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- MakaleYalnız künye2017Late Jurassic Magmatism and Stratigraphy in the Eastern Sakarya Zone, Turkey: Evidence for the Slab Breakoff of Paleotethyan Oceanic Lithosphere
Dokuz, Abdurrahman; Aydinçakir, Emre; Kandemir, Raif; Karsli, Orhan; Siebel, Wolfgang; Derman, Ahmet Sami; Turan, Mehmet
Middle-Late Jurassic Cimmerian events in Turkey have been actively discussed in the past three decades, but proposed tectonic models associated with magmatism, metamorphism, and stratigraphic features remain controversial. To address this issue, Upper Jurassic mafic lavas are investigated at three locations (Alucra, Gumushane, and Olur) in the eastern Sakarya Zone, northeastern Turkey. These lavas are submarine and form planar flows parallel with the bedding plane in the Upper Jurassic carbonate sequence near the base or just below in the clastic sedimentary rocks. The basaltic lavas show calc-alkaline features and possess Nb-Ta values and Nb/U, Nb/La, and Ce/Pb ratios that are greater than those of island arc basalts. Multielement patterns are almost hump shaped, similar to ocean island basalts, which experience Pb depletion and weak negative Nb-Ta, Zr-Hf, and Ti anomalies. The low initial (Sr-87/Sr-86) ratios (0.70372-0.70554), positive initial epsilon(Nd) values (+2.7 to +4.4), and initial Pb isotope ratios that plot between mid-ocean-ridge and ocean island basalts are consistent with a melt derived from subcontinental lithospheric mantle, metasomatized by earlier fluids from subducted sediments and plume materials from the asthenosphere. Moderate Dy/Yb ratios with an average value of 1.8 imply partial melting in the spinel-garnet transition zone at depths of similar to 70-100 km. Slab breakoff is suggested as a geodynamic mechanism that accounts for these geochemical signatures. This inference is also favored by stratigraphic and sedimentologic evidence from the Upper Jurassic-Lower Cretaceous sedimentary rocks, which is consistent with short-lived vertical (epirogenic) movements in the region. Lower-Middle Jurassic sequences are transgressive, suggesting that subduction-related extension opened a backarc basin (Neotethys) in the south of the Sakarya Zone. Upper Jurassic-Lower Cretaceous carbonates point tectonically to tranquility during carbonate deposition in the Neotethys Ocean, which seems to have been achieved by complete closure of the Paleotethys in the north. About 15-20 m.yr. later (Kimmeridgian), after first carbonate deposition, intraplate-typemafic lavas ascended up to the shelf surface of the Neotethys. This was followed by formation of disconformity surfaces and then accumulation of coarse clastic sediments. All this points to a short-lived epirogenic movement that we ascribe to the breakoff of the southward-subducting Paleotethyan oceanic lithosphere in the Late Jurassic.
- MakaleYalnız künye2011Petrochemistry and U-Pb zircon ages of adakitic intrusions from the pulur massif (eastern pontides, ne Turkey): Implications for slab rollback and ridge subduction associated with Cenozoic convergent tectonics in the eastern Mediterranean
Eyuboglu, Yener; Santosh, M.; Chung, Sun-Lin
The well-preserved magmatic arc in the eastern Pontides orogenic belt of northeastern Turkey offers critical clues on convergent margin tectonics associated with the late Mesozoic-early Cenozoic geodynamic evolution of the eastern Mediterranean region. Here we investigate the petrology, geochemistry, and U-Pb zircon chronology of adakitic intrusions from the Pulur region in the southern zone of the eastern Pontides belt. The intermediate to felsic intrusives in the Pulur region are characterized by the presence of abundant plagioclase, amphibole, and biotite phenocrysts. The rocks display extreme light rare earth element enrichment (LaN YbN p13.69-51.08); high Al2O3 (15.46-17.47 N Nwt%), Na2O (3.57-8.43 wt%), Sr (324.8-1468 ppm), and La (13.9-55.8 ppm) concentrations; a high Sr/Y ratio (35-473); and low Y (3-12.6 ppm) and heavy rare earth element concentrations, closely comparing with the typical features of adakites. Zircons extracted from the Pulur adakites show euhedral crystal shapes, oscillatory zoning patterns, andhigh Th/U ratios (up to 3.33) consistent with their magmatic origin. Their 206Pb/238U ages show a range of 55.21-53.07 Ma, correlating the magmatism to the early Eocene. We propose a tectonic model involving the initiation of a slab rollback and simultaneous opening of a slab window during ridge subduction in a south-dipping sub duction zone for the origin of the Tertiary adakitic and nonadakitic magmatism in the eastern Pontides orogenic belt. We also identify a close relation between slab rollback processes and back arc extension, which led to the opening of the Tertiary basins in the southern part of the eastern Pontide orogenic belt. The eastern Pontide orogenic belt is one of the world's largest metallogenic provinces, and it hosts various economic mine ralizations such as volcanogenic massive sulfide, epithermal gold-silver, porphyry copper-molybdenum, skarn-type, and chromite. However, the adakitic intrusions exposed in the southern part of the eastern Pontide orogenic belt are devoid of ore deposits, which leads us to propose that the Lutetian or later nonadakitic granitic intrusions exposed at the north of the Gumushane-Bayburt-Ispir line were probably responsible for the epithermal gold mineralization in this belt.