Petrographic, geochemical and isotopic signatures of the cebre rhyohte (demirözü-Bayburt)

Rocks of the Cebre Rhyolite, outcropping in areas near the Demirözü (Bayburt), were investigated in terms their petrographical, geochemical and isotopic characteristics and then findings obtained have been tried to interpret petrologically and tectonically in the context of this thesis. The Cebre Rhyolite with an outcropping area up to 12 km2 constitutes an example of very few extrusive rocks formed during the Variscan deformations. Zircon U-Pb data suggests that the Cebre Rhyolite was emplaced at around 332.8±4.38 Ma. Hyaloporphyritic and microporphyritic textures are dominant in the unit, in places flow banding texture is also observed. The mineral phases are represented by abundant phenocrysts of quartz and Kfeldspat, and by subordinate cordierite phenocrysts, rare muscovite microphenocrysts and biotite microcrysts set in a fine-granined groundmass. Phenocrysts largely show disequilibrium texture along their margins, which points to an interaction with matrix. Cordierite, completely altered to pinite, includes in places poikilitic zircon. Zircon, titanite, apatite and Fe-Ti oxides are accessory phases. The samples of the Cebre Rhyolite have very high SiO2 (74.23-80.62 wt. %), K2O (3.72-7.42 wt. %) and Al2O3 (10.77-14.11 wt. %) contents, but very low CaO (0.02-0.21 wt.%), Na2O (0.05-0.78 wt. %) and MgO (0.3-0.21 wt.%) contents. The samples are strongly peraluminous with A/CNK values ranging from 1.48 to 2.95 and A/NK from 1.49 to 2.99. Almost all the major and trace elements are negatively correlated with increasing SiO2. They are moderately enriched in LREE relative to HREE (LaN/YbN= 4-11) and show strong negative Eu anomaly (Eu*/Eu=0.04-0.25). Strongly negative Nb, Ta, Sr, P and Ti anomalies are the other features of the samples. Negative correlations of major and trace elements and negative Sr and Eu anomalies along with petrographical features indicate that the fractional crystallization, particularly the K-feldspat and to a lesser extent plagioclase, was played key role in the formation of final composition of the Cebre Rhyolite. Presence of alumina rich phases (cordierite and muscovite) along with geochemical features mentioned above are consistent with an origin located in the crust and point to the contribution of sediments. It is suggested that closure of the Rheic Ocean in the Early Carboniferous by way of arc-continent collision was followed by delamination, which led to asthenospheric upwelling that provided extra heat for melting of crustal material. Keywords: Demirözü, Geochemistry, Cordierite, S-type rhyolitet, Sakarya Zone, U-Pb zircon age, Variscan orogeny