High-K Calc-Alkaline to Shoshonitic Eocene Volcanic Rocks from the Central Iran (Posht-e-Badam Block, Yazd Province)

Abstract The Posht-e-Badam block is situated in the central part of the Central-East Iranian Microcontinent (CEIM), as part of the larger Alpine‒Himalayan orogenic system. Eocene volcanic rocks of the Posht-e-Badam block present very good exposures in the Saghand (Khoshumi Mountain), Allahabad and Chapedony areas (Yazd province, Central Iran). These Eocene volcanic rocks are basic to acidic in composition (SiO2 = 48.9 to 63.4 wt %), but the intermediate samples with trachyandesite composition are the predominant rock type. The main rock-forming phenocrystic minerals are clinopyroxene (augite), amphibole (hornblende and magnesio-hastingsite), plagioclase (labradorite to oligoclase) and biotite (Fe# = 0.48) set in a medium to finegrained groundmass of same minerals plus sanidine and magnetite. Sanidine is restericted to the matrix. The studied rocks show porphyritic, glomeroporphyritic, trachytic, anti-rapakivi, poikilitic and corona as the main textures. Whole rock geochemical data of the studied Eocene volcanic rocks show the Al2O3 and TiO2 values from 14.8 to 17.8 and 0.47 to 1.28 wt %, respectively. The total alkalis (Na2O + K2O) content reaches up to 8.74 wt %, which shows the highly enrichment in alkalis. The K2O value in the analyzed samples is 2.32 to 5.42 wt % (average: 3.57 wt %). The Posht-e-Badam block Eocene volcanic rocks belong to the high-K calc-alkaline to shoshonitic magmatic series. Chondrite-normalized REE patterns and the primitive mantle-normalized multi-element spider-diagram display light rare earth elements (LREE) enrichment relative to heavy rare earth elements (HREE) (La/Lu = 77 to 707), enrichment in large ion lithophile elements (LILE) (Cs, K, Rb, Sr, Ba) and depletion in high field strength elements (HFSE) (Ta, Nb, Ti). These chemical characteristics point to the subduction-related K-rich magmatic rocks. Geochemical criteria of the analyzed samples suggest that the parental magma of these rocks was produced by a low degree of partial melting of a previously metasomatized mantle wedge spinel lherzolite. The occurrence of this potassium-rich volcanism in the Posht-e-Badam block can be ascribed to the former subduction of the Neo-Tethyan oceanic crust beneath the CEIM from Triassic to Eocene.