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 _2–42An ₁) ± 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.

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