Geochemistry and U-Pb zircon geochronology of the Alvand plutonic complex in Sanandaj-Sirjan Zone (Iran): New evidence for Jurassic magmatism

This paper presents geochemical, Sr-Nd isotopic, and U-Pb zircon geochronological data on the Alvand plutonic complex in Sanandaj-Sirjan zone (SSZ), Western Iran. The gabbroic rocks show a trend of a calc-alkaline magma suite and are characterized by low initial (87)Sr/(86)Sr ratios (0.7023-0.7037) and positive epsilon Nd(t) values (2.9-3.3), which suggest derivation from a moderately depleted mantle source. Geochemical features of the granites illustrate a high-K calc-alkaline magma series, whereas the leucocratic granitoids form part of a low-K series. Granites have intermediate (87)Sr/(88)Sr ratios (0.707-0.719) and negative epsilon Nd((t)) values (-1.0 to -3.4), while leucocratic granitoids have higher initial (87)Sr/(86)Sr ratio (0.713-0.714) and more negative epsilon Nd((t)) values (-3.5 to -4.5). Potential basement source lithologies for the granites are Proterozoic granites and orthogneisses, and those for the leucocratic granites are plagioclase-rich sources such as meta-arkoses or tonalites. The U-Pb dating results demonstrate that all granitoids were exclusively emplaced during the Jurassic instead of being Cretaceous or younger in age as suggested previously. The pluton was assembled incrementally over c. 10 Ma. Gabbros formed at 166.5 +/- 1.8 Ma, granites between 163.9 +/- 0.9 Ma and 161.7 +/- 0.6 Ma, and leucocratic granitoids between 154.4 +/- 1.3 and 153.3 +/- 2.7 Ma. Granites and leucocratic granitoids show some A-type affinity. It is concluded that the Alvand plutonic complex was generated in a continental-arc-related extensional regime during subduction of Neo-Tethyan oceanic crust beneath the SSZ. The U/Pb zircon age data, recently corroborated by similar results in the central and southern SSZ, indicate that Jurassic granitoids are more areally extensive in this belt than previously thought. (C) 2010 Elsevier Ltd. All rights reserved.