Quaternary high-Mg ultrapotassic rocks from the Qal’eh Hasan Ali maars, southeastern Iran: petrogenesis and geodynamic implications

Abstract

A set of rare, high-Mg ultrapotassic rocks from the Qal’eh Hasan Ali maars, southeastern Iran, was investigated using phlogopite 40Ar/39Ar geochronology, olivine chemistry, and bulk-rock elemental and Sr–Nd–Hf isotopic geochemistry to decipher their petrogenesis and regional tectonic implications. Phlogopite separates yield inverse isochron 40Ar/39Ar ages from ca. 112 to 119 ka, indicating that magma genesis postdated the onset of the Arabia–Eurasia collision at the Late Eocene–Early Oligocene. The studied rocks are characterized by kamafugitic affinity with relatively low SiO2 and Al2O3 and high CaO and Sr. They contain relatively primitive olivine (Fo85–92) that, on the basis of olivine–liquid Fe/Mg exchange equilibrium, suggests the primary melt to be ultrapotassic with ~13 wt% MgO. Other key geochemical features include extreme enrichment in most incompatible trace elements, depletions in Nb, Ta, P and Ti and enrichment in Pb relative to elements of similar incompatibilities. The Sr–Nd–Hf isotopic ratios of the rocks do not deviate significantly from the bulk silicate Earth (87Sr/86Sr = 0.7055–0.7059, 143Nd/144Nd = 0.5125–0.5126 and 176Hf/177Hf = 0.2827–0.2829). Relatively high Zn/Fe, Gd/Yb, Rb, Rb/Sr and P2O5 and low Yb and P/P* for the rocks are consistent with derivation from a mantle source containing clinopyroxene, phlogopite, apatite and garnet that formed in response to modal metasomatism in the lithospheric mantle. Relatively low Hf/Nd and high Sr/Hf of the rocks indicate that the metasomatized lithologies from which the studied rocks formed were derived dominantly from subducted marly sediments. The Qal’eh Hasan Ali magmatism is best explained by small-scale destruction of the continental mantle in a post-collisional setting, presumably driven by localized convective instability as a result of the Arabia–Eurasia collision.