Zircon U–Pb age and geochemical constraints on the origin of the Birjand ophiolite, Sistan suture zone, eastern Iran

Abstract: The Birjand ophiolite is one of the ophiolitic complexes in the northern Sistan suture zone, eastern Iran, which marks the closure of an enigmatic branch of the Neotethys Ocean (called the Sistan Ocean) associated with the collision between the Lut and Afghan continental blocks. Ophiolitic rocks in the suture zone occur as collisional block-to-block geological terranes, or as blocks within tectonic mélange. We present zircon U–Pb age, and whole-rock geochemical and Sr–Nd isotopic data of the Birjand ophiolite to understand its formation age and magma genesis in this poorly constrained part of the Alpine–Himalayan orogenic belt. Varieties of the ophiolitic rocks include clinopyroxene-bearing harzburgites, harzburgites, dunites, massive and pillow basalts, dolerites, gabbros and leucogabbros. Using laser ablation inductively coupled plasma-mass spectrometry, zircon separates from two leucogabbros yield U–Pb ages of 113±1 and 107±1Ma, which are interpreted to represent the formation ages of the ophiolite. The clinopyroxene-bearing harzburgites can be explained as melting residues after low to moderate extraction of MORB-type melts, whereas the harzburgites require more than one melting event to explain their trace element depletion. Most mafic rocks of the ophiolite display MORB-like trace element patterns and high εNd(t) (+3.4 to +8.4), features consistent with derivation from MORB-source mantle. Magma genesis can be explained by low to moderate degrees of melting of moderately depleted spinel peridotite and variable interaction with enriched mantle melts. The Birjand ophiolite represents a MORB-type ophiolite and part of the Middle Cretaceous oceanic lithosphere of the Sistan Ocean that closed during the destruction of a narrow arm of the Neotethys Ocean. [Copyright &y& Elsevier]