Genesis of vein-quartz deposits in relation to uplift of the Serbo-Macedonian Zone, N. Greece: Evidence from fluid inclusion studies

A major group of quartz veins occur in two-mica gneisses of the polymetamor phic Vertiskos Formation (VF) of the Permian or older Serbo-Macedonian Zone , The host rocks have been affected by five metamorphic (M-1-M-5) and defor mation phases (D-1-D-5). Vein emplacement in the Kastri vein-quartz deposit , Thessaloniki district, occurred along axial planes of the D-3 regional fo lding event. Vein-quartz is characterized by an early episode of ductile de formation (quartz 1). local cata-clasis and dynamic recrystallization (quar tz 2), overprinted by brittle fracturing. Fluid inclusion studies reveal fi ve types of inclusions: (1) Type 1, carbonic CO2 (+/-H2O) inclusions with c alculated densities of 0.85-0.98 g/cc; (2) Type II, low salinity (0.9-14.4 wt % eq. NaCl) aqueous-carbonic H2O-CO2-NaCl (+/- CH4-N-2) inclusions with highly variable aqueous: carbonic phase ratios, bubble - as well as clew-po int T-h between 200 and 400 degreesC, and a wide range of calculated densit ies from 0.74 to 0.99 g/cc; (3) Type III, aqueous H2O-NaCl (+/- CO2) inclus ions (T-h: 130-269 degreesC - salinity: 2.9 to 12.7 wt % NaCl equiv. - dens ity: 0.85-0.95 g/cc); (4) Naturally decrepitated/leaked type IV inclusions, and, (5) Type V inclusions which occupy recrystallized-uns trained quartz 2 boundaries. Types 1, II, and IV occur together within quartz I grains eit her as isolated inclusions or groups, and are interpreted as coeval and mos t likely related to vein filling. Type III occur mainly in healed brittle f ractures crosscutting both quartz I and 2 and are interpreted as post-vein filling hydrothermal fluids, whereas type V are very late. Natural decrepit ation and leakage textures of type IV inclusions are attributed to internal overpressures, and linked to post-D-3 regional decompressive uplift of the VF. Uplift from mid-crustal depths is also supported by deformation micros tructures in quartz which indicate that the vein was forced through the duc tile-brittle transition resulting in the overprinting of ductile strain tex tures by brittle fractures. Wide ranges in aqueous-carbonic inclusion compo sitions, densities, and P-T conditions of entrapment may have been derived by a combination of various processes accompanying decompression, such as p ost-entrapment re-equilibration, syn- to postmetamorphic fluid evolution an d immiscibility. In the absence of independent constraints on the P-T-X con ditions of vein mineralization and a solid relative chronology of inclusion types I, II, and IV, a choice among alternative interpretations for variab le fluid iclusion properties is difficult. However, each process may have b een dependent on decompresssion and uplift. Type III late inclusions may ha ve been trapped late along the uplift path during ongoing decompression. Up lift of VF may be related to the Late Cenozoic evolution of the region char acterized by extensional tectonics.