Ofioliti, 2005, 30 (2), 75-84 75

GEOLOGICAL EVOLUTION OF THE KOZIAKAS OPHIOLITIC COMPLEX (WESTERN , )

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 . 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 occur east of the Genessio village, and west of the tinized peridotites, gabbros, dolerites, pillow-lavas, amphi- Kaloneri village (Fig. 1). They display cataclastic character- bolites, radiolarites, limestones (of Late Triassic, Middle- istics, they have an apparent thickness of about 10 meters Late Jurassic and Late Cretaceous age, estimated on the ba- and consist mainly of fine-grained amphibolites with thin in- sis of microfossil evidence), manganiferous shales, argillitic terlayers of medium-grained ones. At the lower parts, there schists and sandstones. are fragments (about 30-40 cm thick) of metasedimentary Serpentinites are found usually as thrust-bounded blocks rocks, consisting mainly of garnet micaschists. The sub-ophi- with, or overlying other mélange blocks. In places, (e.g. olitic metamorphic sole is tectonically sandwiched between north of Fylira village, Fig. 1) pillowed and massive, vesicu- the underlying ophiolite mélange and an overlying serpen- lar, spylitic alkali lava sequences are locally interbedded tinized peridotite. As a result, these rocks are highly tec- with cherts, pelagic carbonates that contain bivalve frag- tonized, especially close to the contacts. The amphibolites ments (Halobia; Aubouin, 1959) and fragmented basaltic display strong synmetamorphic fabrics such as mineral folia- material. The close association of these lavas with pelagic tion and associated stretching lineation parallel to the basal carbonates (with Halobia) suggests alkaline syn-rift volcan- thrust faults beneath the overlying peridotites. In addition, ism in a continental margin succession (Saccani et al., the garnet micaschists show a well developed foliation and 2003). Furthermore, the similar geochemical characteristics metamorphic textures parallel to those in the amphibolites. of the alkali basalts inside the Koziakas ophiolitic mélange with the Triassic Glykomilia trachyandesites, as well as Mantle peridotites their close association with pelagic carbonates and cherts lead to the hypothesis of eruption in a deep rift basin The Koziakas ultramafic mantle tectonites are, by far, the (Pomonis, 2003; Pomonis et al., 2004; Magganas et al., most voluminous rocks of the Ophiolitic Unit (Fig. 1). They 1997). overthrust the ophiolitic mélange (Fig. 2) and locally the am- Distinctive sedimentary lithologies of the Koziakas ophi- phibolites of the metamorphic sole. These are predominantly olitic mélange include thick-bedded pelagic carbonates and spinel-harzburgite and lherzolite with minor plagioclase- associated carbonate breccias, of Late Triassic and Early bearing lherzolite. Both harzburgite and lherzolite are homo- Jurassic age. These carbonates occasionally comprise nodu- geneous but locally the harzburgite exhibits discontinuous lar, stylolitic, Ammonitico Rosso-type lithologies (Bernoulli banding defined by concentration of orthopyroxene grains. and Jenkyns, 1974). Elsewhere, radiolarian cherts, common- These bands are usually about 2 cm thick, in places up to 10 ly recrystallized and calcite veined, are associated with cm thick and several tens of cm in length. In places, the manganiferous and ferruginous shales. Koziakas ultramafic tectonites are coarse-grained with The tectonized pelitic matrix commonly consists of blocky pyroxenes. The contacts between harzburgites and shale, mudstone and marl. Coarse material such as lithic lherzolites are mainly gradational over a few centimeters. 78

Fig. 2 - Characteristic cross-sections from the Koziakas ophiolite. Locations of these sections are indicated in the geological map of Figure 1. 79

