Cordierite-bearing strongly peraluminous Cebre Rhyolite from the eastern Sakarya Zone, NE Turkey: Constraints on the Variscan Orogeny

Abstract

The Cebre Rhyolite with an outcropping area up to 12km2 is one of the rare extrusions in the Variscan basement of the Sakarya Zone. The unit consists of high-K calc alkaline rhyolites (SiO2=74–82wt.%). Abundant phenocrysts of quartz and K-feldspar are accompanied by subordinate cordierite phenocrysts, rare muscovite microphenocrysts and biotite microcrysts set in a fine-grained groundmass. Three types of rock fragments (xenoliths) have been recognized; (i) porphyritic, (ii) equigranular hypabyssal and (iii) hypocrystalline fragments. Zircon U–Pb dating indicates that the Cebre Rhyolite was extruded at 332.8±4.38Ma, which post-dates the Variscan low temperature metamorphism and pre-dates the emplacement of I-type granitic intrusions (325–303Ma).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. They have very high K2O (3.72–7.42wt.%) and Al2O3 (10.77–14.11wt.%) contents, but very low CaO (0.02–0.21wt.%), Na2O (0.05–0.78wt.%) and MgO (0.3–0.21wt.%) contents. The samples show geochemical affinity with the upper continental crust, e.g., enrichment of large ion lithophile elements (LILEs; K, Rb, U, Th, Pb), depletion of high field strength elements (HFSEs; Nb, Ta, Ti), Sr, P and Eu, but ԐNd(t) values (−3.06 to −8.75) and isotope ratios of Sr(t)(87Sr/86Sr=0.70499–0.70915) and Pb(t) (206Pb/204Pb=16.41–17.570, 207Pb/204Pb=15.54–15.59, 208Pb/204Pb=36.20–37.22) are similar to those of the lower crust. Geochemical and isotope data indicate that the Cebre Rhyolite was generated by melting of metapelitic rocks with some addition of intermediate metaigneous derived magma. As a geodynamic model, we propose that the Variscan Orogeny in Turkey was occurred by collision of Gondwana with an arc/terrane separated from the southern margin of Laurussia. This collision was followed shortly after by splitting of oceanic lithosphere into two pieces and sinking down into asthenosphere. Rapid upwelling of asthenosphere to space emptied by previous oceanic lithosphere provided heat for high temperature metamorphism and anatexis of metasedimentary rocks in the crust.