Middle Permian basic and acidic volcanism in the Istanbul zone (NW Turkey): evidence for post-variscan extensional magmatism

Cumhur Babaoğlu*, Gültekin Topuz, Aral I. Okay, Serhat Köksal, Jia Min Wang, Fatma Toksoy-Köksal

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

4 Citations (Scopus)


The Istanbul Zone (NW Turkey) forms the eastward extension of Avalonia and was subjected to deformation, uplift and erosion for a time period of 40–50 Ma following the collision with the Sakarya Zone during Early to Late Carboniferous. This paper deals with the petrology and age of the volumetrically minor basic and acidic volcanism at the lowermost horizons of Middle Permian continental red beds, which are overlain by Lower Triassic marine sedimentary rocks in the Kocaeli Peninsula. The volcanic activity is represented mainly by amygdaloidal basalt, rhyolite and minor trachydacite. The amygdaloidal basalt was derived from near-primary middle-K calc-alkaline mantle melts with negligible crystal fractionation. On the other hand, the rhyolite and trachydacite compositionally resemble A2-type rhyolites and underwent low-pressure crystal fractionation as indicated by the presence of a significant Eu anomaly. Initial ɛNd values of amygdaloidal basalt range from 0.0 to 1.5 and those of rhyolite-trachydacite are between −0.4 and −3.4. Amygdaloidal basalt and rhyolite-trachydacite are not directly related to each other by crystal fractionation. Amygdaloidal basalt probably represents the product of the near-primary mantle melts from low-degree melting of a spinel peridotitic source, and the rhyolite-trachydacite originated from highly-fractionated products of basic magmas that are slightly more alkaline than amygdaloidal basalt. However, basic and intermediate products of alkaline basic magmas are unknown in this region to date. U-Pb dating of zircons from a rhyolite sample yielded an igneous crystallization age of 261 ± 3 Ma (2σ), suggesting that the date of deposition of the continental red beds goes back to the latest Middle Permian. Based on the transgressive nature of the Permian-Triassic sequence that starts from the Middle Permian continental red beds and grades into Lower Triassic marine deposits, we suggest that the volcanism likely occurred in an extensional setting. This extension was concurrent with the northward subduction of the Palaeo-Tethys beneath the Sakarya and Istanbul zones after the Variscan orogeny. Therefore, the latest Middle to Late Permian volcanism might have occurred during the initial stage of a back-arc extensional setting.

Original languageEnglish
Pages (from-to)3435-3452
Number of pages18
JournalInternational Geology Review
Issue number21
Publication statusPublished - 2023

Bibliographical note

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This paper resulted from the master’s thesis of the first author. Financial support for this study came partially from the Turkish Academy of Sciences (TÜBA) and the National Natural Science Foundation of China (research grants #41888101, #41972065 and #92255303). Assistance from Ayşe Sehra Özkan during fieldwork, from Turgut Duzman, Ezgi Sağlam and İnal Demirkaya during sample preparation, Remziye Akdoğan during the preparation of zircon mount and Selin Süer during Sr-Nd isotopic measurements is gratefully acknowledged. We benefited from discussions with İbrahim Gedik on local geology.

FundersFunder number
National Natural Science Foundation of China41972065, 41888101, 92255303
Türkiye Bilimler Akademisi


    • A-type rhyolite
    • back-arc extension
    • basalt
    • Istanbul Zone
    • Palaeo-Tethys
    • Permian
    • variscan orogeny


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