Biostratigraphic zonation and 40Ar–39Ar ages for the Neotethyan Khoy ophiolite of NW Iran

The Khoy ophiolite in northwestern Iran represents a piece of oceanic lithosphere formed in the Mesozoic Neotethys. This ophiolite consists from bottom to top of well-defined basal metamorphic rocks, peridotites, gabbros and a variety of upper to lower Cretaceous deep-marine rocks. These include pelagic fossiliferous carbonates and basalts as interlayers or exotic blocks. Four hornblende separates from gabbros and amphibolites give values from 108 to 111 Ma. The results from microfossil studies show the presence of late Cretaceous (e.g. Marginotruncana, Globotruncana) and middle to late Eocene (e.g. Globorotalia, Orbulonoides) planktonic foraminifera. The data suggest that although the initial rifting and crystallization of the oceanic crust of the Khoy ophiolite began in the late Albian, the volcanic activities continued through to the late Eocene time. Another possibility for such variation of lithofacies and biofacies and disturbances in the fossil record may be the result of bioturbation and transportation and rapid changes of depositional conditions during eruption of the basaltic lava flows. Pelagic sediments alone have been listed and of the foraminiferas the first three are planktonic, as would be expected. Only Orbulonoides is a benthic form and aberrant. The third possibility is that the Khoy ophiolite may represent a classic supra-subduction-type ophiolite where volcanic members of the different parts of oceanic environment (e.g. MORB, volcanic arc, ocean island) are now juxtaposed.