Petrofabrics of High-Pressure Rocks Exhumed at the Slab-Mantle Interface from the ‘Point of No Return’

Abstract:

The highest pressure typically recorded by metamorphic rocks exhumed from oceanic subduction zones is ~2.5±1 GPa, corresponding to the maximum decoupling depth (MDD) (80±10 km) identified in active subduction zones; beyond the MDD (the ‘point of no return’) exhumation is unlikely. One of the few places where rocks returned from the MDD largely unaltered is Sivrihisar, Turkey: a structurally coherent terrane of lawsonite eclogite and blueschist facies rocks in which assemblages and fabrics record P-T-fluid-deformation conditions during exhumation from ~80 to 45 km. Crystallographic fabrics and other structural features of high-pressure metasedimentary and metabasaltic rocks record transitions during exhumation. In quartzite, heterogeneous microstructures and crystallographic fabrics record deformation and dynamic recrystallization from ~2.6 GPa to ~1.5 GPa, as expressed by transition from prism<a> c-axis patterns through progressive overprinting and activation of rhomb<a> and basal<a> slip. Omphacite, glaucophane, phengite, and lawsonite in quartzite remained stable during deformation. In marble, CaCO₃ deformed in dislocation creep as aragonite, producing strong crystallographic fabrics. This fabric persisted through formation of calcite and destruction of the shape-preferred orientation, indicating the strength of aragonite marble. Omphacite in metabasalt and quartzite displays an L-type crystallographic fabric. Lawsonite kinematic vorticity data and other fabrics in metabasalt are consistent with exhumation involving increasing amounts of pure shear relative to simple shear and indicate strain localization and simple shear near the fault contact between the high-pressure unit and a serpentinite body. This large coaxial component multiplied the exhuming power of the subduction channel and forced rocks to return from the MDD.