U-Pb GEOCHRONOLOGY, Sr-Nd GEOCHEMISTRY, PETROGENESIS AND TECTONIC SETTING OF GANDAB VOLCANIC ROCKS, NORTHEASTERN IRAN

This paper addresses U-Pb geochronology, Sr-Nd geochemistry, petrogenesis and tectonic setting in the Gandab volcanic rocks. The Gandab volcanic rocks belong to the Sabzevar zone magmatic arc (northeastern Iran). Petrographically, all the studied volcanic rocks indicate porphyritic textures with phenocrysts of plagioclase, K-feldespar, hornblende, pyroxene, and magnetite which are embedded in a fine to medium grained groundmass. As well, amygdaloidal, and poikilitic textures are seen in some rocks. The standard chemical classifications show that the studied rocks are basaltic trachy andesite, trachy andesite, trachyte, and trachy dacite. Major elements reveal that the studied samples are metaluminous and their alumina saturation index varies from 0.71 to 1.02. The chondrite-normalized rare earth element and mantle-normalized trace element patterns show enrichment in light rare earth elements (LREE) relative to heavy rare earth elements (HREE) and in large ion lithophile elements (LILE) relative to high field strength elements (HFSE). As well they show a slightly negative Eu anomaly (Eu/Eu* = 0.72-0.97). The whole-rock geochemistry of the studied rocks suggests that they are related to each other by fractional crystallization. LA-MC-ICP-MS U-Pb analyses in zircon grains from two volcanic rock samples (GCH-119 and GCH-171) gave ages ranging of 5.47 +/- 0.22 Ma to 2.44 +/- 0.79 Ma, which corresponds to the Pliocene period. In four samples analysed for Sr and Nd isotopes Sr-87/Sr-86 ratios range from 0.704082 to 0.705931 and Nd-epsilon values vary between +3.34 and +5. These values could be regarded to as representing mantle derived magmas. Taking into account the comparing rare earth element (REE) patterns, an origin of the parental magmas in enriched lithospheric mantle is suggested. Finally, it is concluded that Pliocene Gandab volcanic rocks are related to the post-collision environment that followed the Neo-Tethys subduction.