Fluid flow and deformation partitioning during blueschist exhumation, Syros, Greece

Deformation, metamorphism and fluid infiltration are important processes in crustal evolution. This thesis is concerned with how these processes interact to remodel the crust after orogenesis. Localisation of strain, fluid infiltration and metamorphic reactions all play a part in controlling the rheological properties of rocks deep in the crust and these factors will therefore influence the process of exhumation, and control the structural and metamorphic development of an exhuming terrain. It has been shown that these processes do not act independently. Complex patterns of inter-linked deformation and hydration of metamorphic assemblages are often superimposed, such that orogenic fabrics and mineral assemblages are partially overprinted by exhumation fabrics and assemblages. Thus, a chronology of deformation, metamorphism and infiltration can be determined and by identifying the depths and temperatures at which events occurred, the exhumation process can be unravelled. The island of Syros in the Attic Cycladic blueschist belt of Greece was chosen for study as it is characterised by a high pressure blueschist facies assemblage (T = 450 °C and P >14 kbars) which has been variably overprinted within the greenschist facies stability field. The partial overprinting has resulted in the preservation of high pressure fabrics. This preservation combined with local down-pressure superimposition of deformation fabrics and recrystallisation allows a chronology of events to be determined making Syros an ideal locality for the study of structural and metamorphic evolution during exhumation. Syros consists of interleaved schist and marble units, with local metabasite horizons. The units are layered and generally dip shallowly to the NE. The partial overprinting of metamorphic assemblages has been used to link down-pressure fabric development, fluid infiltration and metamorphism, whilst the interleaving of lithologies has allowed an assessment to be made of the lithological controls on fluid channelling. Direct observations of fluid channelling cannot always be made. Evidence for fluid infiltration is often overprinted during recrystallisation and fabric development. This study has therefore combined field observations with petrographic, geochemical and stable isotope studies to document fluid pathways. By combining field and analytical techniques I have demonstrated that fluid pathways are spatially associated with the partitioning of deformation, and in reactive lithologies with retrogressive metamorphism. In northern Syros the greenschist overprint is localised and spatially associated with brittle fracture-controlled fluid infiltration. Locally, blueschist assemblages are cut by greenschist facies veins, which are surrounded by greenschist retrogressive haloes. I have shown by geochemical analysis that retrogression in vein haloes is the result of fluid infiltration from the vein into the host rock. In haloes containing albite porphyroblasts, hydration is accompanied by Na-influx. These localised areas of retrogression have been used to link fracture controlled fluid infiltration and metasomatic exchange to greenschist recrystallisation. In comparison the south of the island documents a more pervasive greenschist facies overprint. Fabrics associated with greenschist recrystallisation in the south of Syros are locally found to be both ductile and brittle, as well as static (inherited) from the blueschist event. Fluid infiltration into marble bands is documented isotopically by the formation of stable isotope fronts at marble contacts. I have modelled the isotope fronts using chromatographic theory to show that fluid infiltration of marble bands associated with front formation was not by pervasive flow. The spatial arrangement of hydrated silicates in impure marble horizons and the heterogeneities in 6180 composition in the front imply that fluid infiltration was localised. Macroand micro-scale stable isotope and geochemical studies of calcite from the marble bands document localised fluid infiltration in zones of fracture and cataclasis. In marble bands that have been affected by ductile deformation, or that have subsequently recrystallised, the patterns of isotopic heterogeneity are less well defined. This is interpreted to be the result of homogenisation of calcite isotopic compositions during recrystallisation. Deformation fabrics accommodating pure shear, layer-parallel extension are identified in all lithologies. In schist units the fabrics are defined by blueschist and locally in southern Syros by retrogressive greenschist mineral assemblages. It is inferred that this horizontal layer extension was the main mechanism for exhumation of the metamorphic terrain, from peak metamorphism through to greenschist facies. Extensional greenschist veins and faults accommodate extension in the same direction, documenting a continuum of E-W to NE-SW directed extension into the brittle regime. The localisation of extensional strain within the terrain during decompression appears to have controlled fluid pathways, by transiently increasing the permeability of all lithologies. This has led to the resulting spatial distribution of retrogressive metamorphism on Syros.