All exposures of the Koziakas ultramafic tectonites show Molassic formations and Quaternary deposits evidence of deformation. Strain in these rocks is recorded by At the NE part of the study area, Eocene-Miocene molas- a variety of structures. Ductile strain is expressed by the sic sediments unconformably overlie the Koziakas Ophiolite shape and orientation of the minerals, particularly orthopy- Unit. These sediments consist of alternations of marls, silty roxene, giving rise to a characteristic sequence of pervasive marls, sandstones and conglomerates. According to Brunn fabric changes that ultimately lead to mylonites. Folds are al- (1956), the molassic formations occurring in the studied so found. The transition to brittle structures such as brittle area belong to the following series from bottom to top: a) faults and brittle shear zones is commonly associated with Krania series of Late Eocene age, b) Eptachorio series of the development of discrete mylonite zones. Serpentinized Oligocene age and c) Pentalofo-Meteora series of Early ultramafic mylonites (up to 10 m thick) with light grey cata- Miocene age. clastic peridotites are characteristic along the contact between Quaternary basin and fluvial deposits, consisting of of the Koziakas ophiolitic complex and WTU on the west. various sizes and lithological composition -mainly calcare- ous and siliceous cobbles, sands and clays- unconformably Dykes network overlie the Eocene-Miocene molassic sediments and cover to the east the Koziakas ophiolite. These sediments usually A sparse network of gabbro, rodingitized gabbro, pla- form screes, talus cones and alluvial fans, which constitute giogranite and dolerite dykes intrudes the mantle tectonites of the extended Thessalian plateau. (Fig. 2). The smaller dykes, 10-50 cm across, are entirely microgranular; the larger ones, in the meter range or above, retain a microgranular aspect at their margins, but grade in- STRUCTURAL FEATURES side into a doleritic and variously foliated facies ranging in grain size from coarse dolerite to gabbro. The gabbroic The geological structure of the Koziakas ophiolitic com- dykes display N-MORB geochemical characteristics, similar plex is illustrated in three characteristic cross-sections (Fig. to those of oceanic gabbros from other ophiolitic complexes 2). This ophiolite sequence shows evidence of intense defor- (e.g. Kelemen et al., 1997; Tsikouras and Hatzipanagiotou, mation due to multiple tectonic events, which have been im- 1998; Monnier et al., 1999), while the doleritic dykes dis- printed from Jurassic to Quaternary. Statistical plot of the play subalkalic (E-MORB) geochemical characteristics strike of the dykes network, reveals that they have two main The plagiogranite dykes are scarce, with up to 1 meter crosscutting directions: the first strikes NE-SW and dips thickness and a medium-grained isotropic texture. The 50o-70o to the NW and the second strikes N-S to NNW-SSE chemistry of the Koziakas plagiogranite is similar to the with a dip at about 65o-80o to the W (with today astronomic other well-known plagiogranites and display VAG-type references; Fig. 3). The first set, which comprises mainly affinities (Pomonis, 2003). Representative medium-grained plagiogranitic (VAG-type) and gabbroic dykes with N- gabbroic and plagiogranitic dykes transected by doleritic MORB geochemical characteristics, is locally crosscut by dykes, occur between the and Fylira villages (Fig. 1). In the second set comprising E-MORB doleritic dykes. It is some cases, the medium to coarse-grained gabbroic dykes, therefore suggested that the first set of dykes is older and have been extensively rodingitized. In places, however, in- possibly related to the earliest phase of sea-floor spreading tense deformation of gabbroic dykes along shear-zones has during Early - Middle Jurassic, which occurred in a NW-SE resulted in a tectonic lineation with plagioclase-rich do- direction, while the second set of dykes is a minor trend. mains and mylonitic textures. Analogous occurrences have been observed in the Mirdita zone (Albania), where dykes with IAT geochemical affini- Volcanic rocks ties cut the oceanic mafic-ultramafic basement of the west- ern MOR basalts of Jurassic age (Bortolotti et al., 1996; Dolerites, massive basalts and basaltic pillow-lava flows 2002; Hoeck et al., 2002). form distinct volcanic sequences