High grade blueschist metasomatism in a serpentinite melange, Syros, Greece

• Main Author: Robson, Rachel M. L
• Format: Dissertation
• Language: English
• Published: University of Edinburgh 2000

This thesis is concerned with the mineralogy and chemistry of metasomatic reaction zone formed around high-grade blueschist facies blocks (15 kb, 450-500°C) of meta-igneous and metasedimentary lithologies within a serpentinite melange matrix on the island of Syros, Greece. The zones are composed of a reaction rind, cm to 10's of cm thick, containing abundant chlorite, sodic and calcic amphibole and talc, and a zone of altered block. The zones are the result of diffusion-controlled bimetasomatism and, importantly, the products of a discrete, short-lived infiltration event producing amphiboles, which occurred during bimetasomatic zone growth. The generalised bimetasomatic zone sequence is: block | altered block | chlorite | talc ± chlorite ± carbonate ¦ serpentinite matrix with carbonate only occurring in the 'talc ± chloride ± carbonate' zone where the 'block' is carbonate-bearing schist. The position of the original block-matrix contact is generally within the chlorite zone, and is in some cases marked by a relict zone of chromite and Cr-rich pyroxene. The composition and width of the 'altered block' zone was found to vary with block composition. In meta-felsites, with unaltered cores of jadeite + quartz + accessories, the 'altered block' zone is around 20-30 cm wide, and is characterised by a progressive outward decrease in quartz content with a thin zone of omphacite rock occurring at the outermost extent of this zone. In dioritic metagabbros, containing glaucophane-epidote-garnet-pyroxene ± minor quartz, bulk compositional change towards the block edge is less marked but altered block zone-width is greater. The volume proportion of garnet and pyroxene also increases towards the block edge as a result of desilication by the reaction: glaucophane + epidote + omphacite1 = garnet + omphacite2 + SiO2 + H2O in which the omphacite becomes more jadeitic. In glaucophane-epidote-garnet-pyroxene ± minor quartz ferrogabbros, this desilication reaction also occurs, but no consistent bulk compositional changes towards the block edge are measurable. The zone of 'altered block' in ferrogabbros is estimated to be up to 2-3m wide. The 'altered block' zone at edge of carbonate-bearing schists is composed of 'pseudo-eclogite', an omphacite ± epidote ± garnet rock, the product of a metasomatic decarbonation reaction with antigorite as the CO2 sink.