Turkish Journal of Earth Sciences

High-K Ignimbrites from the Zilan Valley (Erciş-Van): Implications to High-Temperature Emplacement from Petrography and Geochemistry






Zilan Valley (Erciş-Van) area is covered by volcanic products of calcalkaline intermediate-acid compositions. Pliocene ignimbrites are one of these products cropping out along the valley, and represented by black and glassy lower level, white-yellowish cream tuffaceous middle level, and less glassy upper level. The ignimbrites vary in composition, crystal content, colour, structural features and show moderate to strong welding textures. They contain plagioclase, alkali feldspar, biotite, augite, quartz, amphibole, Fe-Ti oxides, accessory apatite and zircon, lithic clasts of volcanic rocks, microgranitoids and metasedimentary rocks. Quartz and augite appears to have crystallized first, followed by plagioclase, biotite and then alkali feldspar. Biotites in the rocks are TiO_2 rich (5.5 to 6.7 wt%), suggesting high temperature pyroclastic flow emplacement. The rocks are peraluminous, hig-K rhylitic to trachytic being relatively evolved and lacking of any iron enrichment. Major and trace element variations suggest fractionation of plagioclase, biotite, Fi-Ti oxides and apatite. The PM-normalized incompatible element patterns are characterized by enrichment in all elements from Sr to Y. The geochemical data together with oxygen and strontium isotopes suggest the importance of crustal rocks in the primary magma source were not generated bulk melting of crust but may represent involvement of localised melting of a sialic crust. Conclusively, the ignimbrites originated by mainly fractional crystallization of a hybrid magma involved mantle and crustal components.

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