A polygenetic origin for the Sikhoran ultramafic-mafic complex in South Iran

The nature and geodynamic setting of the Sikhoran ultramafic-mafic complex, southwest Iran, is debated, with multiple competing theories having been proposed in recent years. The Sikhoran ultramafic-mafic complex is located in the southern part of the Sanandaj-Sirjan metamorphic-magmatic zone (SSMMZ) and is comprised of porphyroclastic, transitional and layered ultramafic-mafic sequences of Carboniferous age. These units are cut by isotropic gabbros, diabasic and pegmatoid gabbroic dikes with ages ranging from Permian to the Cretaceous. These bodies also intrude into Upper Paleozoic metamorphic rocks, where they induced high-temperature contact metamorphism during the Late Carboniferous. Here we present new textural descriptions, mineral and whole-rock geochemical analyses, and the results of zircon U-Pb dating of magmatic mafic rocks and meta-morphic host rocks from this region in order to reconstruct the petrogenesis of the Sikhoran ultramafic-mafic complex. Zircon U-Pb ages from layered gabbros indicate Late Carboniferous (320.8 & PLUSMN; 6.4 Ma) crystallization, which is similar to the age of the Upper Paleozoic host gneisses (334.6 & PLUSMN; 4.9 Ma). By contrast, isotropic gabbros that cross-cut the complex have a crystallization age of 178.3 & PLUSMN; 2.3 Ma, and anatectic quartz diorite/plagiog-ranite produced from partial melting of hosting amphibolite in contact with the gabbros yielded Late Triassic-Early Jurassic ages (187.2 & PLUSMN; 2.6 Ma). These data suggest a polygenetic origin and a range of tectonic settings for various parts of the ultramafic-mafic complex. Geochemical modeling shows that mantle plume-related melting (-5-40%) of metasomatized mantle could have formed the ultramafic-mafic layered cumulate com-plex during the Late Carboniferous. A geochemical transition from island arc tholeiite (IAT) isotropic gabbros to cross-cutting enriched mid-ocean ridge basalts (E-MORB)-diabasic dikes occurred between the Late Triassic-Early Jurassic to the Late Cretaceous. The mantle-derived ultramafic parts of the Sikhoran ultramafic-mafic complex have similarities with other known ultramafic-mafic mantle plumes (e.g., Tinaqulllo, Lherz, and Ronda), and its layered ultramafic-mafic segment has similarity with layered intrusions (e.g., Pulur complex in Turkey). The highest observed crystallization temperature in porphyroclastic, transitional and layered ultramafic mantle sections of-1140 degrees C is considered as the mantle potential temperature, since the highest crystallization temperatures are observed in the most forsteritic olivine crystals. Based on amphibole chemistry, the crystalli-zation conditions of transitional to layered ultramafic rocks at 900-1005 degrees C occurred at high H2O contents in a melt between nickel-nickel oxide (NNO)-8.5 and NNO-9.0, which lies along the NNO-2 buffer. These results are consistent with a model in which the opening of the Zagros Neotethys Oceanic basin in the Esfandagheh region initiated in the Late Carboniferous, alongside the formation of an extensional environment that allowed ascent of a subcontinental mantle plume. Subduction initiation of oceanic lithosphere began during the Late Triassic-Early Jurassic and ended in the Late Cretaceous.