Petrological and geochemical study of the Late Cretaceous ophiolite of Khoy (NW Iran), and related geological formations

摘要

This paper, based on 113 new whole rock analyses and about 3500 electron microprobe analyses of the mineral phases, is dedicated to the petrography and geochemistry of the Khoy ophiolites and related formations, NW Iran. It is complementary to a previous paper published in this Journal, where we gave a detailed description of the geology of the Khoy area, including various geological field sections, two geological maps in colour, new micropaleontological data and 27 new ~(40)K-~(40)Ar datings (Khalatbari-Jafari, M., Juteau, T., Bellon, H., Whitechurch, H., Cotten, J., Emami, H., 2004. New geological, geochronological and geochemical investigations on the Khoy ophiolites and related formations, NW Iran. J. Asian Earth Sci. 23, 507-535). Our conclusions are: (a) The petrographic study confirms the field data showing the existence of two ophiolite complexes in the region of Khoy. (b) The Late Cretaceous ophiolitic lavas of the Khoy region exhibit very homogeneous T-MORB-type multi-element plots, suggesting that they were formed at oceanic spreading centers, by partial melting of a depleted mantle source, probably contaminated by one or several regional mantle plumes, responsible for their moderate enrichment in LREE. They do not show any negative anomaly for Nb, Zr or Ti, which allows us to exclude a genesis in a 'supra-subduction' environment. (c) The Late Cretaceous ophiolite of Khoy was created at a slow-spreading oceanic ridge, (d) Cryptic variations along extrusive and layered gabbros sections suggest frequent replenishment and magma mixing events in the magma chambers, (e) The 'supra-ophiolitic turbiditic series' overlying the Late Cretaceous ophiolite was accumulated in a subduction trench running along the northwestern margin of the Iran Block. This trench was fed with detrital volcanic fragments from both sides: T-MORB basalt fragments from the ocean-side, and arc-type basalts from the continent-side. (f) The meta-ophiolites of Khoy probably also represent slow-spreading conditions, and the porphyroclastic to mylonitic tectonites preserved in these metamorphic slices attest to extreme conditions of ductile shearing, characteristic of oceanic fracture zones.