inside the ophiolitic mélange, while they locally thrust over peridotites and pass upwards to basaltic pillow-lava flows occupying the top of the ophiolitic pile. Macroscopic distinction between dolerite and massive basalt is not always easy and they have not been therefore distinguished in the geological map by a clear geological boundary (Fig. 1). The lavas are usually dark green, dark grey, or blue-grey when fresh. Geochemi- cal data point out that the Koziakas volcanic rocks are clas- sified as basalts with N-MORB features, sub-alkalic basalts resembling enriched mid-ocean ridge basalts (E-MORB), al- kali basalts with geochemical characteristics analogous to typical WPB and low-Ti basalts with boninitic affinities. Locally, doleritic and basaltic dykes with E-MORB and boninitic geochemical affinities crosscut the volcanic blocks into the Koziakas ophiolitic mélange. The thrust contact be- tween massive basalt and serpentinized peridotites forms cataclastic zones up to 5m wide, where irregular fragments of these two lithologies interdigitate in a yellowish or green- ish silty matrix. Locally, the pillow-lavas are intercalated with cherts rich in Mn-oxides of hydrothermal origin Fig. 3 - Orientation diagram for poles of 26 network dyke surfaces. Con- (Scarpelis et al., 1992; Pomonis, 2003). tours at 3, 6, 9, 12, 15, 18%. 80

Two main fold axial trends were observed in the Kozi- ridge, as well as during drifting from the ridge. Analogous akas mantle peridotites. Moderately inclined folds trending structures have been described for other mylonitic peridotites NE-SW characterize the first set. This set is variably over- (Nicolas, 1989; Rassios, 1991). Elsewhere, in the Koziakas printed by axial planar cleavage and younger brittle struc- gabbroic dykes, east of the Xyloparoiko village (see Fig. 1), tures. Flattened parallel folds represent the second set with the mylonitic foliation (up to 2 m wide) produced tectonic N-S to NNW-SSE trend. A smaller number of steeply in- lineations with plagioclase-rich domains. clined folds with E-W trend are restricted to the northern Brittle shears are arrayed along thrust contacts, in shear part of the studied area, because of the arcuate shape of the zones, or superimposed upon older brittle structures. The Koziakas mountain range. Statistical plots of the main axial contact of the Koziakas ophiolite with the WTU on the east- trends on stereograms are shown in Fig. 4. ern part is characterized by a NNW-SSE to N-S thrust zone Most of the Koziakas peridotites and serpentinites dis- dipping about 65o-80o to the east (Pomonis, 2003; Doutsos play a foliation with associated mineral stretching lineation et al., 2006). This thrust contact is attributed to the final em- marked mainly by orthopyroxene crystals. Statistical plots placement of the Koziakas ophiolite over the WTU during on a stereographic projection show two main directions the Eocene-Miocene and is represented by 2-5 m wide ser- (Fig. 5a): the first strikes NW-SE with a SW dip and the pentinite mylonitic zones to the north (between Genessio second, which is restricted to the northern part, strikes ESE- and Vitoumas villages, see Fig. 1), and by cataclastic zones WNW with a SSW dip. 10-15 m wide, to the south (between Mouzaki and Fylira Mylonitic structures occur mainly in the basal parts of the villages). The serpentinite mylonitic zones are characterized Koziakas mantle peridotites. They are pervasive mylonite by S-C structures with reverse motion, indicating thrust zones up to 1m wide, striking NNW-SSE with a steep ENE movement towards WSW and NNW at the northern part dip, capturing the ductile to brittle transition precisely. This (see Fig. 2). Smaller low-angle internal thrust striking NW- fabric is presumed to be a result of shearing during mantle SE with a NE dip, defines the slices detected in the Ophio- asthenospheric movement and its uplift toward the oceanic lite Unit (see Fig. 2).

Fig. 4 - Orientation diagrams for poles of: a) 14 fold axes from the first set and b) 29 fold axes from the second set. Contours at 4, 8, 12, 16%.

