Petrogenesis and tectonic significance of Sardasht syenite–monzonite–gabbro–appinite intrusions, NW Iran

Abstract

The Middle Eocene intrusive rocks of Sardasht are located in the north-western part of the Sanandaj–Sirjan Zone (SSZ), NW of Iran. The Sardasht complex is a massive or occasionally layered intrusion composed of mafic (gabbro, appinitic gabbro, and appinite) and felsic (syenite and monzonite) units with respective calc-alkaline and alkaline-shoshonite affinities. Mafic mineral constituents are plagioclase, clinopyroxene, olivine, amphibole and opaque minerals which display granular, poikilitic, ophitic, and reaction textures. Amphibole is the dominant mafic mineral displaying an extraordinary variability in texture and chemical composition characterizing the appinite suites. The felsic units are mainly composed of perthitic alkali feldspar along with low abundances of plagioclase, biotite, amphibole and opaque minerals. The occurrence of mafic minerals with felsic minerals in the syenites are typical of agpaitic texture indicating that mafic aggregates and felsic units of the Sardasht intrusions were injected simultaneously. The abundance of major and trace elements show that mafic intrusions were originated from a metasomatized spinel lherzolite mantle with high concentrations of light rare earth elements (LREEs), Ti, Sr and Ba and low contents of Hf, Zr, Nb, and Ta. Mafic and felsic rocks showed the 87Sr/86Sr ratios ranging from 0.7044 to 0.7049 and from 0.7079 to 0.7095 with e(Nd)40.7 Ma ≈ + 4.1–7.6 and + 4.0–4.6, respectively. Combining petrological, geochemical, and isotopic findings, it is suggested that the Sardasht mafic and felsic intrusions were formed as a result of Neotethys slab break-off and transtension along the SE-trending lateral strike-slip fault zones related to oblique subduction of the Neotethys plate underneath east the SSZ in Early to Middle Eocene. The felsic intrusions were resulted from partial melting of the arc crust base due to the injection of hydrous-mafic magma formed in the mantle wedge into the base.