Petrology, geochemistry and tectonics of the Sabzevar ophiolite, North Central Iran

The Sabzevar ophiolite is a highly dismembered ophiolite complex located along the northern boundary of the central Iranian microcontinent (CIM), and is one of the internal Iranian group of ophiolites and colored melanges. The igneous rocks of this complex consist of peridotites (harzburgite, dunite and lherzolite), serpentinite, minor pyroxenite, gabbros, and a volcanic sequence that exhibits a wide range of composition from basalts and basaltic andesites to rhyodacite–dacites, rhyolites and basanites. Sedimentary rocks include a variety of Upper Triassic to Lower Cretaceous deep- and shallow-marine rocks. These include pelagic fossiliferous carbonates that are mixed with the pillow basalt and basaltic andesite as interlayers or exotic blocks ranging in size from 10 to 100 m. Also present are extensive units of radiolarian chert, which are interbedded within the basalts and basaltic andesites. A combination of petrographic observations and analyses of incompatible trace elements and rare earth elements indicates the presence of at least four different types of extrusive rocks in the Sabzevar ophiolite. The geochemical data clearly identifies some of the extrusive rocks to have formed from three distinct types of basaltic melts; (i) the group-1 basaltic rocks, which formed from an initial melt with N-MORB-like (LREE depleted) chemical signatures and are petrogenetically related to the gabbros, (ii) group-2 basaltic rocks which have E-MORB chemical signatures, and (iii) group-3 basaltic rocks with LREE-enriched signatures and incompatible trace element patterns that suggest an island arc affinity. The data also suggest that the gabbros and the group-1 basaltic rocks are petrogenetically related and produced by fractionation of plagioclase and to a lesser extent clinopyroxene and hornblende. Similar to other studied Iranian ophiolites (e.g. Khoy, Band-e-Zeyarat/Dar Anar, Neyriz, Kermanshah and Baft) the results from the Sabzevar ophiolite demonstrate the presence of relics of mid-ocean ridge basalt (MORB)-like signatures in the extrusive units of Neo-Tethyan ophiolites and suggests that the fossil oceanic lithosphere preserved in these ophiolites developed at an oceanic spreading center. In addition, the Sabzevar ophiolite contains other volcanic rocks that have non-MORB-like geochemical signatures (e.g. within-plate, island arc). The differences between these geochemical signatures could be a result of a differing upper mantle composition, or different degrees of partial melting of the same upper mantle. The presence of sizable Nb anomalies in the extended rare earth element (REE) patterns for andesites and the group-3 basalts is characteristic of volcanic arc magmas, or possibly are products of the ascending magma that was modified by crustal contamination. The presences of the basanites and the silica-undersaturated rocks could be attributed to extrusive activity following ophiolite emplacement. Plate reconstructions suggest that rocks of the Sabzevar ophiolite were part of the Tethyan oceanic crust that was formed during rifting of a narrow but deep seaway that separated the CIM from the Eurasian plate. The ophiolite was emplaced during northeast dipping subduction (i.e. closure) of this segment of the Tethyan seaway (Sabzevar Ocean). [Copyright &y& Elsevier]