Fig. 5 - Orientation diagrams for poles of: a) 68 foliation surfaces from the Koziakas serpentinites (contours at 1, 2, 3, 4%) and b) 51 dip surfaces of faults (contours at 2, 4, 6%). 81

The tectonic fabrics mentioned above are crosscut by three developed in a short-lived embryonic ocean, which never neotectonic sets of steeply dipping faults: the main one con- reached a mature stage (Piccardo et al., 2004; Principi et al., sists of NNW-SSE trending normal listric and reverse faults, 2004). On the other hand, the hypothesis that the Koziakas a secondary younger one includes meter to km-long NE-SW ophiolite could be part of the basement of a true (MORB) reverse slip faults, the third set crosscuts the two older ones oceanic lithosphere as that of the Western Mirdita (e.g. Bec- with NW-SE trending faults. Statistical plot of the dip direc- caluva et al., 1994), of the Pindos Upper Ophiolitic Unit tions of the three main fault systems is shown in Fig. 5b. (Jones and Robertson, 1991; Economou-Eliopoulos and Va- condios, 1995; Saccani et al., 2004) or that of the Dhimaina Ophiolitic Unit in (Bortolotti et al., 2003a; Saccani DISCUSSION et al., 2004) is excluded due to the presence of volcanic rocks with E-MORB, IAT and boninitic geochemical affini- The allochthonous and imbricated nature of the Koziakas ties, implying a SSZ origin. ophiolite represents a significant common feature of all the Strain in the Koziakas peridotites is recorded by a variety ophiolite exposures of the western belt in Greece. This fea- of structures. Ductile strain is expressed by foliation, the ture combined with the presence of a sub-ophiolitic meta- shape and orientation of the minerals, particularly orthopy- morphic sole and an ophiolitic mélange at the base of the roxene, folds and ductile shear zones, including ductile im- ophiolites, reflect their emplacement history. The Koziakas bricate structures. The transition to brittle fabrics such as ophiolitic mélange is part of an extensive mélange belt brittle faults and brittle shear zones is commonly associated found in Greece, Albania and former-Yugoslavia (e.g. Kara- to the development of mylonite zones. The pervasive my- mata, 1988; Jones and Robertson, 1991; Robertson and lonitic zones in the basal parts of the Koziakas mantle tec- Shallo, 2000). Detailed structural work indicates that sub- tonites are presumed to be a result of shearing during sole duction accretion processes may have formed this “block- formation and detachment. Similar structures with analo- in-matrix” mélange as an accumulation of oceanic sedi- gous interpretation occur in the Pindos and Vourinos ophio- ments and crust scraped from the descending slab. These ac- lites (Rassios, 1991; Rassios and Smith, 2000). cretionary processes and subsequent Tertiary re-activation The dominant tectonic fabrics imprinted mainly on the of thrusts were responsible for the pervasive tectonic defor- Koziakas mantle tectonites trend NW to NE and are consis- mation and mixing of the Koziakas mélange. Similar sub- tent with a NE-SW emplacement direction. Similar NE-di- duction-accretion settings are well documented, for exam- rected structures occur in the Pindos, Vourinos and Othris ple, in the adjacent Avdella mélange of the Pindos ophiolite, ophiolites (Robertson et al., 1991; Rassios, 1990; Rassios in the Ermioni complex (Clift, 1996), as well as in the and Smith, 2000). The similarity in orientation for the Pin- Ankara mélange (Tankut et al., 1998) and in Alaska dos, Vourinos, Othris and Koziakas ophiolites could imply (McHugh Complex; Kusky and Bradley, 1999). that they were initially emplaced as a single unit, which has On the other hand, the metamorphic soles have been also not undergone significant internal rotations. widely interpreted as fragments of oceanic crust, volcanic The absence of massive gabbros in the Koziakas Ophi- seamounts and oceanic sediments that were underplated to olitic Unit and their presence as fragments in the sub-ophi- the base of a young hot ophiolite during its initial stages of olitic mélange is attributed to erosion, detachment and thin- intra-oceanic displacement (Spray et al., 1984; Jones et al., ning during obduction and final emplacement of the ophi- 1991). The radiometric dates of the Koziakas metamorphic olitic complex as in other ophiolites (e.g., Lippard et al., sole indicate a