Metamorphism, deformation, and preservation of high-pressure assemblages and textures

In the Sivrihisar region of central Anatolia, blueschist facies metasedimentary and metabasaltic rocks crop out over a wide area, with preserved contacts between lawsonite eclogite, lawsonite blueschist, and other blueschist and eclogite facies rocks. Lawsonite eclogites are predicted by experiments to be a common rock type at depth in subduction zones (45-300 km), but there are only 10 or so known examples of lawsonite eclogites at the Earth's surface. Why are they so rarely exhumed, or why do they so rarely survive exhumation? The Sivrihisar lawsonite eclogites are among the highest pressure examples of this rock type known (P ~ 21-26 kbar), and are spectacularly well preserved.

The Sivrihisar rocks are also extremely interesting because the blueschists are adjacent to quartzites that contain andalusite + kyanite + sillimanite + staurolite ± garnet, and schists that contain chloritoid ± staurolite ± garnet.  We are trying to relate the metamorphic history of these rocks (blueschists, eclogites, and Al2SiO5-bearing rocks) to the subduction, collision, and exhumation processes, as well as other important events in the region (emplacement of metaperidotite; magmatism). Important clues are in the metamorphic assemblages and isograds, deformation fabrics (the rocks are L-tectonites), and other textural features such as marbles consisting of calcite fibers.

Publications

Davis, P.B., and Whitney, D.L. (in press) Petrogenesis and structural petrology of high-pressure metabasalt pods, Sivrihisar, Turkey. Contributions to Mineralogy and Petrology.

Davis, P.B., and Whitney, D.L. (2006) Petrogenesis of lawsonite and epidote eclogite and blueschist, Sivrihisar Massif, Turkey. Journal of Metamorphic Geology, 24, 823-849. Whitney, D.L. & Davis, P.B. (2006) Why is lawsonite eclogite so rare? Metamorphism and preservation of lawsonite eclogite, Sivrihisar, Turkey. Geology, 34, 473-476.

Whitney, D.L. (2002) Coexisting andalusite, kyanite, and sillimanite: sequential formation of three polymorphs during progressive metamorphism near the Al2SiO5 triple point, Sivrihisar, Turkey. American Mineralogist, 87, 405-416.

Theses: Peter B. Davis (Ph.D. 2008), M.A. Aycenk (M.S. 2004)

Garnet-lawsonite blueschist interlayered with lawsonite eclogite at the margin of an eclogite pod, Sivrihisar.

Photomicrograph of garnet-lawsonite blueschist from the pod shown at left. Field of view = 4 mm.