Zircon U–Pb ages and Hf–O isotopic composition of migmatites from the Zanjan–Takab complex, NW Iran: Constraints on partial melting of metasediments

Abstract

We study migmatites and other metamorphic rocks in the Zanjan–Takab region of NW Iran and use these results to report the first evidence of Oligocene core complex formation in Iran. Four samples of migmatites associated with paragneisses, including leucosomes and associated para-amphibolite melanosomes were selected for U–Pb dating and Hf–O isotopic analysis. Zircon cores – interpreted as originally detrital zircons – have variable ages that peak at ca. 100–110Ma, but their sedimentation age – indicated by the youngest 206Pb/238U ages – is ca. 35–40Ma. New zircons associated with incipient melting occur as overgrowths around zircon cores and/or as newly grown grains. Morphologies and internal structures suggest that rim growth and formation of new zircons were associated with partial melting. All four samples contain zircons with rims that yield 206Pb/238U ages of 28–25Ma, indicating that partial melting occurred in Late Oligocene time. δ18O values for zircon rims vary between 8.2 and 12.3‰, significantly higher than expected for mantle inputs (δ18O ~6‰) and consistent with equilibrium with surface materials. Zircon rims yield εHf(t) between 2.2 and 12.4 and two-stage Hf model ages of ~448–562Ma, indicating that the region is underlain by Cadomian–Caledonian crust. According to the Hf–O isotopic values, the main mechanism forming zircon rims was dissolution of pre-existing detrital zircons with reprecipitation of new zircon shortly thereafter. Oligocene ages indicate that partial melting accompanied core complex formation in the Zanjan–Takab region. Extension, melting, and core complex formation in south-central Iran are Eocene in age, but younger ages of Oligocene–Miocene in NW Iran and Turkey indicate that extension was distributed throughout the region during Cenozoic time.