PETROGENESIS OF LAWSONITE AND EPIDOTE ECLOGITE AND BLUESCHIST, SIVRIHISAR, TURKEY

Blueschist and eclogite facies metasedimentary and metabasaltic rocks in the Sivrihisar Massif, Turkey, record a wide range of high-P-low-T (HP-LT) conditions. The massif is part of one of the world's largest HP-LT terrains, the Tavsanli zone of W Turkey, but is distinct from the rest of the zone in its structural position and orientation of mineral lineations. Sivrihisar eclogites contain Omp + Grt + Ph ± Qtz ± Gl ± Lws or Ep, and occur primarily as pods in blueschist facies metabasalt, marble, and quartz-rich rock. We have identified 230 m-scale HP pods in the field area. Blueschists and quartz-rich rocks contain Na-Amp + Ph + Lws and/or Ep ± Grt ± Omp. Eclogite and blueschist have similar NMORB bulk compositions but record different P-T conditions: 12 kbar, 380 C (Lws blueschist); 15-16 kbar, 580-500 C (Lws-Ep blueschist); 18 kbar, 600 C (Ep eclogite); 24-26 kbar, 500 C (Lws eclogite). These Lws eclogites have the highest pressures reported for this rock type, which is predicted to be common in subduction zones but is rarely exposed at the Earth's surface. In some Lws blueschists, Ep inclusions and the partial replacement of matrix Ep by Lws suggest an anticlockwise P-T path. Other rocks contain no Ep as inclusions or a matrix phase, and were metamorphosed entirely within the Lws stability field. Results of the P-T study and map-scale distribution of blueschists and eclogites suggest that rocks recording different maximum P-T conditions were tectonically juxtaposed, although all shared the same exhumation path close to the Lws – Ep stability boundary. The overall structure of the massif is characterized by tectonic slices representing different structural levels of a subducted and exhumed continental margin or marine sequence, with maximum depths of subduction of each slice ranging from 45-75 km. Within the slices, compositionally equivalent, cm-scale layers of eclogite and blueschist formed at the same P-T conditions and represent different extents of prograde reaction controlled by variations in strain history, fO2, or other chemical factors. The formation of pods post-dates eclogite facies metamorphism, but pre-dates the juxtaposition of the map-scale slices at 10 kbar, 300-400 C.