Fluid Processes in Subduction Zones: Insights from the Sivrihisar (Turkey) Subduction Complex

Abstract

High-pressure/low-temperature (HP/LT) metamorphic rocks from exhumed subduction complexes provide us with an accessible record of subduction processes, including metamorphism, deformation, and fluid-rock interaction. This thesis presents the results of an integrated geochronologic (in situ UV laser 40Ar/39Ar phengite geochronology), geochemical (bulk rock and mineral major and trace element analysis), and microstructural (electron backscatter diffraction, X-ray computed tomography) study of the high-pressure rocks from the Tavşanlı Zone, a Late Cretaceous subduction complex exposed in northwest Turkey. In the Tavşanlı Zone, one of only ~19 lawsonite eclogite localities in the world, pristine lawsonite eclogite was exhumed from depths of 80 km as part of a structurally coherent metabasaltic and metasedimentary sequence. Because there are few places on Earth in which rocks have returned largely unaltered from these depths in an oceanic subduction zone, the lawsonite eclogite- and blueschist-facies rocks of the Tavşanlı Zone represent a rare archive of subduction processes occurring at deep forearc depths (45 – 80 km). Although the P-T conditions of metamorphism and deformation of the eclogite- and blueschist-facies rocks of the Tavşanlı Zone are well-known, the timing of HP/LT metamorphic events is uncertain because geochronologic studies of the Tavşanlı blueschists have yielded a wide range of ages (60 – 192 Ma) and the timing of eclogite metamorphism has not been well studied. Detailed studies of fluid-processes and fluid-rock interactions within the terrane have also not been conducted. To that end, in situ UV laser 40Ar/39Ar phengite geochronology was used to determine the timing of peak (maximum pressure) and retrograde metamorphism, as well as evaluate the significance of previously documented intra- and intersample 40Ar/39Ar age ranges in the context of variations in mica composition, protolith (metabasalt vs. metasediment), and textural and structural positions (Chapter 2). In situ major and trace element and microstructural studies of the hydrous mineral lawsonite (Chapter 3), which is ubiquitous in the rocks of the Tavşanlı Zone, are used to evaluate the trace element affinity of lawsonite, as well as its utility as a monitor of metamorphic and deformation processes within subduction zones. Finally, geochemical studies of lawsonite-rich veins and layers (Chapter 4) that approximate the CASH system are used to understand the origins and implications of these volatile and trace element rich assemblages for element and water cycling in subduction zones.