The Lar alkaline igneous complex: genesis and geochemical features of a subduction-related magmatic event in the Sistan belt (SE Iran)

We present a detailed geochemical and petrographic study on samples from the Lar alkaline igneous complex, which is located in the Sistan belt (SE, Iran), a poorly studied area of the Alpine-Himalayan orogenic belt. The complex is composed of intrusive and hypabyssal rocks with shoshonitic to ultrapotassic affinity with variable silica saturation degree. The most alkaline rocks 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, respectively. Lamprophyres and nepheline-syenites show petrographic textures suggesting subsolidus unmixing processes between nepheline and K-feldspar recalling the occurrence of pseudoleucite and represent the most alkaline potassic rocks of the Oligocene magmatism in the Sistan-Lut collisional belt. Incompatible trace element patterns show geochemical features compatible with a subduction-related setting (i.e., LILE enrichments and HFSE depletions), with the alkaline potassic and silica-undersaturated rocks characterized by higher LREE/HREE fractionation (LaN/YbN from 12 to 22) with respect to the silicasaturated and -oversaturated rocks (LaN/YbN from 10 to 12). Preliminary investigation on the mineral chemistry highlights that lamprophyres (mg# 0.70-0.79) are in fact cumulates of olivine (Fo up to 75) and clinopyroxene (mg# up to 0.83), thus attesting the absence of near primary melts in the complex. Ne-syenites (mg# 0.50-0.60) show comparable clinopyroxene compositions (mg# up to 0.82) and equilibrium temperature around 950°C, in agreement with the results of phlogopite-melt geothermometer. The nepheline compositions in the pseudoleucite textures indicate stabilization temperatures under 700°C. The Sr-Nd-Pb isotopic compositions are rather homogeneous and suggest a possible common mantle source for the Lar igneous rocks and that the crustal contamination/assimilation processes affect only the most differentiated products. A geochemical and isotopic-based modelling suggests that the metasomatic agents, which modified the mantle beneath the study area, are mainly constituted by melts derived from carbonaterich sediments, whereas melts from carbonate-poor sediments are subordinate. In the general framework of the Alpine-Himalayan orogen, magmas of the Lar igneous complex show interesting geochemical similarities with those of the post-leucititic phase of the Neapolitan district in the Roman Magmatic Province, suggesting analogies in the magma genesis among different sectors of the Tethyan realm.