Arslan, Mehmet; Temizel, İrfan; Ackerman, Lukáš; Yücel, Cem; Yazar, Emel Abdioğlu

The Cenozoic subalkaline to alkaline volcanic rocks from the Eastern Pontides Orogenic Belt (EPOB) in NE Turkey exhibit wide range of highly siderophile element (HSE) contents and Os isotope compositions that can be related to their mantle sources and parental melt evolution. Although some Eocene rocks show signs of late-stage modification, most of the studied volcanic rocks possess primary HSE and Os systematics. The Eocene volcanic rocks have largely variable PdN/IrN (~0.25–751), PdN/PtN (~0.25–87) and (187Os/188Os)i ratios (~0.113 to 0.51). The Miocene volcanic rocks have high PdN/IrN (~2.6–441), low PdN/PtN (~0.11–1.7) and radiogenic (187Os/188Os)i ratios ranging from ~0.166 to 0.52 while the Mio-Pliocene volcanic rocks show high PdN/IrN of ~7.2–29, low PdN/PtN (~1.2–2.7), and only mildly radiogenic (187Os/188Os)i values between ~0.132 and 0.177. Finally, the Quaternary volcanic rocks have low PdN/IrN (~2.3–3.6), PdN/PtN (~0.9–1.6) and radiogenic (187Os/188Os)i values ranging from ~0.133 to 0.194. Overall, the primitive mantle-normalized HSE patterns of the studied volcanic rocks are characterized by PPGE (Pt, Pd) and Re enrichment with respect to IPGE (Os, Ir, Ru). The variations in HSE and radiogenic Os isotope compositions of the relatively differentiated volcanic rocks indicate that crustal assimilation played active role in their parental melt evolution, especially in the Eocene and Miocene period. Besides, the obtained HSE data also suggest that the parental magmas of all the studied volcanic rocks were generated mainly in the S-saturated conditions. The temporal and spatial HSE and Os isotope variations observed in the studied Cenozoic volcanic rocks reflect the heterogeneity of the mantle source beneath the EPOB, which could be attributed to variations in response of different fluid/melt metasomatism of the underlying SCLM.

Temizel, İrfan; Arslan, Mehmet; Yücel, Cem; Yazar, Emel Abdioğlu; Kaygusuz, Abdullah; Aslan, Zafer

Late Cretaceous felsic plutons in the Eastern Pontides orogenic belt of Turkey are found in varying ages, sizes and compositions. The Late Cretaceous syenitic bodies, namely Direkli and Yeniköy Tepe plutons in the Ordu area, intruded into the Late Cretaceous volcanics and volcanoclastics, and compositionally consist of syenite and minor quartz-syenite with mainly granular and rarely porphyric, poikilitic and perthitic textures. The studied plutons contain K-feldspar, plagioclase, quartz, clinopyroxene, biotite and minor hornblende, and Fe-Ti oxide, apatite, and zircon. Mineral thermobarometer estimations imply that the syenitic plutons were crystallized in P-T conditions representing mid to shallow crustal levels. Petrochemically, these syenitic plutons show I-type, metaluminous-peraluminous transitional (A/CNK = 0.85–1.04) and shoshonitic features. LA-ICP-MS U-Pb zircon dating on the syenitic plutons yielded weighted mean ages of 78.5–72 Ma. Primitive mantle-normalized trace element patterns of the studied plutons are characterized by negative anomalies of Ba, Sr, Nb, Ta and Ti, and enrichment in Rb, Th, K, Ce and La. The chondrite-normalized rare earth element (REE) plots of the studied plutons show moderately enriched concave-shaped patterns (La N /Lu N = 10.3–10.7 for the Direkli pluton; La N /Lu N = 6.8–11.7 for the Yeniköy Tepe pluton) with negative Eu anomalies. The plutons have 87 Sr/ 86 Sr ratios (0.705368–0.706954), 143 Nd/ 144 Nd ratios (0.512600–0.512672), 206 Pb/ 204 Pb ratios (19.29–19.52) and 207 Pb/ 204 Pb ratios (15.65–15.67). The geochemical data, as well as regional geology, suggest that the Late Cretaceous syenitic plutons have formed in an extensional continental arc setting triggered by slab roll-back. The parental magma of syenitic plutons was derived from melts of lower crustal materials (metabasalt/metaandesite) and lithospheric mantle component.