Petrogenesis and Structural Petrology of Lawsonite Eclogite and Blueschist Pods, Sivrihisar, Turkey

Eclogites commonly occur as pods in rocks that record lower pressures than their eclogite inclusions; e.g. eclogite in blueschist. The highest-pressure part of the Sivrihisar Massif of west-central Turkey contains hundreds of pods of eclogite within transitional blueschist-eclogite facies metasedimentary and metabasaltic rocks. These pods range in size from 3 cm to 6 m. Most pods are comprised of one rock type (e.g. lawsonite eclogite), but some pods have a core of eclogite surrounded by alternating cm-scale layers of lawsonite blueschist and eclogite, and transitional eclogite-blueschist assemblages. These lithologically complex pods also contain texturally late lawsonite-rich veins and layers. Within pods, there is no significant difference in bulk composition of eclogite and blueschist. Most layers lack reaction textures, but some blueschists contain texturally complex garnets and glaucophane rims on omphacite, suggesting an eclogite to blueschist transition. Phase diagrams (pseudosections) were calculated for bulk compositions from several samples from core and margin of one meter–scale pod. Lawsonite eclogite in the pod core equilibrated at 22-24 kbar, ~520 C, and lawsonite eclogite and blueschist at the margin equilibrated at similar temperatures but at a wider range of pressures (17-24 kbar). The complex eclogite-blueschist pod is foliated, lineated and folded, and omphacite in the pod core has a different lattice preferred orientation compared to omphacite at the pod margin. Petrologic and structural data are consistent with formation of pods by folding and dissection of eclogite layers at high-P, and localized retrogression of pod margins during exhumation through blueschist facies conditions. This study shows that the pod texture seen in many high-pressure/ low-temperature terranes can be formed within the subduction channel. The results are applicable to understanding the genesis of high-P pods in subduction complexes and the mechanisms/conditions of overprinting of high-P assemblages and textures during exhumation.