Structure and petrology of Tauride ophiolites and mafic dike intrusions (Turkey): Implications for the Neotethyan ocean
Cretaceous Neotethyan ophiolites occur in four east-west–trending subparallel zones within the Tauride tectonic belt in southern Turkey. The ophiolites of the Inner, Intermediate, and Outer zones tectonically overlie the Mesozoic platform carbonates of the Tauride belt and are commonly underlain by a Cenomanian ophiolitic melange. These ophiolites consist mainly of tectonized mantle rocks, mafic-ultramafic cumulates, and gabbros, and commonly lack sheeted dike complexes and extrusive rocks of a complete ophiolite sequence. Metamorphic soles that are several hundred meters thick occur as thrust-faulted slices beneath these ophiolites and show well-developed metamorphic field gradients. Ophiolitic units and the metamorphic soles are intruded by mafic dike swarms that are truncated at the contact with the underlying melange unit. Dike rocks are made of subalkalic basalt to andesite typical of evolved island-arc tholeiites; they display large compositional variations, with SiO 2 content between 50 and 60 wt% and MgO between 8 and 4 wt%, and contain higher Ti augite phenocrysts and significantly less calcic plagioclase than their host cumulates. The majority of the analyzed dike rocks show a slight depletion in light rare earth elements (REE) with low La/SmN ratios and are depleted in both high-field strength (HFS) and heavy REEs, while enriched in large-ion-lithophile elements (LILE) relative to normal mid-ocean ridge basalt (MORB). These characteristics suggest a mantle source that underwent previous melt extractions and subsequent metasomatism by LILE- and light REE-enriched fluids. Geochemical modeling of trace elements shows that melting occurred at relatively low pressures under hydrous conditions and that it may have required the existence of an asthenospheric window, in which the dike magmas developed through tapping and mixing of melts generated within a rising melting column starting slightly within the garnet stability field, or in a transitional zone between the garnet and spinel stability fields at about 60 km depth. This asthenospheric window was probably created during subduction of a Neotethyan ridge system; magmas ascending from the melt column within this window generated dikes that crosscut the metamorphic soles and were injected into the overlying mantle wedge and oceanic lithosphere. The new 40 Ar/ 39 Ar hornblende dates of 92–90 Ma and 90–91 Ma from the metamorphic soles and dike swarms, respectively, show that evolution of these two geologic units was closely related in time and space and that they formed at the same intraoceanic subduction zone within the Inner Tauride seaway. These data suggest that the Tauride ophiolites within the three zones to the north originated from the same root zone situated north of the Tauride carbonate platform, and that they constitute remnants of a single ophiolitic nappe sheet derived from the Inner Tauride seaway within the Neotethyan ocean.