Tectonometamorphic and hydraulic processes along afossil subduction plate interface in the northern MirditaOphiolites (Bajram Curri, Albania)

Abstract

The Western Vardar ophiolite crops out along the entire Balkan Peninsula. In northeast Albania near Bajram Curri, the Jurassic plate interface reveals a metamorphic sole below peridotites of the Mirdita Ophiolite. There, I mapped an area of around 35 km2 along the southeast-dipping fossil plate interface and performed various micro-scale analyses such as polarisation microscopy, Raman-spectrometry, Raster-electron microscopy and Micro-x-ray fluorescence on metamorphic sole as well as mantle lithologies. Three major units are distinguishable. The mantle harzburgites overthrust a tectonic melange which itself can be separated into a non-metamorphic broken formation and a metamorphic sole. The metamorphic sole itself was accreted at a lower structural level than the broken formation and later emplaced on top of it. The tectonic melange is tightly folded with fold axes plunging gently towards NNW or NNE. The metamorphic sole shows an inverse metamorphic field gradient up to greenschist-facies conditions near Bajram Curri and granulite-facies conditions near Shkelzen. Granulite-facies conditions are preserved as relictic blasts with albite, margarite & muscovite as pseudomorphs after feldspar; as well as garnet and monazite. S-c’-fabrics in calcschists show top to the west shear sense direction, suggesting westward emplacement of the overlying ophiolites. All lithologies in the metamorphic sole are characterised by a later lower-greenschist-facies overprint, evolving from dynamic to static recrystallisation. Stilpnomelane, quartz and white mica form a typical mineral assemblage giving temperatures between 280°-450°C and pressures between 4.3-5.5 kbar. Right above the southeast-dipping plate interface, foliated upper plate harzburgites with southeast dips crop out. Mylonitic layers preserve a mineral assemblage with major olivine and minor orthopyroxene, clinopyroxene, as well as chromium-rich spinel (Cr# 72). Clinopyroxene lamellae in Aluminium- depleted (Al2O3 0.85 wt%) orthopyroxene clasts give lower temperature limits for recrystallisation of 1042°-1063°C under low-pressure conditions. The mantle source was either already depleted or the primary harzburgite was melted in a second stage. In the later evolving ultramylonitic layers, the mineral phases exhibit average grain sizes of 5 µm. There, phases of two clinopyroxene with low-calcium and high-calcium content respectively occur. Serpentinisation decreases upsection and developed in three texturally distinct types that bear different serpentine minerals and opaque phases.