Transition from I-type to A-type magmatism in the Sanandaj–Sirjan Zone, NW Iran: an extensional intra-continental arc FATEMEH SARJOUGHIAN 1 * , ALI KANANIAN 2 , MICHAEL HASCHKE 3 and JAMSHID AHMADIAN 4 1 Department of Earth Sciences, Faculty of Sciences, University of Kurdistan, Sanandaj, Iran 2 School of Geology, College of Science, University of Tehran, Tehran, Iran 3 Umwelt und Ingenieurtechnik GmbH Dresden, Dresden, Germany 4 Department of Geology, Payame Noor University, Tehran, Iran The South Dehgolan pluton, in NW Iran was emplaced into the Sanandaj–Sirjan magmatic–metamorphic zone. This composite intrusion comprises three main groups: (1) monzogabbro–monzodiorite rocks, (2) quartz monzonite–syenite rocks, and (3) a granite suite which crops out in most of the area. The granites generally show high SiO 2 content from 72.1%–77.6 wt.% with diagnostic mineralogy consisting of biotite and amphibole along the boundaries of feldspar–quartz crystals which implies anhydrous primary magma compositions. The granite suite is metaluminous and distinguished by high FeOt/MgO ratios (av. 9.6 wt.%), typical of ferroan compositions with a pronounced A-type affinity with high Na 2 O+K 2 O contents, high Ga/Al ratios, enrichment in Zr, Nb, REE, and depletion in Eu. The quartz monzonite–syenites show intermediate SiO 2 levels (59.8%–64.5 wt.%) with metaluminous, magnesian to ferroan characteristics, intermediate Na 2 O+K 2 O contents, en- richment in Zr, Nb, REE, Ga/Al, and depletion in Eu. The monzogabbro–monzodiorites show overall lower SiO 2 content (48.5%–55.9 wt.%) with metaluminous and calc-alkaline compositions, relatively lower Na 2 O+K 2 O contents, low Ga/Al ratios, and FeOt/MgO (av. 1.6 wt.%) ratios, low abundances of Zr, Nb, and lower REE element concentrations relative to the granites and quartz monzonite–syenites. These geo- chemical differences among the three different rocks suites are likely to indicate different melt origins. We suggest that the South Dehgolan pluton resulted from a change in the geodynamic regime, from compression to extension in the Sanandaj–Sirjan zone during Mesozoic sub- duction of the Neo-Tethys oceanic crust beneath the Central Iranian microcontinent. Copyright © 2015 John Wiley & Sons, Ltd. Received 18 May 2014; accepted 16 November 2014 KEY WORDS subduction; extension; petrogenesis; A-type and I-type granitoids; Dehgolan; Sanandaj–Sirjan Zone 1. INTRODUCTION The South Dehgolan pluton forms part of a linear belt of intrusions in the Sanandaj–Sirjan zone (SSZ) about 80 km northeast from the Zagros orogenic belt in northwestern Iran. The northwest–southeast trending Zagros fold–thrust belt (Fig. 1A) is an integral part of the Alpine–Himalayan orogenic system which resulted from Arabian–Central Iranian microplate continental collision after subduction of the Neo-Tethys oceanic crust beneath Central Iran (Dewey et al., 1973; Sengör, 1984). During Middle to Late Triassic, coeval with the closure of the Palaeo-Tethys in the North, a rifting episode along the Zagros belt resulted in the opening of a new ocean called Neo-Tethys. The timing of inception of subduction is inferred to be Late Triassic–Early Jurassic (Berberian and Berberian, 1981), or Late Jurassic (Mohajjel et al., 2003). This led to a metamorphic event associated with Upper Triassic emplacement of intrusive bodies within this zone. Closure of the oceanic domain was marked by the obduction of ophiolites along the main Zagros thrust in the late Cretaceous (Agard et al., 2005), and the final closure of the Neo-Tethys and collision of Arabia and Central Iran took place during Neogene times (Berberian and Berberian, 1981; Berberian et al., 1982). The ~1500 km northwest–southeast trending 150–250 km wide SSZ zone is structurally parallel to the Zagros fold– thrust belt (Alavi, 1994) and consist of Mesozoic and Palaeozoic rocks (Berberian, 1977) which separate the stable Central Iran block from the Afro-Arabian Plate (Stöcklin, 1968). SSZ zone rocks include abundant deformed and undeformed Mesozoic plutons and volcanic rocks. At pres- ent, the SSZ structurally behaves like a rigid block moving northward at 14 mm/year relative to stable Eurasia (Vernant et al., 2004). The SSZ was intruded by numerous igneous complexes ranging from gabbro to granite compositions, typical of active continental margins. Zircon U–Pb ages *Correspondence to: F. Sarjoughian, Department of Earth Sciences, Faculty of Sciences, University of Kurdistan, Sanandaj, Iran. E-mail: fsarjoughian2@gmail.com Copyright © 2015 John Wiley & Sons, Ltd. GEOLOGICAL JOURNAL Geol. J. (2015) Published online in Wiley Online Library (wileyonlinelibrary.com). DOI: 10.1002/gj.2637