Early Cretaceous (Albian) intra‐oceanic subduction in northern branch of Neotethys in NW Iran: Zircon U–Pb geochronology and geochemistry of ophiolitic metagabbros from the Chaldoran area

Abstract

The Chaldoran ophiolite in NW Iran and at the Turkish border is a part of the larger Khoy ophiolite. Serpentinized peridotites, pillow basalt, cumulate and isotropic gabbro, pelagic limestone, and radiolarite along with volcano‐sedimentary units are the main rock types in the area. These rocks are mixed together in some places as tectonic and “coloured” mélanges. The gabbro consituent of this ophiolite is either fresh or is metamorphosed up to upper greenschist facies. These meta‐basic rocks contain relatively large crystals of plagioclase and clinopyroxene, partially changed to amphibole. Metamorphic epidote is formed in some samples. Whole‐rock chemistry of these rocks shows a basaltic magmatic composition. Low SiO2 contents along with relatively high MgO and FeOtotal contents testify for a mantle source for the parental magma of the studied samples, intermediate between tholeiitic and calc‐alkaline series. LREE and HFSE are slightly depleted and LILE show enrichment in the studied samples and Ta, Nb, Ti show negative anomalies with respect to MORBs. All chemical features are compatible with a supra‐subduction zone setting for the rocks. The studied rocks are similar to primitive arc and less evolved rocks, geochemically. Also the samples show both MORB‐like magmas, formed at the initial stage of a subduction and IAT chemical features. U–Pb dating of zircons on the gabbro sample gave 107.5 ± 1.3 Ma (Albian) crystallization age. The metabasites from the Chaldoran ophiolite are similar to intra‐oceanic ophiolite‐related igneous rocks from the Zagros suture zone in Iran and NE Iraq and equivalent rocks from the Neotethyan sutures in east Turkey and Armenia and basic rocks from the Late Cretaceous ophiolites of the outer Zagros and Taurus, which are related to the subduction initiation (SI) stage.