Petrological and geochemical constraints on the origin of the Nehbandan ophiolitic complex (eastern Iran): Implication for the evolution of the Sistan Ocean

摘要

The Nehbandan ophiolite complex (NOC) crops out in the Sistan suture zone, which marks the boundary between the Lut and Afghan continental blocks. The NOC is composed of various ophiolitic lithotypes included in a tectono-sedimentary melange, which are commonly interpreted as remnants of the oceanic lithosphere of the Sistan Ocean. Three different sequences (or associations) of ophiolitic rocks can be recognized in the NOC: (1) mantle peridotites consisting of clinopyroxene- (cpx-) rich harzburgites and depleted harzburgites; (2) olivine websterite-pyroxenite-gabbronorite sequence; (3) wehrlite-troctolite-cumulate gabbro-isotropic gabbro-basalt sequence. Petrographic observations, mineral chemistry, whole-rock chemistry, and rare earth element (REE) modelling carried out on the different rock associations led to the following conclusions: (1) the wehrlite-troctolite-cumulate gabbro-isotropic gabbro-basalt association represents a portion of oceanic crust generated in a mid-ocean ridge setting; (2) the cpx-rich harzburgites represent the residual mantle after 5-20% removal of mid-ocean ridge basalt-type (MORB) melt This residual mantle was subsequently enriched in light REE (LREE) by subduction-derived fluids in a supra-subduction zone (SSZ) setting and it compositionally represents the typical mantle source for boninitic melts; (3) the olivine websterite-pyroxenite-gabbronorite association represents a portion of oceanic crust generated in an intra-oceanic arc setting; (4) the depleted harzburgites represent the residual mantle after 10-30% removal of boninitic melts in an intra-oceanic arc setting. The data presented in this paper provide new constraints for the tectonic evolution of the Iranian sector of the Neo-Tethys. In fact, in contrast with previous geodynamic models, the occurrence of SSZ ophiolites in the NOC implies that the phase of convergence between the Lut and Afghan blocks, which led to the closure of the Sistan Ocean, was accompanied by the development of an intra-oceanic arc setting.