The First Petrographic Textural Data of Sferulitic Rhyolites Observed in the west of İdrisyayla (Kırka-Eskişehir)

Abstract

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 Kırka region, south of Eskişehir, within the Kırka-Afyon-Isparta Volcanic Province, Western Anatolia, consists predominantly of Lower Miocene volcanic and pyroclastic sequences with rhyodacitic-rhyolitic in composition, lacustrine limestones, borate-bearing claystones, tuff, stromatolitic limestones 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 in the west of İdrisyayla in Kırka (Eskişehir) region were described for the first time with this study. The rhyolites are grayish light brown in color, have hypocrystalline hyalopilitic-holohyaline texture and present spherical structures defined as spherulites. These rocks present a completely flowing structure and are composed of volcanic glass containing rarely euhedral plagioclase 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 presents continuity in the inner texture of the spherulitic structures. This study suggests that the sferulitic structures in the İdrisyayla (Kırka-Eskişehir) region were 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 of 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 the axial cavities due to the locally folding of the flow bands and the following radial mineral growths representing the liquid phase, were also changed the laminated texture in the marginal phase of the rhyolite.