Petrogenesis and tectono-magmatic significance of volcanic and mantle rocks from the Albanian-Greek ophiolites: Implications for the Triassic-Jurassic evolution of the Dinaride Tethyan branch

The Albanian-Greek ophiolites crop out in two major ophiolitic zones: 1) the External Ophiolites (EO) which can be subdivided into: a) a westernmost belt (EOa): W Mirdita, Pindos, Koziakas, Othrys; and b) an easternmost belt (EOb): E Mirdita, Vourinos, Argolis. 2) the Internal Ophiolites (IO) which can be subdivided into: a) Almopias ophiolites (IOa) and b) Guevgueli ophiolites (IOb). Many stratigraphic and petrological similarities can be observed between ophiolites from the IO and EO.
   The Albanian-Greek ophiolites and sub-ophiolitic mélanges include eight different types of volcanic rocks: 1) Triassic, within-plate alkaline rocks (WPB) which are included in both EO and IO mélanges; 2) Triassic high-Ti mid-ocean ridge basalts showing enriched compositions (E-MORB) (EO and IO mélanges); 3) Triassic (EO and IO mélanges) and Jurassic (EOa) high-Ti mid-ocean ridge basalts showing normal compositions (N-MORB); 4) Jurassic medium-Ti basalts (MTB) (EOa); 5) Jurassic low-Ti, island arc tholeiitic (IAT) rocks (EOb and IOa); 6) Jurassic very low-Ti (boninitic) rocks (EO and IO ophiolitic and mélange sequences); 7) Jurassic back-arc basin basalts (BABB) (IOb); 8) Triassic (EO and IO mélanges) and Jurassic (IOb) calc-alkaline rocks (CAB). They also include two types of mantle peridotites: 1) depleted lherzolites (EOa) and 2) harzburgites showing various degrees of depletion (EOa, EOb, IOa). The main aim of this work is to identify the possible petrogenetic mechanisms, in terms of mantle sources, primary melt generation, and mantle residua, for the distinct lava groups and their related tectonic settings of formation, in order to propose a reconstruction of the tectonic evolution of the Mesozoic Dinaride Tethys as follows:
   1) From the Late Palaeozoic-Early Triassic, extensional tectonics affecting the Gondwana trigged the rifting of the continental lithosphere. The magmatic activity included: (a) CAB rocks generated from a sub-continental mantle formerly modified by SSZ geochemical components; (b) alkaline WPBs deriving from an OIB-type mantle source.
   2) During the Middle-Late Triassic, the uprising of primitive asthenosphere led to the generation of the Tethyan oceanic lithosphere. This stage is associated with the formation of: (a) N-MORBs derived from ~10-20% partial melting of primitive asthenosphere; (b) alkaline WPBs most likely erupted in seamounts or off-axis areas; (c) E-MORBs generated from ~12% partial melting of primitive asthenosphere influenced by the OIB-type component. The residual MORB mantle is represented by the depleted lherzolites cropping out in the EOa.
   3) From the Early-Middle Jurassic, the tectonic regime was dominated by intra-oceanic convergence associated with the development of MTBs and IATs which derived respectively from ~10% and 10-20% partial melting of the MORB residual mantle with variable addition of subduction components. Afterwards, the progressive slab roll-back led to mantle diapirism and incipient intra-arc spreading associated either with 10-20% partial melting of the MTB and IAT residual mantle (harzburgite) or with ~30% partial melting of the MORB residual mantle (depleted lherzolite), both enriched in LREE by subduction-derived fluids. These partial-melting events produced the boninitic magmas in both fore-arc and inner arc and left, as residual mantle, the depleted harzburgites which are commonly found in the EOa, EOb and IOa.
   4) During the Late Jurassic, a magmatic arc developed onto the Eurasia continental realm, leading to the formation of CAB rocks by ~15-20% partial melting of depleted peridotite mantle significantly enriched in Th and LREE by subduction-derived fluids. Soon after, extension in the back-arc region led to the uprising of primitive (MORB-type) asthenosphere, which was enriched in Th and LREE by the nearby subduction. 10-20% partial melting of this mantle source produced the BABBs cropping out, with CAB intercalations, in the IOb.