The origin of amphibolites from metamorphic soles beneath the ultramafic ophiolites in Evia and Lesvos (Greece) and their geotectonic implication

Abstract

The dismembered Jurassic ophiolitic complexes of the two Greek islands, Evia and Lesvos show significant similarities concerning their general structural, petrographic and geochemical characteristics. However, their geotectonic position is quite different, the Evia ophiolites belonging to the “Western Ophiolite Belt” of the internal Hellenides, whereas the Lesvos ophiolites seem to be part of the “Eastern Ophiolite Belt”, representing a possible link to the Pontides. Both ophiolites consist of large ultramafic masses, which are emplaced onto continental crust. The overthrust zones comprise flysch and ophiolitic mélange as well as amphibolitic metamorphic soles beneath the ultramafics. The identification of the protolithic character of the amphibolites as well as their metamorphic and deformation history allows a clarification of their complex emplacement processes from their original extensional oceanic ridge environment within the Pindos and Vardar marginal basins into a compressional supra-subduction zone, island arc and continental environment. On the basis of relict magmatic minerals and textures as well as on major and trace element bulk chemical composition, the medium to coarse grained Evia amphibolites are interpreted as former gabbroic and cumulate rocks similar to equivalent amphibolites dredged from numerous fracture zones in the Atlantic oceanic crust. Horizontal and vertical displacements which are able to produce such foliated and mylonitized gabbroic rocks suggest the existence of major deep reaching transform faults, which may be considered as one of the most important locations for the breakup of oceanic crust and lithosphere and subsequent incipient subduction. The Evia amphibolites could have been formed by a two-stage process: first, as gabbros that formed part of the gabbroic oceanic crust and secondly representing as small gabbroic and cumulate bodies within the ultramafic mantle. Either of these were transformed into low-grade epidote amphibolites under oceanic metamorphic facies conditions and deformed by transform fault displacement mechanism. Subsequently, within the Pindos intra-oceanic basin, these transform faults also facilitated obduction of slices of oceanic mantle and crust onto an accretionary wedge. In this respect, the Evia ophiolite complex shows a very similar emplacement process to that of the Othris, Vourinos and Pindos ophiolites, which can be interpreted using a combination of an intra-oceanic transform fault thrust and an obduction model. In this respect, the “Evia transform fault–overthrust model” sheds new light onto the discussion of the age of formation of oceanic crust, as well as on the timing of subduction and obduction. The so called “sole amphibolites“, especially those of only low-grade epidote-amphibolite facies may have already been formed during mid-ocean floor spreading and transform fault movements and not later during the ophiolite emplacement processes. Therefore all of the ages of oceanic crust and amphibolites, both from sedimentary and paleontological evidence as well as from radiometric data indicate a specific time window between the generation of the older parts of oceanic crust around 170 Ma (Bajocian) and their emplacement processes at approximately 155 to 150 Ma. This leads to the conclusion that the Pindos marginal oceanic basin could have at least an age spread of 15 to 20 Ma. In contrast, the Lesvos amphibolites are fine to medium grained with granoblastic and nematoblastc textures with relics of clinopyroxene and plagioclase. Cataclastic fabrics are also present as well as retrograde mineral assemblages from greenschist to prehnite facies metamorphic overprinting. According to bulk chemical analyses, these amphibolites are identified as MORB-type oceanic basalts as well as SSZ (supra-subduction zone/IAT —island arc tholeiite) basalts and olivine gabbros. The Lesvos amphibolites are seen as basaltic and gabbroic slices, which were incorporated into a supra-subduction, gabbro bearing island arc environment, overridden by larger slices of depleted lherzolite and harzburgite mantle. Since the amphibolites show in many cases highly deformed fabrics and a rather high-grade metamorphism as well as MORB basaltic compositions, an intra-oceanic thrusting cannot be excluded. Their origin can rather be placed in the Vardar oceanic basin than in the Pindos basin.