Discovery of a Neo-Tethyan ophiolite in Northern Iran: Evidence for its formation at a slow–spreading center

M. Salavati, A. Kananian, M. Noghreian, A. Darvishzadeh, and A. Samadi Soofi

The Southern Caspian Sea ophiolite complex (SCO) is an almost complete oceanic lithospheric section including from bottom to top, layered ultramafic cumulates, layered gabbros, isotropic gabbros, sheeted dike and basaltic lavas with a pelagic limestone cap yielding an Late Cretaceous age.

Layered ultramafic cumulate rock is composed of clinopyroxenite, wehrlite, dunite and massive- to layered-mafic cumulate rocks consisting of gabbro and norite. Disaggregated, this ophiolite complex includes a small sheeted dike complex and a preponderance of pillow lavas over sheet flows in the volcanic section. Geological observations such as the absence of well-developed layered gabbro, the presence of pyroxenite dikes, the presence of small pockets of sepentenized dunites, the absence of well-developed dike complex, the absence of chromatic pods, well-developed extended point source volcanism onto pillow lavas, the presence of gabbro pegmatoidic dikes in layered and isotropic gabbro suggest that the SCO is a lherzolite ophiolite type (LOT) and was formed in a slow-spreading center.

The available field and geochemical data on SCO (the presence of highly magnesian clinopyroxene (Mg# =81-90), homogeneous composition of clinopyroxene, absence of zoning in clinopyroxene and low Mg# in coexisting olivine, the geochemical data for the volcanic, mafic–ultramafic cumulate rocks and REE features (Nb,Ti and Zr negative anomalies)) show that this ophiolite complex was formed in medium- to high-pressure from the basaltic magma in a subduction-related marginal basin such as an island arc/suprasubduction zone tectonic setting.