Trace element composition of rutile and Zr-in-rutile thermometry in meta-ophiolitic rocks from the Kazdağ Massif, NW Turkey

Abstract

In northwest Turkey, ophiolitic meta-gabbros are exposed on the Kazdag Massif located in the southern part of the Biga Peninsula. Trace element composition of rutile and Zr-in-rutile temperatures were determined for meta-gabbros from the Kazdag Massif. The Zr content of all rutiles range from 176 to 428 ppm and rutile grains usually have a homogeneous Zr distribution. The rutile grains from studied samples in the Kazdag Massif are dominated by subchondritic Nb/Ta (11–19) and Zr/Hf ratios (20–33). Nb/Ta and Zr/Hf show positive correlation, which is probably produced by silicate fractionation. The Nb/Ta and Zr/Hf ratios increase with a decrease in Ta and Hf contents. The core of rutile grains are generally characterized by low Nb/Ta ratios of 17–18 whereas the rims exhibit relatively high Nb/Ta ratios of 19–23. Trace element analyses in rutile suggest that these rutile grains were grown from metamorphic fluids. The P-T conditions of meta-gabbros were estimated by both Fe–Mg exchange and Zr-in-rutile thermometers, as well as by the Grt-Hb-Plg-Q geothermobarometer. The temperature range of 639 to 662 °C calculated at 9 kbar using the Zr-in-rutile thermometer is comparable with temperature estimates of the Fe-Mg exchange thermometer, which records amphibolite-facies metamorphism of intermediate P-T conditions. The P-T conditions of meta-ophiolitic rocks suggest that they occur as a different separate higher-pressure tectonic slice in the Kazdag metamorphic sequence. Amphibolite-facies metamorphism resulted from northward subduction of the Izmir-Ankara branch of the Neo-Tethyan Ocean under the Sakarya Zone. Metamorphism was followed by internal imbrication of the Kazdag metamorphic sequence resulting from southerly directed compression during the collision.