Eclogites and blueschists of the Sistan Suture Zone, eastern Iran: A comparison of P–T histories from a subduction mélange

The Sistan Suture Zone of eastern Iran is a Cretaceous to Tertiary orogenic belt that formed by closure of a northwards-projecting arm of the Neo-Tethys ocean. Disrupted ophiolitic rocks within the subduction complex include a tectonic mélange of metamorphic rocks dominated by metabasites, metacherts, and metaspilites. The latter rocks occur as small (<200 m diameter) fault-bounded lenses within a low-grade schistose matrix. The metabasite lenses include variably overprinted eclogites, blueschists, and epidote–amphibolites. High-P assemblages in the metabasites include (i) Grt+Omp+Mg-Gln+Phe+Pg+Qtz+Rt, (ii) Grt+Omp+Zo+Phe+Qtz+Rt, and (iii) Grt+Gln+Zo+Ep+Pg+Qtz. Lawsonite was probably stable during prograde metamorphism, as testified by pseudomorphs in garnet. The eclogitic assemblages evolved, in some cases via a blueschist stage, to an epidote–amphibolite–facies assemblage: Bar+Ep+Ab±Bt±Chl±Qtz+Ttn. Pervasive epidote–amphibolite–facies retrogression occurred locally and was facilitated by deformation during exhumation. The mélange matrix was deformed and equilibrated under greenschist-facies conditions. Recorded eclogite–facies pressures range from 1.9 to 2.3 GPa, implying subduction of oceanic crust to depths of ca. 65–80 km. Peak-metamorphic temperature/depth ratios were in the range 6.6–9.5 °C/km. Eclogite–facies metamorphism occurred under conditions of high aH2O, implying the presence of a discrete aqueous fluid phase. Aqueous fluids were also present during retrogression. Significant differences exist between the peak-metamorphic P–T conditions recorded by samples from different lenses in the mélange, indicating that the lenses were detached from subducting lithosphere at different depths. P–T paths of eclogite–facies rocks were ‘clockwise’ with ca. 1.5 GPa of almost isothermal decompression.