U–Pb geochronology and geochemistry of Bibi-Maryam pluton, eastern Iran: Implication for the late stage of the tectonic evolution of the Sistan Ocean

Abstract The Bibi-Maryam pluton crops out in the Sistan suture zone, eastern Iran. This pluton is a 1.5 × 2 km stock composed of leucocratic tonalite, granodiorite and granite. U–Pb zircon geochronology of a leucogranite indicates an emplacement age of 58.6 ± 2.1 Ma (95% confidence). The Bibi-Maryam rock suite is sodic with elevated Na2O/K2O (2.9 to 5.5), Sr/Y (15.6–62.2), La/Yb (13.3–22.2), and low MgO (0.86–1.81) abundances. It lacks significant Eu anomalies. Because of these geochemical characteristics, Bibi-Maryam rocks are similar to high-SiO2 adakites. Trace element modeling indicates that the Bibi-Maryam adakitic rocks could be produced by 5–8% non-modal batch partial melting from a source with composition of 95% N-MORB + 5% sediment in the presence of 35–40% amphibole + 5–10% garnet + 55–60% clinopyroxene + 1% apatite + 1% rutile. This source mineralogy is similar to hornblende eclogite or garnet amphibolites. Collectively, these data provide new constraints for the evolution of the Sistan suture zone and suggest that the Bibi-Maryam pluton formed via slab melting in an oceanic arc and pre-plate collision tectonic setting. This implies that the closure of the Sistan Ocean and Lut–Afghan continental blocks collision happened after the Bibi-Maryam emplacement at 58.6 ± 2.1 Ma.