Petrogenesis of Late Campanian Alkaline Igneous Rocks in Eastern Anatolia: Magmatism related to a subduction transform edge propagator (STEP) fault?

ABSTRACT

In eastern Anatolia, the Divriği-Hekimhan Magmatic Province (DHMP) includes ~77–69 Ma alkaline rock units which are located to the NW of the Baskil Arc of ~85–74 Ma. The magmatic rocks are composed of nepheline (Ne)–to quartz (Q)–normative alkaline basaltic to trachytic/syenitic units. Among them, the basaltic rocks are composed of plagioclase (Or_2–11Ab_32–51An_39–64) + clinopyroxene (Wo_47–51En_35–42Fs_9–16) + Fe-Ti oxide ± alkali feldspar (Or_57–98Ab_2–42An₁) ± biotite ± olivine. Their ⁸⁷Sr/⁸⁶Sr_(I) ratios and ɛNd_(I) values vary in the ranges of 0.70591–0.70871 and−3.2–1.6, respectively. The subvolcanic trachytic rocks are composed of perthitic alkali feldspar phenocryst in a matrix of feldspar (Or_45–61Ab_38–54An_0–2), biotite, and Fe-Ti oxides. The trachytic volcanic rocks are made up of feldspar (Or_38–63Ab_34–59An_1–4) in a fine-grained matrix. Their ⁸⁷Sr/⁸⁶Sr_(I) ratios and ɛNd_(I) values vary in the ranges of 0.70532–0.70952 and−3.2–0.7, respectively. The syenitic rocks in the region contain both quartz- and nepheline-sodalite-bearing syenites. Geochemical features reveal that the Ne-normative basaltic magmas have undergone mafic mineral fractionation coupled with crustal contamination to produce the Q-normative derivatives. Enhanced differentiation of the Ne- and Q-normative fractionated magmas via feldspar-dominated fractionation created the silica-undersaturated and -oversaturated trachytic magmas, respectively. During the feldspar-dominated differentiation, the re-melting of accumulated alkali feldspars in the magma chamber likely gave rise to the formation of trachytic rocks with alkali feldspar-like whole rock compositions. The final products of the Ne-normative magmas are represented by the phonolites and foid-syenites with silica-undersaturated eutectic compositions. A geochemical evaluation of the basaltic rocks revealed that the alkaline magmatism mainly originated from a shallow asthenospheric mantle source which had previously been metasomatized by oceanic to continental subduction. We suggest that the DHMP was formed in response to STEP fault-controlled rolling back of the northward subducting slab of the Baskil Arc, which created a localized gap for asthenospheric upwelling.

KEYWORDS:

• Eastern Anatolia
• alkaline magmatism
• STEP fault
• geochemical modelling

Acknowledgments

We thank Ayça Yıldırım, Kubilay İsmet Çakar, Efe Gürsoy, Göktuğ Yavuz, and Yiğit Kocaayan for their bits of help during the sample preparation. Dr Serhat Köksal is acknowledged for his support with the isotopic measurements. Dr Hossein Azizi and an anonymous reviewer are thanked for their constructive suggestions. Dr Robert J. Stern is thanked for his editorial handling.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Supplementary material

Supplemental data for this article can be accessed online at https://doi.org/10.1080/00206814.2023.2201830

Additional information

Funding

This study was supported by the Scientific and Technological Research Council of Turkey (TÜBİTAK; grant number: ÇAYDAG-119Y187), the Department of Scientific Research Projects of Dokuz Eylül University (grant number: 2019.KB.FEN.006), and the Turkish Academy of Sciences (TÜBA-GEBİP/2018).