eastern part of matrix to clasts is variable, but generally low. In places, the the mountain range and comprises an ophiolitic mélange and blocks are chaotically jumbled against one another with thin a subophiolitic metamorphic sole. Massive dolerites, massive screens of matrix filling the inter-block spaces. Elsewhere, basalts and basaltic pillow-lava flows overthrust the mantle the mélange consists of few blocks set in extensive outcrops tectonites (harzburgites, lherzolites and plagioclase-lherzo- of tectonized matrix. The mélange also contains slices of lites), which are in turn thrust over the ophiolitic mélange turbidites, composed of terrigenous, fine- to medium- and the metamorphic sole. In places the pillow-lavas are in- grained, carbonate-cemented sandstone and shale. Most tercalated with cherts rich in Mn-oxides. A sparse network of contacts within the ophiolitic mélange are highly foliated dykes including gabbro, rodingitized gabbro, plagiogranite and tectonic. However, igneous contacts are occasionally and dolerite, intrudes the mantle tectonites. preserved within large doleritic and basaltic blocks, com- Our own structural observations (described below) sug- monly juxtaposed to highly strained rocks. Characteristic gest that the Koziakas Ophiolitic Unit overthrust to the west exposures of the ophiolitic mélange are found close to the and to the north the clastic formation of the Koziakas subunit Xylopariko village (Fig. 1), where slices of foliated serpen- in contrast to what reported in Papanikolaou and Lekkas tinized peridotites, pillow-lavas, radiolarites and limestones (1979) ideas, that consider the southern part of the Koziakas of Late Triassic and Middle-Late Jurassic age, are inter- ophiolite to rest conformably on this subunit. Steeply dipping changed with large blocks of massive olivine-gabbro. cataclastic and mylonitic shear-zones confirm a thrust origin along the contact between the ophiolitic mélange and the Sub-ophiolitic metamorphic sole clastic formation to the south and between the mantle tec- tonites and the clastic formation to the north (Fig. 2). The Koziakas sub-ophiolitic metamorphic sole consists of amphibolites and metasedimentary rock fragments. The am- phibolites include MORB-type protoliths with IAT influ- Ophiolitic mélange ence, suggesting a back-arc basin environment for their ori- The ophiolitic mélange occupies the dominant area of the gin (Pomonis et al., 2002). K-Ar determinations on horn- Koziakas Ophiolitic Unit and is a multicoloured formation blende separates from the amphibolite suggest a Late Juras- composed of a chaotic matrix intermixed with exotic frag- sic age (174±3 Ma and 161±1 Ma, Pomonis et al. 2002), ments. The fragments are mainly angular and vary in size similar to those inferred for other ophiolites of northern-cen- from several centimetres up to tens of meters (exotic tral mainland Greece. Two small outcrops of metamorphic blocks). The exotic fragments include serpentinites, serpen- rocks