MINERALIZED VEINS RESULTING FROM FLUID-FLOW IN DECOLLEMENT ZONES OF THE SICILIAN PRISM - EVIDENCE FROM FLUID INCLUSIONS

Imbrications took place at the frontal part of the Sicilian accretiona ry prism during the Plio-Quaternary. The structural setting shows a su perposition of southward directed thrust-sheets bounded by two decolle ment levels. Syntectonic veins were sampled in thrust fault zones loca ted at the Frontal part of the prism. In the lower thrust fault zone, hydraulic breccias include quartz and calcite veins. The initial cryst allisation fluids have been entrapped in these veins and preserved as different generations of fluid inclusions. Primary intracrystalline aq ueous fluid inclusions are composed of low salinity water (near 0.8 wt % equ.NaCl) depleted in gas and display homogenisation temperatures i ncreasing from quartz to calcite to values up to 200 degrees C. In cal cite veins, hydrocarbon fluid inclusions containing light aliphatic oi l indicate the occurrence of organic matter in the tectonic wedge with a thermal evolution comprised within the oil window. The pressure req uired is the lithostatic pressure, in agreement with the conditions im plied by the tectonic setting accounting for the formation of hydrauli c breccias. It ranges from 140 degrees C-0.8kbar for quartz up to 235 degrees C-1kbar for calcite deposition. Intersection of oil and aqueou s solution isochores lead to comparable values. In addition the occurr ence of pyrobitumen particles in the veins indicates that the fluids r eached temperatures higher than 150 degrees C. These high temperature fluids are localized in the lower fault zone. In the upper fault zone, syntectonic calcite veins only display low temperature fluid inclusio ns. The results of X-ray studies on the foliated host-rock of the vein s provide evidence for a highly localized incipient thermal transforma tion of the clay minerals (I/S of R(3) type) at the contact with the h ydraulic breccia, while the overall mass of the surrounding shales rem ained unaffected (I/S of R(0) type). The strict localization of the ve ins in the fault zones suggest that dewatering of the Sicilian wedge o ccurred along high-permeability decollements and faults. In the lower fault zone, warm fluid circulation of fresh water was responsible for the hydraulic fracturing and the subsequent vein formation restricted to the basal decollement, where a thermal desequilibrium occurred betw een the fluids and the shale matrix. This may be interpreted as the re sult of large-scale advection of fluids arising from the deeper intern al zone of the prism and flowing along the basal decollement.