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- MakaleAçık Erişim2023Geochemical and radiometric data for mafic rocks from the Guleman Ophiolite (SE, Turkey): New insights on the geodynamic evolution of the southern Neo-Tethyan ocean
Sapancı, Özgür; Tokat, Gürkan; Eraslan, Nuh Kadri; Karakaya, Abdullah Onur; Gücer, Mehmet Ali; Çimen, Okay
The Guleman Ophiolite is located in the southeast of Turkey and mainly consists of harzburgites and dunites hosting economically viable chromitite bodies, and lesser amounts of cumulate and isotropic gabbro, diabase dykes, pyroxenite and locally plagiogranite. This study reports the combined whole-rock geochemical and radiogenic isotope (Sr and Nd) data for gabbro and diabase dykes, and in-situ zircon U-Pb geochronological data for plagiogranite and gabbro from the Guleman Ophiolite. Two different geochemical groups were determined based on immobile minor and trace elements, and radiogenic isotope characteristics of gabbro and diabase dykes investigated here. The first group is represented by boninites and displays highly depleted elemental patterns compared to NMORB, and spoon-shaped rare earth element profiles. Contrarily, the second group resembles back-arc basin basalts and is characterized by NMORB-like rare earth element patterns associated with negative Nb anomalies. The boninitic samples are characterized by less radiogenic 143Nd/144Nd(i) (0.51229 to 0.51267; ƐNd = −4.8 to +2.7) and more radiogenic 87Sr/86Sr(i) (0.70549 to 0.70758) isotope compositions compared to the back-arc basin basalt-like samples (143Nd/144Nd(i) = 0.51289 to 0.51296; ƐNd = +7.0 to +8.4; 87Sr/86Sr(i) = 0.70496 to 0.70629), indicating derivation from distinct and/or heterogeneous mantle source regions within a supra-subduction zone. The LA–ICP–MS zircon U-Pb analyses yielded concordant late Cretaceous ages for gabbro (82 ± 2.9 Ma) and plagiogranite (79.9 ± 0.8 Ma) samples. The overall geochemical, isotopic and geochronological data presented here suggest that the Guleman Ophiolite consists of forearc and initial back-arc spreading sections of oceanic crust, which may have been formed in an intra-oceanic subduction zone within the Berit Ocean (northern branch of the southern Neo-Tethyan ocean). Moreover, the generation of continental arc magmatism (i.e. Baskil and Esence Granitoids) was well-documented on the active margin of the Malatya-Keban Metamorphics to the north. Therefore the closure of this oceanic basin should have resulted in multiple coeval northward subduction events during the late Cretaceous.
- MakaleYalnız künye2010Late Cretaceous high-K volcanism in the eastern Pontide orogenic belt: Implications for the geodynamic evolution of NE Turkey
Eyüboǧlu, Yener
High-K volcanics were produced in two different Late Cretaceous cycles during the infilling of back-arc basins of the eastern Pontides. The first cycle, represented by shoshonitic trachyandesites and associated pyroclastics, containing high K2O (2.74-4.81 wt-%) and NaK2O (3.60-5.51 wt-%), overlies a mid-Cretaceous ophiolitic-olistostromal melange formed during the rifting stage of a back-arc basin (Neotethys). 40Ar39Ar dating of biotite from trachyandesite at the base of the first cyle indicates that shoshonitic volcanism in the far south of the eastern Pontides started in the early Campanian. Volcanic rocks of this cycle are overlain by upper Campanian-Maastrichtian rudist-bearing reefal limestones. The second cycle of high-K volcanism is represented by analcime-bearing volcanic rocks erupted in a Neotethyan lagoonal environment. These volcanic rocks, intercalated with continental detritus, are characterized by high a2O (3.22-7.16 wt-%), now concentrated in secondary analcime crystals. Their K2O contents also range between 0.83 and 6.05 (wt-%). All of these units are disconformably overlain by Eocene turbidites with a basal conglomerate. Volcanic rocks belonging to both cycles show various enrichment degrees in LILE, HFSE, LREE, and HREE with respect to primitive mantle and chondrite [(La/Lu) cn=11.10-25.89]. Negative Nb, Ta, Zr, Hf, and Ti anomalies are typical of these subduction-related arc magmas. Geochemical similarities between the two volcanic cycles suggest that trachyandesites and analcime-bearing volcanics were derived from similar enriched mantle sources, and that they formed in the same geotectonic setting of a back-arc basin environment of the eastern Pontide magmatic arc during the Late Cretaceous. In addition, Nd-Sr and Pb isotope ratios of the investigated volcanic units indicate that their mantle melt sources were similar. The new geological, geochemical and isotopic data suggest that southward subduction of the Palaeotethyan oceanic crust can be separated into two main stages for the late Mesozoic Pontide orogenic belt. Early southward subduction ended by slab break-off during the mid-Cretaceous, resulting in upwelling and injection of asthenospheric mantle that caused the opening of the eastern Pontide back-arc basin (Neotethys). Resumption of southward subduction began with intense TH-CA bimodal volcanism in the northern front of the arc during the Turonian-Coniacian. The intensity of this Late Cretaceous magmatism in the north decreased towards the south, and a transition to monogenic calcalkaline-alkaline (CA-A) Campanian andesitic volcanism developed in the southern zone. Yet farther south, andesitic igneous activity graded into the Campanian-Maastrichtian shoshonitic and ultrapotassic volcanism in the Bayburt-Maden, Tokat, Amasya, and Gümüshaciköy areas. This zoning of the Late Cretaceous arc magmatism (TH-CA, CA-A and shoshonitic-ultrapotassic, from north to south) also supports the concept of southward subduction during late Mesozoic evolution of the eastern Mediterranean region.