Middle to Late Jurassic age (Pomonis et al., 1986). Moreover, the occurrence of the sparse dyke network 2002), which is similar to those reported from amphibolites of the Koziakas ophiolite is not so frequent as in a typical of the western ophiolite belt (e.g. Thuizat et al., 1981; Spray sheeted-dyke complex. However, this case is common, espe- et al., 1984; Hatzipanagiotou et al., 1994; Koepke et al., cially in the ophiolites of the Western Mediterranean, where 1985). This result is consistent with the hypothesis that all the sheeted dyke complex seems to be substituted by sparse the metamorphic sub-ophiolitic soles of the western ophio- dykes accompanied by massive diabase (Coleman, 1977). lite belt formed due to the same major geological event These features are typical of oceanic crust developed in a (Spray et al., 1984; Jones et al., 1991; Hatzipanagiotou et slow-spreading ridge, where the magmatic budget is reduced. al., 1994). However, biochronological studies along the Al- The significant proportion of plagiogranitic dykes (1-2 banides - Hellenides chains indicate the existence of Mid- %vol.) with VAG-type geochemical affinities and the pres- dle-Upper Triassic and Lower Jurassic ophiolitic rocks with ence of doleritic dykes with mainly E-MORB characteristics, MORB geochemical affinities (e.g. Danelian and Robertson, which intrude the mantle tectonites, imply mixing of a de- 2001; Bortolotti et al., 2001; 2003b). pleted mantle source with a hydrous phase (Pomonis, 2003). The mantle sequence of the Koziakas ophiolite is domi- Furthermore, the occurrence of rocks with boninitic affinities nated by residual harzburgitic, lherzolitic and subordinate and lavas with geochemical characteristics ranging from N- plagioclase-bearing lherzolitic mantle tectonites. They dis- MORB to IAT (Pomonis, 2003; Saccani et al., 2003) in the play a variable degree of serpentinization, although primary magmatic cover unit overthrusting the peridotites, or as dis- mineral phases are usually well preserved, indicating that al- tinctive volcanic blocks in the ophiolitic mélange, provides a teration was largely incomplete. The Koziakas mantle tec- good evidence for a back-arc setting rather than a typical tonites are similar to the ultramafic components of all the ocean-ridge one (Pearce et al., 1984b). Analogous MORB- western ophiolites, especially to the adjacent Pindos and type rocks with IAT affinities are also present in Pindos, Othris ophiolites, but they lack economic-value chromite Vourinos, (Jones et al., 1991; Pe-Piper et al., 2004) and Oth- deposits. This absence could be attributed to pervasive duc- ris ophiolites (Valsami et al., 1994; Rassios, 1990; Barth et tile cataclasis that has disseminated otherwise economic de- al., 2003), of supra-subduction zone origin (Smith, 1993) posits throughout a greater volume of rock, as it has been suggesting a close genetic relationship between them and the suggested for Pindos and Othris ophiolites (Rassios and Koziakas ophiolite. On the other hand, recent studies in Al- Smith, 2000). It is also possible that the Koziakas ophiolite banian ophiolites suggest that such different magmatic 82 groups (MOR- and SSZ-type volcanics) have originated tween the West Thessaly Unit (WTU) and the Pelagonian from fractional crystallization from different primary basalts continent. A compression episode during Late Jurassic-Ear- that were generated, in turn, from partial melting of mantle ly Cretaceous resulted in the beginning of oceanic detach- sources progressively depleted by previous melt extractions. ment and subduction. The subduction plane, which plunged According to this model, the initiation of subduction process- eastwards according to the general tectonic transport of the es takes place in proximity to an active MOR and leads to Hellenides, Dinarides and Albanides Units (Jones and contemporaneous eruptions in a fore-arc setting of MORB- Robertson, 1991), caused obduction of the Koziakas ophio- type basalts, generated from the extinguishing MOR and the lite to the east, over the Pelagonian margin. In Early Ter- initiation of SSZ-type volcanics in the SSZ mantle wedge tiary, the collision of Africa with the Eurasia plate resulted from a moderately depleted mantle source (Bortolotti et al., in a new compressional episode that closed the Pindos 2002). Such a model seems less possible