Petrogenesis of two post-collision ultrapotassic volcanic suites from northwest and central Iran

The petrogenesis of two post-collision ultrapotassic alkaline volcanic suites has been investigated with special reference to the identification of the source components, source mineralogy and the source melting processes. These are the Sarai volcano of northwest Iran and the Rayen maars in central Iran, both of which belong to the Tabriz-Bazman continental margin arc.; Magmatic activity at Sarai started at ∼10.3 Ma and ceased ∼8.7 Ma ago. Early removal of olivine and the fractionation of clinopyroxene and phlogopite from a primary leucitite magma in a shallow magma chamber led to the formation of a spectrum of silica-undersaturated (leucitite) and silica-saturated (trachyte) rocks. Post-eruption leucite-analcime transformation changed the older (∼10.3 Ma), more evolved leucitites and leucite phonolites to analcimites and analcime phonolites, respectively, but did not affect the less evolved, younger (∼9.1 Ma) leucitites. This generated two distinct alkaline rock series at (1) an ultrapotassic series, including leucitites and minettes, and (2) a potassic series, including analcimites and analcime phonolites.; Ultrapotassic volcanic ejecta were erupted from maars in the Rayen area between 5000 and 50,000 years ago. The ultrapotassic volcanic rocks (rayenite) show a wide range of textures from porphyritc to cumulate, and all textures in between. They have consistent mineral chemistry and do not show significant geochemical diversity.; Volcanic rocks from both suites are characterized by arc-related geochemical signatures: high large-ion lithophile element (LILE) concentrations, high LILE/HFSE (high field strength element) and high LREE/HREE (light rare earth element/heavy rare earth element) ratios, relative depletion of Ti, Nb, and Ta and high Al₂O₃, compared to basalts from other tectonic settings. The Sr, Nd and Pb ratios of the Sarai samples are comparable to other Roman province-type high-K rocks, but those of rayenites are close to the values of the bulk silicate Earth.; Involvement of the continental crust in the petrogenesis of these rocks has not been significant. The Sarai samples provide evidence for a sediment contribution to their source region, but there is no geochemical evidence of sediment subduction for the rayenites. A phlogopite- and clinopyroxene-rich and olivine-poor source (i.e., phlogopite-clinopyroxenite) is suggested for the ultrapotassic rocks of both localities. The source phlogopite did not melt (react out) completely. Residual garnet in the source buffered the Yb and HREE concentrations of the high-K rocks. It is concluded that the mantle wedge beneath the Zagros orogenic belt retained its subduction-related geochemical features, long after subduction ceased ∼55 Ma ago.