Zircon SHRIMP U-Pb Ages, Geochemical, and Sr-Nd Isotopic Constraints on the Petrogenesis of the Middle Eocene Calc-Alkaline Andesitic Rocks: Implications for Continental Arc Magmatism and Slab Break-off in NE Iran

Abstract

The Torbat-e-Heydariyeh andesitic rocks (THA) are part of the Cenozoic continental arc magmatic system of the northern branch of the Neotethys Ocean (NE Iran). Columnar jointing is the most significant feature of these rocks and they also show porphyritic, vitrophyric, and vitroglomeroporphyric textures. Plagioclase, clinopyroxene, ±orthopyroxene are the major mineral phases. The SHRIMP U-Pb zircon dating yielded an age of 41.00 ± 0.69 Ma for the rocks (Middle Eocene, Bartonian). Geochemically, they are of medium- to high-K calc-alkaline affinity. Primitive mantle-normalized diagrams exhibit enrichment in large ion lithophile elements (LILE), such as Cs and Rb, and also depleted in high field strength elements (HFSE) and heavy rare earth elements (HREE), with prominent negative anomalies of Ti, Nb, Y, and Yb, suggesting a tectonic setting of an active continental margin. The chondrite-normalized REE diagram displays enrichment of light rare earth elements (LREE; La_N/Yb_N = 5.37–6.66) and small negative Eu anomalies (Eu/Eu* of 0.69–0.78). Thorium enrichment implies the reaction between the mantle wedge and the melt of subducting oceanic slab, and/or subducting sediment. The role of subducted sediments along with subducted oceanic lithosphere is evident in these magmatic rocks using Ba/La versus Th/Nd and Ba/Th versus La_N/Sm_N diagrams. The ε _Nd(t) and (⁸⁷Sr/⁸⁶Sr)_i values vary between −0.1 to +0.2 and 0.704 89 to 0.705 01, respectively, and are compatible with parental melts from subduction of the lithospheric mantle. We suggest that the THA rocks were produced by the partial melting of the metasomatized lithospheric mantle, which corresponds to slab break-off of the northward subducted Neotethys oceanic slab in an extensional setting. The hot asthenospheric mantle upwelling triggered by the Neotethys slab break-off would severely heat the physically mixed mantle wedge peridotite and therefore caused partial melting to produce the Middle Eocene volcanic rocks in NE Iran.

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