Geological and Geochemical Characteristics of Late Ordovician Volcanic Levels of the Gözdağ Formation in the İstanbul-Zonguldak Tectonic Unit (NW Turkey): Implications for Global Events During the Ordovician

Öz

The İstanbul-Zonguldak Tectonic Unit is regarded as the easternmost fragment of Avalonia-Carolina and designated as Far East Avalonia. Its stratigraphy is characterized by discontinuous sedimentation from Late Ediacaran to Late Carboniferous. In the western part of the block, known as İstanbul Terrane, the Gözdağ Formation is represented by lagoonal sedimentary rocks consisting of shale-sandstone with limestone of Middle Ordovician-Lower Silurian age. Here, I report on stratigraphic positions and petrographical and geochemical data of fine- and coarse-grained tuffs and lavas in the Late Ordovician strata of the Gözdağ Formation. The fine- and coarse-grained tuffs have pyroclastic and the lavas have porphyritic, vitrophyric and aphanitic textures. The fine-and coarse-grained tuffs are of Sandbian and Katian ages, and the lavas have Hirnantian ages, according to the stratigraphic positions of the Late Ordovician volcanic rocks. The fine-grained tuffs have high potassium calc-alkaline, and the coarse-grained tuffs and lavas have a calc-alkaline character. They are devoid of noticeable with-in plate components, as deduced by the presence of obvious negative Nb anomalies, and they have subduction signatures. In conjunction with data from the literature, the Sandbian fine-grained tuffs were deposited in a lagoonal depocenter in the İstanbul-Zonguldak Tectonic Unit in the earliest Late Ordovician due to multiple Plinian-type eruptions during the last phase of the Taconic orogeny, which formed between the Piedmont Terrane and Laurentia. The Katian coarse-grained tuffs were products of volcanic activities formed in the arc settings during the last stage of depletion of the Teisseyre-Tornquist Ocean, lying between Avalonia and Baltica. The Hirnantian lavas were formed by flowing in a lagoonal depocenter of the İstanbul-Zonguldak Tectonic Unit during the soft-docking of Avalonia and Baltica, known as the pre-Caledonian orogeny.

Anahtar Kelimeler

• Far East Avalonia
• Gözdağ Formation
• İstanbul-Zonguldak Tectonic Unit
• Ordovician
• Volcanic Rocks

İstanbul-Zonguldak Tektonik Birliği'ndeki Gözdağ Formasyonu'nun Üst Ordovisiyen Yaşlı Volkanik Seviyelerinin Jeolojik ve Jeokimyasal Özellikleri (KB Türkiye): Ordovisiyen Sırasındaki Küresel Olaylar İçin Çıkarımlar / Geological and Geochemical Characteristics of Late Ordovician Volcanic Levels of the Gözdağ Formation in the İstanbul-Zonguldak Tectonic Unit (NW Turkey): Implications for Global Events During the Ordovician

Öz

İstanbul-Zonguldak Tektonik Birliği, Avalonya-Karolina'nın en doğudaki parçası olarak kabul edilir ve Uzak Doğu Avalonya'yı temsil eder. Stratigrafisi ayrıca Üst Ediyakaran'dan Üst Karbonifer'e kadar kesintili sedimantasyon ile karakterize edilir. Bloğun İstanbul Birliği olarak bilinen batı kesiminde Gözdağ Formasyonu, Orta Ordovisiyen-Alt Silüriyen yaşlı şeyl-kumtaşı kireçtaşından oluşan lagünel tortul kayaçlarla temsil edilmektedir. Burada, Gözdağ Formasyonu'nun Üst Ordovisiyen tabakalarındaki ince- ve kaba-taneli tüflerin ve lavların stratigrafik konumları ile petrografik ve jeokimyasal verilerini rapor ediyorum. İnce- ve kaba- taneli tüfler piroklastik, lavlar ise porfirik, vitrofirik ve afanitik dokuludur. Üst Ordovisiyen yaşlı volkanik kayaçların stratigrafik konumlarına göre, ince- ve kaba-taneli tüfler Sandbiyen ve Katiyen, lavlar ise Hirnansiyen yaşlıdır. İnce-taneli tüfler yüksek potasyumlu kalk-alkali, kaba-taneli tüfler ve lavlar kalk-alkali karaktere sahiptir. Fark edilebilir negatif Nb anomalilerinin varlığından anlaşıldığı gibi, bariz plaka içi bileşenlerden yoksundurlar ve yitim imzalarına sahiptirler. Literatürden elde edilen verilerle bağlantılı olarak, Sandbiyen ince-taneli tüfler Piedmont Birliği ile Lavrensiya arasında en erken geç Ordovisiyen' de oluşan Takonik orojenezinin son evresi sırasında çoklu pliniyen-tipi patlamaların bir sonucu olarak İstanbul-Zonguldak Tektonik Birliği'ndeki bir lagün alanında çökelmiştir. Katiyen kaba-taneli tüfler Avalonya ile Baltika arasında uzanan Teisseyre-Tornquist Okyanusu'nun tüketilmesinin son aşamasında yay ortamında meydana gelen volkanların ürünleridir. Hirnansiyen lavları ise Kaledoniyen öncesi orojenezi olarak bilinen Avalonya ve Baltika'nın yumuşak yanaşması sırasında İstanbul-Zonguldak Tektonik Birliği' nin bir lagün ortamında akarak oluşmuştur.

Anahtar Kelimeler

• Gözdağ Formasyonu
• İstanbul-Zonguldak Tektonik Birliği
• Ordovisiyen
• Uzak Doğu Avalonya
• Volkanik Kayaçlar

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