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ABSTRACT

We discuss geochemical, chronological, and field data from the extrusive sequence and individual diabase and sheeted dikes in the Upper Cretaceous Khoy, Kermanshah, Fannuj, Nosratabad, south Fariman, northwest Fariman, Dehshir, and Sabzevar ophiolite massifs of Iran. The extrusive sequences include pillow lava, sheet flow, hyaloclastite, hyaloclastic breccia, and interbeds of chert and pelagic limestone with Late Cretaceous microfauna. The Khoy, northwest Fariman, and Sabzevar massifs also include Upper Cretaceous–Lower Paleocene supra-ophiolitic volcanic and volcanic-sedimentary rocks that formed in a trough near the basin where the extrusive sequence formed. The Khoy pillow lava displays transitional (T) mid-ocean-ridge basalt (MORB) characteristics but no chemical contribution from the components released from the subducted slab. On the other hand, the diabase dikes that cut the Khoy extrusive sequence show signatures of subduction-zone magmatism and contribution from the melt released through the partial melting of the subducted slab. While lava in the Harsin (Kermanshah) extrusive sequence in west Iran displays enriched (E) MORB and plume (P) MORB characteristics, the pillows in the Fannuj, northwest Fariman, Dehshir, and Sabzevar extrusive sequences indicate the contribution of both fluids and melt from the subducted slab. The Nosratabad and south Fariman ophiolites also show evidence for either melt or fluids, respectively. Partial melting of the subducted slab sedimentary cover may have formed the acidic pillow lava and sheet flow in the Fannuj and Nosratabad extrusive sequence, respectively. Some pillows in the Nosratabad, Sabzevar, northwest Fariman, and to a lesser extent, Dehshir extrusive sequence display oceanic-island basalt (OIB) geochemical characteristics. Mantle plumes or asthenospheric flow that probably moved up through weak zones of the subducted slab may have affected the partial melting of the mantle wedge above the slab. The combined OIB and suprasubduction characteristics suggest the role of rollback of the subducted slab in the magmatism of the northeast Iranian ophiolites. The clear MORB-like geochemical characteristics in the extrusive sequence of the ophiolites in northwest and west Iran, and the suprasubduction-zone characteristics in the diabase and extrusive sequence of the ophiolites in southeast and east Iran, west of the Lut block, and northeast Iran may represent a major Late Cretaceous transition from a MORB to a suprasubduction-zone setting.

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