High-K Calc-Alkaline Eocene Volcanic Rocks from the Anarak Area (Central Iran): A Key Structure for the Early Stages of Oceanic Basin Closure and the Beginning of Collision

Abstract

Abstract: The combination of geochemical analyses and geochronological data of Eocene volcanic rocks reveals a gradual shift in the dominant magmatic series from calc-alkaline to high-K calc-alkaline and shoshonitic in Central Iran. Transitional high-K calc-alkaline Eocene volcanic rocks with predominantely trachy-andesite composition are exposed in western part of the Central-East Iranian Microcontient (CEIM) (NE of Anarak, Isfahan Province, Central Iran). Petrography and mineral chemistry analyses indicate that the rock-forming minerals of these trachy andesites are clinopyroxene (diopside and augite, Mg# = 0.840–0.94), plagioclase (andesine and oligoclase, An18–39), sanidine (Or65–69), amphibole (Magnesio-hastingsite and magnesio-hornblende, Mg# = 0.576 to 0.787), and phlogopite (Mg# = 0.54‒0.58) with minor amounts of opaque minerals (magnetite and titanomagnetite), zircon, titanite and apatite. Secondary minerals, as the result of alteration, are calcite, chlorite (diabantite), kaolinite, hematite and prehnite. Geochemical analyses of whole rock samples show that these rocks are characterized by SiO2 content of 55.1 to 62.5 wt %, Al2O3 amounts of 14.3 to 16.70 wt %, K2O contents of 2.92 to 4.83 wt % and TiO2 values of 0.55 to 0.72 wt %. They are enriched in alkalis (Na2O + K2O up to 10.43 wt %) and large ion lithophile elements (LILE) (Cs, K, Rb, Sr, Ba), depleted in high field strength elements (HFSE) (Ta, Nb, Ti) and exhibit weak negative Eu anomaly (Eu/Eu* = 0.70‒0.92). These Eocene volcanic rocks present strong enrichment in light rare earth elements (LREE) relative to heavy REE (HREE) (La/Lu ratio up to 131.29), and a flat HREE pattern. All these chemical cheriteria point to the subduction-related high-K calc-alkaline magmatic rocks. The occurrence of this potassic volcanism can be attributed to the former subduction of the CEIM confining oceanic crust beneath the CEIM from Triassic to Eocene. Geochemical features of these trachy andesites suggest that the parental magma were possibly derived from relatively low degrees of partial melting of a mantle wedge spinel lherzolite experienced strong metasomatism by fluids/melts released from the Neo-Tethys subducted slab. These high-K calc-alkaline volcanic rocks are generated at the late stages of the orogeny and after the cessation of subduction. © 2021, Pleiades Publishing, Inc.