LAWSONITE PORPHYROBLAST TEXTURES AS INDICATORS OF METAMORPHIC AND DEFORMATION CONDITIONS IN A SUBDUCTION ZONE

The textural characteristics of lawsonite in eclogite and blueschist provide information about high-pressure (HP) metamorphism and deformation in subduction zones and help explain the rare occurrence of lawsonite eclogite in exhumed HP rocks. In the Sivrihisar Massif, Turkey, the exceptional preservation of lawsonite in eclogite and blueschist allows evaluation of the mechanisms and conditions of deformation under HP conditions. In thin sections cut perpendicular to foliation and parallel to lineation, the orientation and aspect ratio of 160-1040 lawsonite crystals were measured in 10 samples collected along a N-S traverse, from the steep fault contact with a metaperidotite body in the north to the southern margin of the lawsonite blueschist/eclogite unit. Some samples were collected from map-scale lawsonite blueschist layers; others from the margins of meter-scale pods rimmed by blueschist and cored by eclogite. Determination of the porphyroblast critical aspect ratio allows a quantitative assessment of kinematic vorticity (Wk), a measure of the relative components of pure vs. simple shear. Samples analyzed from blueschist layers record a major component of pure shear (Wk = 0.44-0.50) or similar amounts of pure and simple shear (Wk = 0.70-0.80). Aspect ratio vs. angle-to-foliation plots for blueschist and eclogite samples from pod margins and a fault zone blueschist sample record smooth distributions consistent with simple shear likely related to pod rotation and fault displacement. Although it is not certain how deformation textures relate to P-T paths, the lawsonite textures likely correspond to deformation under HP conditions. If the lawsonite textures reflect deformation during a HP part of the exhumation path, the record of pure shear is consistent with models of extrusion along a subduction channel. Syn-subduction extrusion accounts for rapid exhumation along a low geothermal gradient and is consistent with the preservation of lawsonite.