The Late Miocene Kuh-e-Siah Volcano of Arc-Front of Urumieh‒Dokhtar Magmatic Arc (Gavkhuni Lagoon, Yazd Province, Iran): Petrochemical Characteristics and Whole-Rock Geochemistry

The Late Miocene Kuh-e-Siah volcano situated in nearly central part of the Urumieh‒Dokhtar magmatic arc (UDMA) and north of the Gavkhuni Lagoon (East of the Varzaneh, south‒east of Isfahan, Iran). Most of the whole-rock samples are basaltic andesite, but small volumes of basalts found only in some locations. The main textures of the studied rocks are porphyritic, microlitic porphyritic, glomeroporphyritic, poikilitic, vesicular, amygdaloidal, interstitial and trachytic. The main phenocrysts are plagioclase, olivine, clinopyroxene and orthopyroxene. They are euhedral to subhedral in shape and sometimes showing rounded rims. Some olivine crystals are corroded or iddingsitized. Cr-spinel found mainly as inclusions in phenocrysts or in the groundmass. The matrix of these volcanic rocks is microcrystalline and it is made of the same minerals observed as phenocrysts. Some of phenocrystic orthopyroxenes have a rim of pigeonite. Secondary minerals are chlorite, calcite and magnetite. Cavities filled by calcite. Some spinel grains have a ferrite chromite and magnetite rim. Electron microprobe analyses indicate that plagioclases are andesine to bytownite (An = 48.2‒82.5), olivines are chrysolite (Mg# = 0.78‒0.81), orthopyroxenes are enstatite (Mg# = 0.67‒0.79), clinopyroxenes are augite (Mg# = 0.80‒0.87), spinels are chromite (Cr# = 0.31‒0.51) and chlorites are diabantite (Mg# = 0.32‒0.64) in chemical composition. Laser-ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) chemical analyses of pyroxenes reveal that clinopyroxenes more enriched in trace elements than the orthopyroxenes. Whole-rock chemical analysis reveal that they are characterized by SiO2 (51.90‒55.30 wt %), TiO2 (0.72‒0.84 wt %), Al2O3 (17.4‒18.1 wt %), Na2O (3.6‒4.0 wt %), and K2O (0.98‒1.16 wt %). The chondrite-normalized rare earth elements (REE) patterns of the samles represent enrichment of light rare earth elements (LREE) relative to medium earth elements (MREE) and heavy rare earth elements (HREE), and slightly negative Eu anomaly (Eu/Eu* = 0.86‒0.97; average: 0.89; Eu* = (Sm.Gd)0.5) points to the plagioclase fractionation during magma differentiation rather than accumulation. The primitive mantle-normalized spider diagram shows enrichment of the large ion lithophile elements (LILE) Cs, Rb, Ba, Th and U, and negative anomalies of high field strength elements (HFSE) Ti, Nb and Ta. Whole-rock geochemical data indicate that these volcanic rocks have calc-alkaline nature and geotectonically belong to the continental volcanic arc setting. These lavas are the result of mantle wedge spinel lherzolites partial melting triggered by Neotethyan oceanic crust subduction beneath of the Central Iran. The Late Miocene normal calc-alkaline magmatism occurred before than the Plio‒Quaternary slab-derived adakitic melts in the central part of the Urumieh–Dokhtar magmatic arc.