for the Koziakas Ocean. During this episode the Koziakas ophiolite was ophiolite, as the amphibolites of the sub-ophiolitic metamor- back-thrust to the west over the WTU, which was in turn phic sole include MORB-type protoliths with IAT influence, thrust westwards over the Pindos flysch. suggesting that the initial stages of intra-oceanic displace- ment took place on an already SSZ-type influenced ocean floor (Pomonis et al., 2002). CONCLUSIONS The presence of gabbroic and plagiogranitic dykes in the Koziakas mantle peridotites, implies an evolutionary history The geology of the Koziakas ophiolite is illustrated in a that involves also metasomatic processes. The gabbroic simplified stratigraphic section (Fig. 6). The Koziakas basal dykes reflect an open system and indicate melt percolation Unit occupies the stratigraphically lowermost members and within the peridotites. An extensive study concerning the belongs to the WTU. It comprises alternated medium-bed- petrogenetic evolution of the Koziakas ophiolitic complex, ded sequences of radiolarites, shales, siltstones and oolithic showed that preferential enrichments of Hf over Zr and Ta limestones of Early - Middle Jurassic age. In places, some over Nb in the plagioclase lherzolite relative to the harzbur- structures analogous to turbiditic currents were observed. gite could imply that metasomatic enrichment controls the This formation occupies the central parts of the Koziakas budget of these elements (Pomonis et al., 2005; submitted). range while the ophiolite crops out in the eastern part. The whole Koziakas ophiolite underwent an extensive The Ophiolite Unit overthrust the Koziakas basal Unit ocean-floor metamorphic episode, resulting in the develop- and consists, from bottom to top, of a well-developed ophi- ment of typical assemblages (i.e. prehnite, actinolite, chlo- olitic mélange, a sub-ophiolitic metamorphic sole, serpen- rite and hornblende). However, in most cases the igneous tinites, serpentinized peridotites, peridotites (harzburgite, texture has been preserved due to hydrothermal circulation lherzolite and plagioclase lherzolite), massive dolerites, with low water/rock ratio (Evarts and Schiffman, 1983). massive basaltes and pillow lavas interlayered with Mn- All these features are consistent with a geotectonic set- rich radiolarites. The ophiolitic mélange comprises a chaot- ting similar to the Pindos and Vourinos ophiolitic complex- ic, multicoloured mixture of rock-fragments of variable es of the western ophiolite belt in the Hellenides (Jones and lithology, shape and size in the form of tectonic blocks, Robertson, 1991). According to this model, the Koziakas surrounded by a highly tectonized matrix. These blocks in- ophiolite suite probably formed in a small backarc basin sit- clude serpentinites, peridotites, gabbros, dolerites, basaltic uated at the eastern margin of the greater Pindos Ocean, be- pillow-lavas, amphibolites, radiolarites, limestones,

Fig. 6 - Tectonic-stratigraphic column of the Koziakas ophiolite. 83 argillitic schists and sandstones. Small outcrops of sub- Bernoulli D. and Jenkyns H.C., 1974. Alpine, Mediterranean and ophiolitic metamorphic rocks, which comprise amphibo- North Atlantic Mesozoic facies in relation to the early evolution lites and metapelites, overthrust the ophiolitic mélange. The of Tethys., In: R.H. Dott and R. H. Shaver (Eds.), Symposium ultramafic rocks of the Ophiolite Unit are in thrust contact on modern and ancient geosynclinal sedimentation. Soc. Econ. with the ophiolitic mélange and represent the upper mantle Mineral. Paleont., Spec. Publ., 19: 129-160. section. A sparse network of dolerite, gabbro (locally Bortolotti V., Carras N., Chiari M., Fazzuoli M., Marcucci M., Photiades A. and Principi G., 2003a. The Argolis Peninsula in rodingitized) and plagiogranite dykes up to 0.5 m wide the palaeogeographic and geodynamic frame of the Hellenides. with a NE-SW and a N-S to NNW-SSE direction intrude Ofioliti, 28: 79-94 the mantle peridotites. The Koziakas ultramafic rocks are Bortolotti V., Chiari M., Marcucci M., Photiades A. and Principi affected by intense deformation, recorded by a variety of G., 2003b. Triassic radiolarian assemblages from the cherts as- ductile and brittle tectonic fabrics. Massive dolerites, mas- sociated with pillow lavas in Argolis peninsula (Greece). Ofi- sive basalts