ULTRALOW PRESSURE AND TEMPERATURE UHP METAMORPHISM OF LAWSONITE ECLOGITE AND BLUESCHIST, SIVRIHISAR, TURKEY

The Sivrihisar (Turkey) subduction complex is a rare example of preservation of mineral assemblages and deformation microstructures from the lawsonite eclogite facies. Sivrihisar rocks record metamorphism and deformation at pressures up to 2.5 GPa, and some rocks also record part of the decompression path. The complex is dominated by marble and quartzite, with lesser amounts of metabasalt in layers and lenses, and therefore consists of low-density (buoyant) material compared to oceanic-crust dominated subduction complexes. Nevertheless, the Sivrihisar metasedimentary-dominated units were subducted to at least 70 km depth before rapid exhumation

Sivrihisar quartzite contains sodic amphibole (including colorless Mg-glaucophane zoned to more Fe-rich rims) + phengite + garnet + lawsonite + piemontite +/- omphacite, and displays a characteristic single-girdle quartz c-axis pattern that is distinct from cross-girdle c-axis patterns of quartz in rocks that record lower P and/or higher T conditions, e.g. epidote blueschist facies rocks. Rod-shaped calcite pseudomorphs after aragonite in marble responded to later overprinting by shearing, which affected the obliquity of the rods relative to foliation but preserved the pseudomorph texture.

Although lawsonite and epidote-bearing units may represent distinct tectonic slices, the presence of pseudomorphs after lawsonite and garnet in epidote blueschist and greenschist facies metabasaltic rocks may be evidence that all exhumed parts of the Sivrihisar complex experienced lawsonite eclogite conditions, with preservation of lawsonite eclogite only in the NW part of the complex. Comparison of pristine lawsonite eclogite assemblages, mineral compositions, and textures with those in neighboring rocks that record lower P and/or higher T conditions shows how (U)HP deformation microstructures can be recognized in quartzite and marble.