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1 Department of Geology, Miami University, Oxford, OH 45056, USA
2 Department of Geology, University of California, Davis, CA 95616, USA
The Kizildag ophiolite in southern Turkey is a remnant of the Neo-Tethyan oceanic crust and displays well-preserved magmatic and tectonic structures of seafloor spreading origin. The ophiolite consists of two structurally distinct massifs that are separated by the NW-striking high-angle Tahtaköprü fault. The main massif to the west contains a serpentinized peridotite core adjoined on the southeast by the normal fault-bounded plutonic sequence and sheeted dyke complex in a structural graben. The dyke-gabbro boundary within this graben is in places faulted along a low-angle detachment surface and is locally marked by a transition zone with mutual intrusive relations between the dykes and isotropic gabbros and plagiogranites. This igneous boundary contains numerous proto-dyke intrusions marking a well-preserved root zone of the sheeted dyke complex and may represent the roof of a fossil magma chamber. Mineralized oceanic faults within the dyke complex form two major subsets. Dyke-parallel normal faults form horst and graben structures and locally flatten with depth acquiring a listric geometry. Dyke-perpendicular faults display steep dips and subhorizontal slickenside lineations, suggesting their oblique- to strike-slip nature. The graben structure containing the plutonic sequence and the sheeted dyke complex is analogous to those documented in the Troodos ophiolite and may similarly represent a fossil spreading axis. The second massif east of the Tahtaköprü fault consists mainly of serpentinized peridotites directly overlain by lava flows, rotated dyke blocks, and gabbros. Sulfide mineralization along some fault planes in the extrusive rocks indicates that hydrothermal systems were associated and operated synchronously with magmatic and tectonic extensional processes. Stratigraphic relations and the structural architecture in this massif suggest that the Kizildag oceanic crust underwent crustal denudation and unroofing of the upper mantle as a result of tectonic extension at a spreading centre. The Tahtaköprü fault separating the two massifs is an accommodation zone that permitted differential movements between the adjacent ridge segments during generation of the Neo-Tethyan oceanic lithosphere. The general structure of the ophiolite suggests its evolution via seafloor spreading and an asymmetric simple shear extension along a slow-spreading centre. The sheeted dykes and pillow lavas have relatively high SiO2 and Al2O3 and low FeO and TiO2 concentrations and show limited FeO and TiO2 enrichments with decreasing MgO contents. These compositional properties differ markedly from typical tholeiitic suites from the ocean floor and arc settings, but correspond closely to those documented from the sheeted dyke complex and the lower volcanic suite of the Troodos ophiolite. The major element compositions suggest low pressure and high degree of melting from a depleted mantle source. The trace element concentrations are markedly depleted in both high-field strength and rare-earth elements and relatively enriched in largeion-lithophile elements compared to normal MORB. It is inferred that the Kizildag ophiolite formed in a tectonic setting where melting occurred at relatively low pressures within the stability field of spinel and proceeded to high melt fractions possibly by progressive depletion and melt removal. A limited positive correlation between the extent of melting and light rare-earth enrichment can be related to infiltration by fluids or melts derived from a more fertile source or possibly a subducting slab fragment. Comparison of the structure and petrology of the Troodos and Kizildag ophiolites and the regional geology suggest their evolution along a seafloor spreading system within the Southern Neo-Tethys. Spreading probably occurred in short segments and resulted in development of the Cretaceous Neo-Tethyan seaway as a marginal basin between the Tauride platform in the north and Afro-Arabia in the south.
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