Geochemical and geochronological evidence for a Middle Permian oceanic plateau fragment in the Paleo-Tethyan suture zone of NE Iran

Abstract

The mafic–ultramafic Fariman complex in northeastern Iran has been interpreted as a Paleo-Tethyan ophiolitic fragment with<br><br>subduction- and plume-related characteristics as well as a basin deposit on an active continental margin. Contributing to this<br><br>issue, we present geochemical, geochronological, and mineralogical data for transitional and tholeiitic basalts. Thermodynamic<br><br>modeling suggests picritic parental magmas with 16–21 wt% MgO formed at plume-like mantle potential temperatures<br><br>of ca. 1460–1600 °C. Rare pyroxene spinifex textures and skeletal to feather-like clinopyroxene attest to crystallization from<br><br>undercooled magma and high cooling rates. Chromium numbers and TiO2<br><br>concentrations in spinel are similar to those in<br><br>intraplate basalts. 40Ar–39Ar dating of magmatic hornblende yielded a plateau age of 276 ± 4 Ma (2σ). Transitional basalt<br><br>with OIB-like trace element characteristics is the predominant rock-type; less frequent are tholeiitic basalts with mildly<br><br>LREE depleted patterns and picrites with intermediate trace element characteristics. All samples show MORB-OIB like<br><br>Pb/Ce, Th/La, and Th/Nb ratios which preclude subduction-modified mantle sources and felsic crustal material. Tholeiitic<br><br>basalts and related olivine cumulate rocks show MORB-like initial εNd values of + 9.4 to + 6.2 which define a mixing line<br><br>with the data for the transitional basalts (εNd ca. + 2.6). Initial 187Os/188Os ratios of 0.124–0.293 support mixed sources with<br><br>a high proportion of recycled mafic crust in the transitional basalts. High concentrations of highly siderophile elements are<br><br>in agreement with the high mantle potential temperatures and inferred high-melting degrees. It is argued that the Fariman<br><br>complex originated by melting of a mantle plume component as represented by the OIB-like transitional basalt and entrained<br><br>asthenosphere predominant in the MORB-like tholeiites. Two lines of evidence such as association of the Fariman complex<br><br>with pelagic to neritic sedimentary rocks and the tectonic position at the boundary of two continental blocks defined by<br><br>ophiolites and accretionary complexes of different ages suggest formation in an oceanic domain. Thus, we interpret it as a<br><br>fragment of an oceanic plateau, which escaped subduction and was accreted as exotic block in the Paleo-Tethyan suture zone.