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

Abstract

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 aH 2O, 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.