A-type granite in the Boein-Miandasht Complex: Evidence for a Late Jurassic extensional regime in the Sanandaj-Sirjan Zone, western Iran

Abstract

The Boein-Miandasht Complex (BMC) in the central part of the Sanandaj-Sirjan Zone, western Iran, is composed of metamorphic rocks, and gabbro, gabbro-diorite, and granite intrusions, in which co-magmatic mingling structures are commonly found. Zircon U-Pb ages demonstrate that crystallization of the granitic rocks occurred during the Late Jurassic (158 ± 5 Ma to 156 ± 4 Ma). Whole-rock granite geochemistry indicates low abundances of MgO (0.43–1.16 wt%), CaO (0.92–2.34 wt%), P2O5 (0.12–0.20 wt%) and Sr (72.2–254 ppm), high contents of SiO2 (66.9–72.0 wt%), alkalis (Na2O + K2O = 8.04–8.94 wt%), Rb (153–213 ppm), high field-strength elements (HFSE), rare earth elements (REE), and high FeOt/MgO ratios. The rocks are calc-alkalic to alkali-calcic and metaluminous to slightly peraluminous, typical characteristics of ferroan A-type granites. Chondrite-normalized REE diagrams display slight enrichment of light REE, flat HREE patterns and significant negative Eu anomalies. The primitive mantle-normalized multi-element variation diagrams show negative Eu, Nb, Sr, P and Ti anomalies. Both REE and other incompatible elements are consistent with A-type granite affinities for these rocks. The thinning of the continental crust and injection of hot mafic magmas within the crust were probably responsible for the partial melting of the lower crust to produce A-type granitic magmas in the region. This process is consistent with an extensional tectonic regime in the central part of the Sanandaj-Sirjan Zone during the Late Jurassic.