Jadeite-chloritoid-glaucophane-lawsonite blueschists in northwest Turkey:: unusually high P/T ratios in continental crust

Sodic metapelites with jadeite, chloritoid, glaucophane and lawsonite form a coherent regional metamorphic sequence, several tens of square kilometres in size, and over a kilometre thick, in the Orhaneli region of northwest Turkey. The low-variance mineral assemblage in the sodic metapelites is quartz + phengite + jadeite + glaucophane + chloritoid + lawsonite. The associated metabasites are characterized by sodic amphibole + lawsonite +/- garnet paragenesis. The stable coexistence of jadeite + chloritoid + glaucophane + lawsonite, not reported before, indicates metamorphic pressures of 24 +/- 3 kbar and temperatures of 430 +/- 30 degreesC for the peak blueschist facies conditions. These P-T conditions correspond to a geotherm of 5 degreesC km(-1), one of the lowest recorded in continental crustal rocks. The low geotherm, and the known rate of convergence during the Cretaceous subduction suggest low shear stresses at the top of the downgoing continental slab.