The Kasian volcanic rocks, Khorramabad, Iran: Evidence for a Jurassic Intra-Oceanic island arc in Neo-Tethys ocean

Abstract

The Kasian volcanic body is located in the eastern margin of the Zagros thrust belt, close to the Sanandaj-Sirjan metamorphic zone. These volcanic rocks are mainly composed of andesite and andesite-basalt rocks with porphyritic, hypocrystalline porphyritic, hyalo-porphyritic and hyalo-microlitic porphyritic textures. Analyses of the distributions of major, rare earth and trace elements reveal a tholeiitic nature and evidence such as enrichment of Pb and LILE (e.g., U, Rb, Ba), depletion in HFSE (e.g., Nb, Ti, Y), slight enrichment of LREE relative to HREE and trace elements discrimination plots reveal island arc affinity for the Kasian volcanic rocks. Some characteristics like, low Nd/Pb and Ce/Pb values (average 8.76 and 12.70, respectively), high U values and low Nb/U ratios (average 3.52) indicate enrichment of mantle wedge by contribution of slab-derived fluids during dehydration of subducting slab of Neo-Tethys oceanic lithosphere. Moreover, the results show these volcanic rocks to have fractionated as they ascended to higher crustal levels. The results of this study are consistent with the new tectonic scenario for the Sanandaj-Sirjan zone, which suggests that during ocean–ocean subduction (from Jurassic to Cretaceous) an immature island arc developed before the closure of Neo-Tethys ocean.