Eclogites are typically found as lenses in high-grade gneiss or blueschist facies rocks that have been tectonically exhumed. In many high-P terranes, the contact between eclogite pods and host rocks is poorly exposed, making it difficult to evaluate the petrologic/tectonic relationship between eclogites and host rocks. This study describes mineral assemblages and garnet zoning across an intact eclogite-blueschist contact, with the goal of reconstructing and comparing reaction histories and P-T paths.

The samples are from the Sivrihisar segment of the Tavsanli zone, a belt of metasedimentary/metavolcanic rocks from a subducted and exhumed continental margin in central Turkey. Sivrihisar blueschists that host m-scale eclogite pods contain garnet + glaucophane + lawsonite + quartz ± omphacite, and experienced incipient eclogite facies conditions. Eclogites are comprised of garnet + omphacite + rutile + quartz + white mica ± epidote. In pristine eclogite pods, garnets are < 1 mm in diameter, inclusion-free, and characterized by an increase in Mg from core to outer core, and a dramatic decrease in Mg at the rim. Up to the rim, changes in garnet composition occur in euhedral, concentric zones, but the low-Mg rim/overgrowth is more irregular, and discontinuous in some garnets. Ca zoning in some eclogite garnets is oscillatory. Blueschist garnets are larger than eclogite garnets (1-3 mm), contain aligned inclusions of epidote and quartz, and are more Fe-rich than garnets in the adjacent eclogite pods. Zoning trends in blueschist and eclogite garnets are similar: Mg increases from core to outer core, with an abrupt decrease at the rim.

Blueschist garnets near the pod/host interface are fractured and disaggregated, suggesting that deformation was focused at the boundary. The lack of zoning associated with garnet fractures may indicate that this deformation was late relative to metamorphism. This deformation may have occurred during incorporation of the eclogite into the blueschist, or may have been a later event that localized at the boundary because of a mechanical contrast between eclogite and blueschist. Our current interpretation is that blueschists and eclogites experienced similar prograde and retrograde P-T-deformation paths/conditions, and that some deformation occurred late along the pod/host boundary.