Petrogenesis of the Niğde Mafic Complex, Turkey: implications for the tectonic and geomorphic evolution of Central Anatolia

Abstract

The Central Anatolian Crystalline Complex (CACC) is a high-grade metamorphic terrain that developed through a protracted history of collision, ophiolite obduction and arc magmatism during the closure of the Neotethys Ocean. The Central Anatolian Ophiolite (CAO) was emplaced onto the CACC during Cretaceous Neotethyan closure but is now highly dismembered. The Niğde Mafic Complex (NMC) is an association of metamorphosed and variably deformed gabbro, diabase, plagiogranite and ultramafic rocks in the southernmost metasedimentary massif of the CACC - the Niğde Massif. The Niğde Mafic Complex is described in detail for the first time, including its structural, petrologic and geochemical features. Mafic, ultramafic and felsic rocks from two adjacent regions, the Ecemiş Mafic Complex (EMC - east of the Niğde Massif - in the Ecemiş Fault corridor) and the Northern Region (isolated mafic-ultramafic outcrops north of the Niğde Massif) are also examined and compared. In addition, gabbro cobbles from basins adjacent to the Niğde Massif are investigated and their likely provenance is suggested. Whole-rock XRF and ICP-MS data indicate a common, suprasubduction zone, ophiolite origin for rocks of the NMC, EMC and Northern Region, and confirm their correlation to the Central Anatolian Ophiolite. Despite variable mineral assemblages, isotropic gabbro from all three regions has similar geochemical features, including depleted LREE (LaN/YbN=0.40-0.89), high Mg numbers (61-87) and low TiO2 (0.13-0.45 wt. %) that indicate a highly depleted mantle source, consistent with a forearc (or pre-arc) origin. Mineral compositions were determined through microprobe analysis. Gabbro mineral assemblages include magnesian orthopyroxene (En64-79) and calcic plagioclase (An74-95), which are typical of suprasubduction zone, water-saturated conditions for magma genesis. Post-kinematic muscovite, garnet and tourmaline-bearing granitoids intruded the NMC and are likely related to the crustally derived Üçkapılı Granite, limiting the age of ophiolite obduction and deformation to before ~85 Ma. Results of hornblende-plagioclase geothermometry for the NMC indicate mid-upper amphibolite facies deformation and metamorphism at temperatures ranging from ~560 to 785°C, similar to conditions reported for the adjacent metasedimentary rocks of the Niğde Massif. Variations in mineral compositions and assemblages among the NMC, EMC and Northern Region reveal a metamorphic gradient whereby the NMC experienced mid-upper amphibolite-facies conditions associated with ductile deformation, and the EMC and Northern Region display evidence for low-grade metamorphism without significant ductile deformation. Overall these findings reveal that the Central Anatolian Ophiolite was locally deformed and metamorphosed in the mid-crust along with tectonically underlying metasedimentary units. The Ulukıșla and Ecemiş Basins, bordering the Niğde Massif locally contain abundant gabbro cobbles in Tertiary conglomerates. Gabbro cobbles from the Oligocene Çukurbağ Formation in the Ecemiş Basin have a mixture of island-arc tholeiite and boninitic geochemical affinities, whereas all cobbles from Messinian conglomerates of the m3pl unit have boninitic geochemistry, exemplified by Ti/V ratios of less than 10. Based on the presence of metamorphic hornblende and the similarity of geochemical features, gabbro cobbles of the m3pl unit may have been sourced in the Niğde Mafic Complex or in other parts of the CAO. This has implications for our understanding of the landscape evolution of the Niğde Massif region including the former extent of the Niğde Mafic Complex.