The Lar alkaline igneous complex: Age, mineralogy, chemistry, and isotopes of a potassic post-collisional event in the Eastern Iran Magmatic Belt (Sistan Suture Zone, East Iran)

This study investigates the Cenozoic magma genesis in the East Iran Magmatic Belt through a detailed petrographic, geochemical, and radiometric analysis of igneous rocks from the Lar alkaline complex in the Sistan Suture Zone (East Iran). The Lar complex consists of intrusive and hypabyssal rocks showing shoshonitic to ultrapotassic affinities with variable degrees of silica saturation. The most alkaline silica-undersaturated lithotypes include lamprophyres, nepheline-syenites, and felsic dykes of phonolitic composition, whereas silica-saturated and over-saturated rocks are represented by syenites, trachytic dykes and monzonites. U-Pb and K-Ar geochronological constraints indicate that intrusive and hypabyssal igneous rocks of the Lar complex were generated in a time span of ca. 3 Ma in the Early Oligocene (from 32.5 ± 0.4 to 29.3 ± 0.6 Ma). Lamprophyres and nepheline-syenites show pseudoleucite textures due to subsolidus unmixing forming nepheline and K-feldspar intergrowths. Incompatible trace element patterns show geochemical features compatible with a subduction-related setting ( i.e. , large ion lithophile elements -LILE- enrichments and high field strength elements -HFSE- depletions), with the alkaline potassic and silica-undersaturated rocks characterized by higher light/heavy rare earth elements (LREE/HREE) fractionation (LaN/YbN from 12 to 22) with respect to the silica-saturated and -oversaturated rocks (LaN/YbN from 10 to 12). The initial Sr-Nd-Pb isotopic composition of the Lar igneous rocks is rather homogeneous (87Sr/86Sri = 0.704295–0.706222, 143Nd/144Ndi = 0.512627–0.512748, 206Pb/204Pbi = 18.82–18.90, 207Pb/204 Pbi = 15.62–15.65, 208Pb/204Pbi = 39.00–39.16) suggesting a common mantle source and limited crustal contamination/assimilation, which only affects the most differentiated products. The geochemical and isotopic features of the Lar magmas suggest that mantle metasomatism occurred as partial melts derived mainly from carbonate-rich sediments. The formation of the Lar igneous complex is attributed to post-collisional magmatism. This magmatism was triggered by asthenospheric upwelling that caused melting within the overlying mantle. Crucially, the mantle composition had been modified (metasomatized) by materials derived from the subducted Sistan Neo-Tethyan oceanic slab.