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

Öz

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 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 İdrisyayla in Kırka (Eskişehir) 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 İdrisyayla (Kırka-Eskişehir) 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.

Anahtar Kelimeler

• Devitrification
• lithophysae
• rhyolite
• spherulite
• Kırka
• Eskişehir

İdrisyayla Volkanitlerinde (Kırka-Eskişehir) Gözlenen Sferulitik Riyolitlere ait İlk Petrografik Dokusal Veriler / The First Petrographic Textural Data of Sferulitic Rhyolites Observed in the İdrisyayla Volcanites (Kırka-Eskişehir)

Öz

Sferulitik (küresel) yapılar özellikle alkali magmatik kayaçlarda gelişen, eşmerkezli olarak zonlu kristal kümeleri olarak tanımlanırlar ve genellikle farklı mineral katmanları veya kabukları şeklinde ortaya çıkarlar.

Batı Anadolu’da Kırka-Afyon-Isparta Volkanik provensi içinde Kırka (Eskişehir) bölgesinde gözlenen volkanosedimenter seri baskın olarak Erken Miyosen yaşlı dasitik-riyolitik bileşimli volkanikler, gölsel kireçtaşları, borat-içerikli kiltaşları, tüf, stromatolitik kireçtaşları ve riyolitler ile bazaltlardan oluşmaktadır. Bu çalışma İdrisyayla kuzeyindeki Neojen seri içerisinde yüzlek veren sub-alkali riyolitik lav domunun kenar zonunda gözlenen sferulitik yapıların iç oluşum mekanizmaları tanımlanmıştır. Riyolitler, grimsi açık kahve renkte, hipokristalen hiyalopilitik-holohiyalin ve sferulit özellikler sunmaktadır ve tamamen akma yapısı sunan, nadir olarak gelişmiş özşekilli plajiyoklas, sanidin ve kuvars mikrokristalleri içeren volkanik camdan oluşmaktadır. Riyolitlerin dış yüzeyinde gözlenen küresel yapılar yaklaşık 10-15 cm boyutlarına kadar ulaşmaktadır. Neredeyse tam yuvarlağa yakın şekil sunan sferolitler lav akma bantları içinde konsantrik halkalar şeklinde görünmektedir. Sferulitik yapıların iç dokusunda volkanik cam fazının akma bant dokusu devamlılık sunmaktadır.

Söz konusu sferulitik yapılar, etkilendiği yüksek sıcaklık ve hızlı soğuma sonucunda, devitrifikasyon mekanizmasıyla ortaya çıkmıştır. Sferulitik riyolitlere ait petromineralojik özellikler, erken magmatik süreçte, silise aşırı doygun sıvının, nispeten farklı yoğunluktaki silikat sıvı fazları içinde kristalleştiğini ve bir merkezden itibaren ışınsal kuvars-feldispat kristal büyümelerinin, bu riyolitlerde sferulitik yapının oluşumuna neden olduğunu göstermektedir. Bunun yanında, yerel olarak akma bantlarının kıvrımlanması ile oluşan eksen boşluklarındaki katı faz mineral büyümesi ve devamında sıvı fazdan ışınsal mineral büyümelerinin geliştiği litofiz benzeri yapıların da riyolitin kenar fazında laminar dokuyu değiştirdiği önerilmektedir.

Anahtar Kelimeler

• Devitrifikasyon
• küresel yapılar
• litofiz
• riyolit
• Kırka
• Eskişehir

Kaynakça

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