| Summary: | 碩士 === 國立臺灣大學 === 地質科學研究所 === 105 === The Neo-Tethyan orogenic evolution of eastern Turkey in the Arabia-Eurasia collision zone involves two tectonic units, i.e., the Pontides in north and the Anatolide-Tauride in south, separated by at least two ophiolite belts representing relicts of the oceanic crust from the northern and southern branches of Neo-Tethys, respectively. To better understand the complex geologic history of Eastern Pontides, particular during the Cretaceous and Eocene when the northern branch of Neo-Tethys may have closed, a combined geochronological and geochemical analysis of main magmatic outcrops was conducted. The result, including specifically zircon U-Pb age and geochemical data of 26 samples, allows us to divide the magmatism into three stages: (1) Early Cretaceous (ca. 144-128 Ma) calc-alkaline rocks, composed of gabbro to granodiorite lithologies; (2) Late Cretaceous (87-77 Ma) tholeiitic to high-K calc-alkaline rocks, composed of diversified lithologies from diorite to granite and from basalt to rhyolite; (3) Early Eocene (55-54 Ma) adakitic rocks, exclusively of felsic composition such as granodiorite and rhyolite; and Late Eocene (43-40 Ma) high-K clac-alkaline rocks, also of diversified lithologies. The Early Cretaceous magmatism, which can be correlated to broadly coeval granitoids emplaced in Armenia, is attributed to the subduction following opening of the northern branch of Neo-Tethys. The second stage of magmatism that exhibits significant lithological and geochemical variations is interpreted as the products of ending subduction and subsequent closure of the northern branch of Neo-Tethys and the opening eastern Black Sea Basin. Under this framework, the two suites of Eocene magmatism are post-collisional activities that resulted from partial melting of the lower part of a collision-thickened juvenile crust and the subduction-metasomatized continental lithospheric mantle, respectively.
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