Transition from I-type to A-type magmatism in the Sanandaj-Sirjan Zone, NW Iran: an extensional intra-continental arc

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 SiO2 content from 72.1%-77.6wt.% 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. 9.6wt.%), typical of ferroan compositions with a pronounced A-type affinity with high Na2O+K2O contents, high Ga/Al ratios, enrichment in Zr, Nb, REE, and depletion in Eu. The quartz monzonite-syenites show intermediate SiO2 levels (59.8%-64.5wt.%) with metaluminous, magnesian to ferroan characteristics, intermediate Na2O+K2O contents, enrichment in Zr, Nb, REE, Ga/Al, and depletion in Eu. The monzogabbro-monzodiorites show overall lower SiO2 content (48.5%-55.9wt.%) with metaluminous and calc-alkaline compositions, relatively lower Na2O+K2O contents, low Ga/Al ratios, and FeOt/MgO (av. 1.6wt.%) ratios, low abundances of Zr, Nb, and lower REE element concentrations relative to the granites and quartz monzonite-syenites. These geochemical 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 subduction of the Neo-Tethys oceanic crust beneath the Central Iranian microcontinent. Copyright (c) 2015 John Wiley & Sons, Ltd.