Petrochemistry of ultrapotassic tephrites and associated cognate plutonic xenoliths with carbonatite affinities from the late Quaternary Qa’le Hasan Ali maars, central Iran

Abstract

The Quaternary Qa’le Hasan Ali (QHA) maars in central Iran occur at the intersection of the north–south Nayband fault, which defines the western boundary of the Lut micro-continental block, and a system of northwest–southeast faults, subparallel to the Zagros suture zone, that formed during the Arabian–Eurasian collision. These post-collisional maars intrude Eocene volcanic rocks of the Urumieh–Dokhtar magmatic belt, which was generated by the subduction of Neotethys oceanic lithosphere below Iran. The highly potassic, Ti-phlogopite+Mg-rich (Fo89–92) olivine+diopside-augite+aegirine–augite basanite tephrites forming the tuff rims of the QHA maars contain tephrite-coated plutonic xenoliths, some of which are interpreted as co-genetic with the tephrites based on their similar mineralogy and Sr isotopic composition (87Sr/86Sr=0.70590). Cognate plutonic xenoliths have up to ∼20vol% calcite, considered to be magmatic calcite because of (1) its grain size, which is similar to feldspars and aegirine–augite pyroxenes in these rocks, (2) the occurrence of fine-grained inclusions of pyroxene and apatite within these calcite grains, and (3) the similarity of the Sr-isotopic composition of this calcite with the other minerals in these rocks. The fact that the magmatic calcite has remained intact and did not volatilize during the transport of these xenoliths to the surface in the hot tephrite magma implies a short transit time, indicating that they are samples of a shallow plutonic complex, as does the presence of anorthoclase in these plutonic xenoliths. Their high modal proportion of magmatic calcite suggests that this shallow plutonic complex has affinities with carbonatites. The magmatic calcite-bearing plutonic xenoliths have high LREE/HREE ratios and contain REE-rich allanite (with up to ∼20wt% LREE) and britholite (∼60wt% LREE) that make up ∼3 modal percent of the most calcite-rich samples. Similar to many post-collisional highly potassic rocks formed in the Alpine–Himalayan collision zone, the highly potassic basanite tephrites of the QHA maars have high 87Sr/86Sr and low C̵Nd (−1.3 to −3.4), distinct from convecting asthenosphere, and they are depleted in Nb and Ta relative to Ba and La. They formed by small degrees of partial melting of lower continental lithospheric mantle metasomatized by both H2O and CO2-rich fluids derived from subducted Neotethys oceanic crust and sediments during the collision of the Arabian and Eurasian plates.