Kizildag Ophiolite Research Articles

A new model for chromitite formation in ophiolites: Fluid immiscibility

Although the involvement of hydrous fluids has been widely invoked in formation of podiform chromitites in ophiolites, there is lack of natural evidence to signify the role and mechanism of fluids. In this study, a new model for the genesis of podiform chromitite is proposed on basis of revisits of comprehensive petrological, mineralogical and geochemical results of the well-preserved Kizildag ophiolite and the well-characterized Luobusa chromite deposit. In this model, ascending magmas intruding oceanic lithospheric mantle would presumably form a series of small magma chambers continuously connected by conduits. Tiny chromite nuclei would collect fluids dispersed in such magmas to form nascent droplets. They tend to float upward in the magma chamber and would be easily transported upward by flowing magmas. Chromite-rich droplets would be enlarged via coalescence of dispersed droplets during mingling and circulation in the magma chamber and/or transport in magma conduits. Crystallization of the chromite-rich liquid droplets would proceed from the margin of the droplet inward, leaving liquid entrapped within grains as precursor of mineral inclusions. With preferential upward transportation, immiscible chromite-rich liquids would coalesce to a large pool in a magma chamber. Large volumes of chromite would crystallize in situ , forming podiform chromitite and resulting in fluid enrichment in the chamber. The fluids would penetrate and compositionally modify ambient dunite and harzburgite, leading to significant fractionations of elemental and isotopic compositions between melts and fluids from which dunite and chromitite respectively formed. Therefore, fluid immiscibility during basaltic magma ascent plays a vital role in chromitite formation.

Island arc tholeiite to boninitic melt evolution of the Cretaceous Kizildag (Turkey) ophiolite: Model for multi-stage early arc–forearc magmatism in Tethyan subduction factories

The Kizildag ophiolite in Turkey is a remnant of the late Cretaceous suprasubduction zone (SSZ) oceanic lithosphere that was developed in Southern Tethys, and is part of a peri-Arabian ophiolite belt including the Troodos (Cyprus), Baër-Bassit (Syria) and Semail (Oman) ophiolites in the eastern Mediterranean region. The internal structure, petrology and geochemistry of the Kizildag intrusive and extrusive rocks show a magmatic progression from island arc tholeiite (IAT) to boninitic compositions. Similar structural architecture and geochemical trends in the coeval Troodos and Baër-Bassit ophiolites suggest that this multi-stage SSZ magmatism was common in most Tethyan ophiolites. The sheeted dikes and the majority of extrusive rocks in Kizildag are composed of basalt, basaltic andesite and andesite with relatively low La/Sm N (0.5–0.7), low MgO (7–9 wt.%) and high TiO 2 (0.7–1.0 wt.%) and exhibit high Dy/Yb N ratios. These IAT suites also include rare, younger dacitic dikes showing strong enrichment in the LREE. The late-stage basaltic dikes and lavas (sakalavites) have high La/Sm N (0.8–1.0), high MgO (9–11 wt.%) and low-TiO 2 (0.3–0.4 wt.%) contents and display strong depletion in both LREE and HREE. This suite of rocks represents the boninitic component of the Kizildag ophiolite that developed from shallow and greater degrees of melting of hydrous and highly refractory peridotites beneath the extended forearc environment. The progressive evolution of Kizildag magmas from IAT to boninitic compositions in time and space was a result of variable mixing, aggregation, and differentiation of geochemically different melt batches, which formed in different levels (polybaric) within the melting column. Rapid slab rollback-induced corner flow, mantle return flow, and asthenospheric diapirism (providing more fertile melt) directly affected the evolution of this melting column and may have triggered advanced degrees of shallow melting of the hydrous and highly depleted forearc mantle, producing refractory liquids of the boninitic stage. These geochemical patterns of the Kizildag ophiolite reflect a multi-stage evolution of SSZ magmatism in Tethyan subduction factories that is analogous to the earlier arc volcanism in the Eocene–Oligocene Izu–Bonin–Mariana system.

Structure, petrology and seafloor spreading tectonics of the Kizildag Ophiolite, Turkey

Abstract 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 SiO 2 and Al 2 O 3 and low FeO and TiO 2 concentrations and show limited FeO and TiO 2 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.

Structure of the Kizildag ophiolite, a slow-spread Cretaceous ridge segment north of the Arabian promontory

The Cretaceous Kizildag ophiolite in southern Turkey is part of the Neotethyan ophiolite belt around the Arabian promontory and consists of two structurally distinct massifs that are separated by a northwest-striking high-angle oblique fault. This fault is interpreted as an accommodation zone that permitted differential movements between the adjacent crustal blocks during the evolution of the ophiolite. The main massif to the west includes an antiformal peridotite core adjoined on the southeast by the normal-fault-bounded plutonic sequence and sheeted dike complex, which collectively form a structural graben reminiscent of slow-spreading ridge axes. The inferred spreading axis and axis-parallel fault contacts are intersected and offset at right angles by tear faults that are geometrically and kinematically analogous to transfer faults in modern oceanic lithosphere at slow-spreading ridge environments. The secondary massif to the east consists mainly of serpentinized peridotites directly overlain along tectonic and/or intrusive contacts by lava flows, rotated dike blocks, and plutonic rocks, and can be interpreted as a segment of oceanic lithosphere evolved at and near the accommodation zone. The general structure of the ophiolite suggests its evolution via magmatic accretion and subsequent tectonic extension along a low-angle shear zone at a slow-spreading ridge environment.