The First Petrographic Textural Data of Sferulitic Rhyolites Observed in the Idrisyayla Volcanites (Kirka-Eskisehir)

Spherulitic structures are classically defined as concentrically zoned crystal clusters that form in igneous systems (especially in alkaline igneous rocks), and usually occur as different mineral layers or shells. The volcano-sedimentary series observed in the Kirka region, south of Eskisehir, within the Kirka-Afyon-Isparta Volcanic Province, Western Anatolia, consists predominantly of Lower Miocene volcanic and pyroclastic sequences with rhyodacitic-rhyolitic composition, lacustrine limestones, borate-bearing claystones, tuff, stromatolitic limestones and rhyolites and basalts. The formation mechanisms of the spherulitic structures observed in the marginal zone of the sub-alkaline rhyolitic lava dome outcropping within the Neogene series west of Idrisyayla in Kirka (Eskisehir) region are described for the first time with this study. The rhyolites are grayish light brown in color, have hypocrystalline hyalopilitic-holohyaline texture and have spherical structures defined as spherulites. These rocks have a completely flowing structure and are composed of volcanic glass containing rare euhedral plagioclase, sanidine and quartz microcrystals, the spherical structures observed on the surface of the rhyolites are up to 10-15 cm in size. The spherulites, which are almost round in shape, appear as concentric rings within the lava flow bands. The flow band texture of the volcanic glass phase is continuous with the inner texture of the spherulitic structures. This study suggests that the spherulitic structures in the Idrisyayla (Kirka-Eskisehir) region formed by the devitrification mechanism as a result of high temperature and rapid cooling. The petro-mineralogical features of the spherulitic rhyolites show that during the early magmatic stage, the oversaturated liquid crystallized in silica-rich fluid phases with relatively different densities, and radial quartz-feldspar crystal growths from a center caused the formation of the spherulitic structure. It is also suggested that the lithophysae-like structures formed by the crystal growths representing the solid phase in axial cavities due to the local folding of flow bands and the later radial mineral growths representing the liquid phase also changed the laminated texture in the marginal phase of the rhyolite.