Genesis and distribution of Ca-Al Silicates in Aegean blueschists and implications for fluid flow in subduction zones

High grade blueschist metamorphism on the Aegean island of Syros involves the widespread growth of the caic-silicate minerals lawsonite, zoisite and epidote / clinozoisite in a wide range of lithologies. In calcareous metasediments the only realistic lawsonite-generating reaction appeared to be one involving calcitebreakdown and thus for significant reaction progress CO2 would have to have been flushed from the rock by the infiltration of a water-rich fluid. Previous studies had noted local concentrations of lawsonite notably at the margins of meta-igneous bodies which apparently supported this hypothesis (Ridley 1982, Barr 1989). However, stable isotope and T-XCO2 modelling studies by Barr (1989) suggested that local buffering of fluid composition on a cm to m scale occurred, apparently ruling out widespread pervasive fluid flow. This apparent paradox between abundance of lawsonite in the metasediments and the syn-metamorphic fluid regime prompted a comprehensive study of possible controls on the growth of lawsonite. The distribution of lawsonite was mapped in the metasediments. Structural mapping was used to elucidate the strain history of the syn-blueschist deformation. Detailed petrographic and microfabric analysis of relevant samples was undertaken to document their petrologic and microstructural development and to identify lawsonite forming reactions. Changes in bulk rock composition accompanying the growth of lawsonite were assessed using XRF analysis coupled with the Isocon method of Grant (1986). Finally oxygen isotope analysis of quartz separates was used in an attempt to assess the extent of syn-metamorphic fluid flow. The results show that in the greyschist units of northern Syros, lawsonite growth is controlled by a syn-deformational, fracture-controlled fluid infiltration event, traces of which are visible as quartz veins. Ca-bearing, water-rich fluid pervasively infiltrated the schists adjacent to the fractures resulting in the growth of large amounts of lawsonite by metasomatic reactions involving the breakdown of mica. The scale of fluid transport and therefore the extent of metasomatic alteration is controlled by the deformation and compositional variation. These observations confirm the suggestion in the previous studies that lawsonite growth is at least partially controlled by the infiltration of an externallyderived fluid. The preservation of small scale variation in fluid composition within significant parts of the metasediments is explained by the channelized nature of the event. The timing of the infiltration event relative to the strain history suggests that it is synchronous with the transfer of the metasediments to the hanging wall of the subduction zone by the successive underplating required to generate uplift and so promote un-roofing. Thus, the results of the study shed light on important aspects of the fluid regime in subducted rocks during a critical phase of their tectonic history. Indeed, it is suggested that the process of underplating, which will inevitably perturb the local thermal structure, may be responsible for initiating the infiltration event by causing the generation of the fluid, or increasing the permeability of the infiltrated schists or both. Consideration of the inferred petrogenetic development of subducted material suggests that the fluid is produced by the dewatering of the slab during the transition from blueschist to eclogite assemblages. The similarity of the alteration to rodingitisation suggests that it may have interacted with ultramafic material prior to entering the greyschists.