Petrogenesis of the two phases of intrusive rocks at Chodarchay, NW Iran: using trace and rare earth elements

Abstract

The Chodarchay area is located 50 km east of the city of Zanjan, within the Tarom-Hashtjin subzone, in the Western Alborz structural zone of NW Iran. Igneous rocks in the area include intrusive granitic (phases I and II), and volcanic rocks. Geochemical characteristics of the granitic rocks were determined using their trace and rare earth element (REE) compositions. The phase I intrusive rocks at Chodarchay include quartz monzonite and quartz syenite, while the phase II mainly consists of granite and alkali granite. These intrusive rocks have SiO2 contents ranging between 60.11 and 69.14 wt%, high K2O + Na2O (8.20 to 10.21 wt%) and K2O/Na2O ratios of 1.24 to 2.56. Petrological and geochemical studies indicate that the granitoids have high-K to shoshonitic affinity, they are metaluminous to mildly peraluminous and characterize as I-type granitoids. The investigated granitoids (phases I and II) have fractionated REE patterns characterized by enrichment in light REEs and varying degrees of depletion in heavy REEs. Large-ion lithophile element (LILE) enrichment, high-field strength element (HFSE) depletion, and distinct positive Pb anomaly in the primitive mantle-normalized trace element patterns of the rocks imply subduction-related arc magmatic signatures. It is suggested that the granites formed in a volcanic arc to post-collision setting from a garnet spinel lherzolite source.