Geochemical and petrogenetic features of plagiogranite dykes in the Koziakas ophiolitic melange (W. Thessaly, Greece)

摘要

Plagiogranite and dolerite dykes with boninitic affinities intrude thrust-bounded blocks of serpentinized harzburgite at the eastern part of the Koziakas ophiolitic melange. Geochemically they are characterized as oceanic plagiogranites with trondhje-mitic composition and metaluminous affinities. Low ZREE and trace element contents indicate derivation from a depleted mantle source. Geochemical modelling shows that the Koziakas plagiogranites could have originated by 5-20 % partial melting of a gab-bro precursor. Locally, an early-stage high-T hydrothermal alteration event seems to have caused the anorthitization of magmatic plagioclase. This hydrothermal event was overprinted by a low-T retrograde alteration. The overall field data and geochemical affinities suggest a supra-subduction zone origin for the studied plagiogranite dykes, which is compatible with the geotectonic environment of the Koziakas ophiolite.