FLUID-ROCK INTERACTION AT THE SLAB-MANTLE INTERFACE DURING SUBDUCTION AND EXHUMATION: THE RECORD IN MÉLANGE VS. COHERENT HP/LT TERRANES

Exhumed high-pressure/low-temperature (HP/LT) metamorphic rocks record fluid-rock interaction that took place at or near the slab-mantle interface of oceanic subduction zones and are thus geochemical archives of element and volatile cycling between subducted lithosphere and the overlying mantle wedge. HP/LT rocks are tectonically exhumed in oceanic subduction zones as mélange (blocks in serpentinite, talc-chlorite, or sediment matrix) or as more coherent terranes comprised of km-scale layers of oceanic crust and associated sediment. Both mélange and coherent terrains record intense deformation and significant fluid-rock interaction among metamorphosed basalt/gabbro, sediment, and ultramafic rocks. In mélange, which is common in Pacific and Caribbean subduction complexes, HP/LT blocks have metasomatic rinds indicating extensive interaction with ultramafic rocks and sediments; in some cases cores of blocks also show geochemical tracers of this interaction and the matrix records evidence for mechanical mixing and chemical interaction with blocks. Coherent terranes, which are more characteristic of the Tethyan realm, also contain metasomatic rocks, typically at metaultramafic/sediment± basalt contacts. In addition, coherent terranes such as Sivrihisar (Turkey) contain extensive HP vein networks and geochemical evidence in bulk-rock and mineral compositions for interaction among rock types, including serpentinite, calc-schist/qz-micaschist, and metabasalt (lawsonite eclogite/blueschist). The abundance of carbonate rocks in Tethyan subduction zones may influence the chemical and physical evolution of these systems, from fluid-rock reaction and phase stability to rheology and exhumation style. In this contribution we compare geochemical and structural records of mélange and coherent terranes, with a focus on lawsonite eclogite-bearing complexes, so as to understand the origin of mélange vs. coherent terranes and to determine the extent and nature of fluid-rock reaction within the slab and between slab and mantle at depths of ~40-80 km in oceanic subduction zones.