Epidote - lawsonite coexistence in blueschist-facies block from the Tavsanli Zone - Turkey: petrological implications

Metamorphic evolution of an epidote-lawsonite blueschist sample characterized by the coexistence of lawsonite and epidote from Sivrihisar area (Taysanli Zone) was studied herein in terms of petrology and mineral equilibria. Based on the textural evidence and phase composition, 2 prograde stages, defined by assemblage-I and -II and 1 retrograde stage were recognized. Assemblage-I indicates epidote-blueschist facies conditions (12 +/- 1 kbar / 485 +/- 10 degrees C). Assemblage-II is characterized by the coexistence of epidote and lawsonite (17 +/- 1 kbar / 515 +/- 10 degrees C) corresponding to the interface of lawsonite blueschist and epidote blueschist facies. Phase diagram calculations and mineral compositions revealed that along this interface, an equilibrium field with lawsonite and epidote is stable. This closed-equilibrium field is controlled by high aH(2)O and an elevated Fe3+/Al ratio of minerals. Pressure-temperature (P-T) estimates and textural observations indicated a counter-clockwise path during the subduction and exhumation history. The preservation of lawsonite and epidote during the retrograde stage pointed to the fact that the path followed the stability field of lawsonite and epidote during exhumation.