Rare earth element signatures of barren magmatic rocks in Ardestan-Arak axis in UDMA, Iran

Ardestan-Arak axis is situated in the Urmia-Dokhtar magmatic belt and Sanandaj-Sirjan zone. Exposed magmatism in Urmia-Dokhtar magmatic belt in study area often includes plutonic (granodiorite, granite, and gabbro with calc-alkaline composition) and volcanic rocks (rhyolite, dacite, trachydacite, andesite, and basalt with porphyry texture). Geochemical analysis shows that the primary magma have characteristics of I-type magmatism which produced in volcanic arc. All volcanic rock samples collected have SiO 2 values from 50 to77 wt% and are metaluminous. NMORB-normalized trace element spider diagrams and chondrite-normalized REE patterns show negative Nb, Ta, and Ti anomalies, enrichment in large ion lithophile elements (LILEs), and light rare earth elements (LREEs) relative to high field strength elements (HFSEs) which are characteristics of calc-alkaline composition of the rocks in subduction zone. Plutonic and volcanic rocks in the Ardestan-Arak axis have slightly LREE-enriched pattern with (La/Yb)n variation in 2.36–10.92 and 2.7–7.7, respectively, which support a subduction nature for most of the samples. Also, low Sr/Y in magmatic rocks (with typical values of the Sr/Y ratio Less than 30) is similar to non-adakite and barren-type igneous rocks in subduction zone in the Urmia-Dokhtar Magmatic Arc. Also, the geochemistry results of plutonic rocks in study area show minor negative to slightly positive Eu/Eu* anomaly varying from 0.9 to 1.2, similar to unprodutive magmas. On the other hand, the average copper grade at the plutonic and volcanic rocks at Ardestan-Arak axis is 32 and 29 ppm, respectively. Another evidence related to barren-type magmas in Ardestan-Arak axis is enrichment in Cs, Rb, Ba, Th, U, Nb, K, and Pb and depletion in Zr, Ti, and P in the NMORB-normalized diagram. These similar values also indicate a similar source for the plutonic and volcanic rocks in Ardestan-Arak axis or show the mantle enrichment processes before melting. Therefore, it could be concluded that magmatism in the study area was not favorable for the formation of porphyry copper mineralization.