Preservation of Blueschist-facies Minerals along a Shear Zone by Coupled Metasomatism and Fast-flowing CO2-bearing Fluids

Abstract

Two types of blue halo (types I and II) composed of blueschist-facies minerals are centered around a brittle, normal shear zone in greenschist-facies rocks on the island of Syros, Aegean Sea, Greece. The shear zone is steeply dipping and cuts a near-horizontal layer of greenschist-facies rocks (albite + epidote + actinolite + chlorite + quartz). Type I and II blue haloes are 0·3 m and c. 1 m wide respectively, and are seen on both sides of the shear zone. The inner type I haloes are composed of nearly pure glaucophane schist and were formed by metasomatic addition of Na2O and SiO2, and to a lesser extent of K2O and large ion lithophile elements (LILE), coupled with loss of CaO, Al2O3 and MnO. The outer type II haloes consist of a carbonated blueschist-facies assemblage (glaucophane + calcite + phengite + epidote + garnet + quartz). These experienced only slight metasomatic changes (i.e. addition of K2O and LILE), which cannot alone explain halo formation. We present petrological, geochemical and thermodynamic evidence that this assemblage was preserved at greenschist-facies conditions because XCO2 was elevated by flow of a CO2-bearing fluid along the shear zone, which was approximately contemporaneous with greenschist-facies hydration in the surrounding rocks. We further note that the flux of CO2-bearing fluid along the shear zone was rapid with respect to the fluid flux in the surrounding rocks. Mass-balance calculations reveal that the fluid flux within the shear zone was at least 100–2000 times greater than the fluid flux within the surrounding rocks. Mineral textures show greenschist-facies minerals replacing blueschist minerals in the type II haloes, supporting our interpretation that the blueschist-facies minerals were preserved during greenschist-facies retrogression. A simplified P–T vs XCO2 pseudosection confirms that preservation of carbonated blueschist can occur at greenschist-facies conditions in the presence of a CO2-bearing fluid.

Preservation of high‐P rocks coupled to rock composition and the absence of metamorphic fluids

“…However, HP–LT mineral assemblages formed due to crustal thickening associated with subduction are often partly or entirely replaced at lower pressure and higher temperature conditions associated with exhumation (Ernst, ). Preservation of HP–LT rocks can reflect an absence of metamorphic fluids (Schliestedt & Matthews, ), restriction of fluid availability to specific P–T conditions (Schorn, ) or fluid composition effects (Breeding, Ague, Bröcker, & Bolton, ; Kleine, Skelton, Huet, & Pitcairn, ). The presence or absence of metamorphic fluids as well as fluid composition effects, usually reflecting channelized fluid flow, provides possible explanations for the occurrence of HP–LT (eclogites and blueschists) rocks in close proximity to rocks that record metamorphism at lower pressure and/or higher temperature (greenschists, amphibolites and granulites) (Austrheim, ; Glodny, Austrheim, Molina, Rusin, & Seward, ; Glodny, Kühn, & Austrheim, ; Glodny, Ring, & Kühn, ; Jun & Klemd, ; Proyer, ; Young & Kylander‐Clark, ).…”

“…vein‐fractures, shear zones); whereas, preservation of HP–LT mineral assemblages occurs in the parts of a rock volume that have been dry (Matthews & Schliestedt, ; Parra, Vidal, & Jolivet, ). Less commonly, metamorphic fluids preserve HP–LT mineral assemblages by maintaining a fluid composition which stabilizes the HP–LT assemblage relative to the replacement assemblage (Kleine et al., ). Despite this important kinetic role played by metamorphic fluids, few systematic studies of the interplay between metamorphic fluid flow and other factors (e.g.…”

High‐temperature, decompressional equilibration of the eclogite facies orogenic root (Western Gneiss Region, Norway)

“…Andersen, Jamtveit, Dewey, & Swensson, ; Hacker, ; Haifler & Kotková, ), how the great depths can be recorded (e.g. O'Brien & Rötzler, ; Ravna & Terry, ) and how the HP and UHP rocks can be preserved on their way back to surface conditions (Austrheim, Erambert, & Engvik, ; Kleine, Skelton, Huet, & Pitcairn, ; Štípská, Powell, & Racek, ). The dynamics in the deep orogenic root have been traced in rocks (e.g.…”

Controls on fluid movement in crustal lithologies: evidence from zircon in metaconglomerates from Shetland

“…; Kleine et al . ). Zircon morphology provides a sensitive monitor of fluid pathways in crustal rocks capable of recording a range of different interactions.…”

“…Other workers have shown that porosity and permeability may be controlled by mineralogy (Arghe et al 2011;Ferry et al 2013), and equilibrium wetting angles (Holness 1993;Price et al 2004), but there are many reports of a structural control of fluid pathways (e.g. Skelton et al 1995;Abart et al 2002;Pitcairn et al 2010;Kleine et al 2014). Zircon morphology provides a sensitive monitor of fluid pathways in crustal rocks capable of recording a range of different interactions.…”