Correlation of Neotethyan rifting related peperitic basaltic occurrences in the Dinarides, displaced fragments of the Dinarides and in the Hellenides

Volcanological, mineralogical, petrological and geochemical studies on pillow lavas, peperites and hyaloclastite breccias as parts of basaltic extrusions of Triassic age into uconsolidated deep water sediments were carried out in the Jurassic mélange of the Darnó Unit (NE Hungary), which is a displaced fragment of the Dinarides. The results were compared with similar occurrences in the Kalnik Mts. (Croatia) and Vareš-Smreka (Bosnia and Herzegovina) in the Dinarides and in the Stragopetra Mts. (Greece) in the Hellenides. Earlier sedimentological and lithostratigraphical studies suggested advanced rifting-related origin to these formations; however petrological and petrochemical comparisons are made in the framework of the present study.
   At the studied localities at most six different volcanological facies can be distinguished (coherent pillow, closely packed pillow, in-situ hyaloclastite breccia, pillow fragmented hyaloclastite breccia, isolated pillow breccia and peperitic basalt); in the Kalnik Mts., a complete submarine lava mould-flow complex with all of these facies is known. Comparing the other localities to this complete system, the volcanological facies can be identified even in occurrences characterized with small blocks. In the studied Darnó Hill quarries, the closely packed pillow, the hyaloclastite breccia and the peperitic facies were observed, suggesting an originally distal position in the submarine lava flow. At Vareš-Smreka and Stragopetra only the peperitic facies is known. This facies bears high importance, as it forms when the lava arrives into the water soaked sediment; mingling the basalt with the limey mud is a clear evidence of early rift-type formation (i.e. formation above the CCD level), and this can be used in the field to distinguish these associations from the Jurassic ophiolites (formed below the CCD level) occurring in the same mélange.
   Petrographic features show similarities among the basalts of the studied localities; the textures are sphaerolitic and variolitic, while the main rock forming components are albitized plagioclase with skeletal crystal habit, calcite/chlorit /serpentine pseudomorphs after olivine, opaque minerals (pyrite, chalcopyrite, hematite) and microcrystalline material as groundmass while clinopyroxene and glass are rare.
   Results of the fluid inclusion studies and examination of the hydrothermal minerals at all the studied localities show that extended fluid circulation system did not develop, but rapid cooling was characteristic in the seawater-dominated hydrothermal system. Several stages of the alteration can be distinguished; after the chloritization of the groundmass, the hydrothermal infillings (quartz, chlorite, epidote, prehnite, pumpellyite, calcite and zeolite) of the amygdales, veins, pyjamas-type basalts’ mineral bands and earlier feeding channels of lava lobes have formed, then finally the low-temperature layer-silicates precipitated. Fluid inclusion and chlorite thermometry data suggest shallow depths of 1.4-4 km for the fluid/rock interaction processes. Hence the advanced rifting-related origin is also more supported than the mid-oceanic ridge-related setting.
   Petrochemical features of the studied rocks show mainly within-plate basalt characteristics, while the MORB-features are subordinate. The studied Triassic basalts are forming a group easily distinguishable from the Jurassic basalts of the same mélange on the different discrimination diagrams. The high Zr/Y ratios (above 4) are also characteristic to the within plate basaltic volcanics. Thus the geochemical data also support that the Triassic pillow basalts, containing pelagic carbonate peperitic facies, are related to the advanced stage rifting of the Dinaridic-Hellenidic Neothethys. However the good correlation among the different studied occurrences and their genetic relationship are also shown with the help of the REE pattern which show slight enrichment from La to Gd in comparison to the Jurassic ophiolites.