and pillow lavas interlayered with Mn-rich ra- oliti, 26: 75-76. diolarites overthrust the ophiolitic mélange and the mantle Bortolotti V., Kodra A., Marroni M., Mustafa F., Pandolfi L., Prin- peridotites. cipi G. and Saccani E., 1996. Geology and petrology of ophi- Small outcrops of molassic formations (Oligocene- olitic sequences in the Mirdita region (Northern Albania). Ofi- Miocene) from the MesoHellenic trench are restricted to the oliti, 21: 3-20. NE part of the Koziakas range. They cover unconformably Bortolotti V., Marroni M., Pandolfi L., Principi G. and Saccani E., 2002. Interaction between mid-ocean ridge and subduction the ophiolite complex, following a general NNW-SSE direc- magmatism in Albanian ophiolites. J. Geol., 110: 561-576. tion. The stratigraphic sequence of the Koziakas ophiolite Brunn J., 1956. Contribution à l’étude géologique du Pinde septen- ends with Neogene to Quaternary deposits, which form the trional et d’une partie de la Macédoine Occidentale. Ann. Géol. Thessaly basin and overlie unconformably both the ophi- Pays Héllen., 7, 258 pp. olitic rocks and the molassic sediments. Clift P.D., 1996. Accretion tectonics of the Neotethyan Ermioni In the Ophiolitic and Koziakas basal Units the earliest Complex, Peloponnesos, Greece. J. Geol. Soc. London, 153: tectonic activity probably occurred during Middle Jurassic - 745-758. Early Cretaceous. This period is characterised by ductile Coleman R.G., 1977. Ophiolites. Ancient oceanic lithosphere? structures, such as folds with a NE-SW axial trend and Springer, Berlin, Heidelberg, New York, 229 pp. mineral lineations with NE-SW direction that are cut by Danelian T. and Robertson A.H.F., 2002. Neotethyan evolution of secondary brittle structures. Mylonitic zones with NW-SE eastern Greece (Pagondas Mélange, Evia Island) inferred from Radiolarian biostratigraphy and the geochemistry of associated strike have been also observed to interrupt older folds at extrusive rocks. Geol. Mag., 138: 345-563. their base and create fault bent folds. The second tectonic Doutsos T., Koukouvelas I. K. and Xypolias P., 2006. A new oro- episode that occured probably during Middle to Late genic model for the External Hellenides. In: A. H. F. Robert- Eocene, is characterised by ductile-brittle deformation. son, D. Mountrakis and J.-P. Brun (Eds.), Tectonic evolution of During this period, thrusts with a NW-SE direction formed, the Eastern Mediterranean regions. Geol. Soc. London, Spec. and also tight and isoclinal folds with NNW-SSE to N-S Publ., (in press). axial trend and faults with NNW-SSE and ENE-WSW Economou-Eliopoulos M. and Vacondios I., 1995. Geochemistry strike. Finally, normal faults with NW-SE strike have been of chromitites and host rocks from the Pindos ophiolite com- located in Tertiary deposits of the Thessaly basin. They in- plex, northern Greece. Chem. Geol., 122: 99-108. terrupt all the above-mentioned tectonic structures and rep- Evarts R.C. and Schiffman P., 1983. Submarine hydrothermal resent the latest tectonic activity probably during Pliocene- metamorphism of the Del Puerto ophiolite, California. Am. J. Sci., 283: 289-340. Pleistocene. Ferrière J., 1974. Nouvelles données stratigraphiques sur le massif The similar radiometric ages and the comparable geolog- du Koziakas. C. R. Acad. Sci. Paris, 278(D): 995-998. ical characteristics of the Koziakas ophiolitic complex with Ferrière J., 1982. Paleogeographies et tectoniques superposées its adjacent ophiolite suites of Pindos, Vourinos and Othris dans les Hellénides internes: les massifs de l’Othris et de imply that they probably developed during the same margin- Pélion. Soc. Geol. Nord. Publ., 8: 970 al basin volcanic arc regime in the Pindos ocean basin. Hatzipanagiotou K., Pe-Piper G. and Pyrgiotis L., 1994. Sub-ophi- olitic amphibolite soles from the Dafnos Pylia-Kedros area, western Thessaly, Greece. N. Jb. Mineral. Mh. H., 9: 391-402. ACKNOWLEDGEMENTS Hoeck V., Koller F., Meisel T., Onuzi K., Gjata K. and Kneringer E., 2002. The south Albanian ophiolites: MOR versus SSZ We gratefully acknowledge Prof. A.H.F. Robertson and ophiolites. Lithos, 65: 143-164 Jaeger P., 1979. Géologie du Massif du Koziakas et de la chaîne du Prof. G. Principi for a thorough review of the manuscript. 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Received, April 27, 2005 Accepted, November 16, 2005