Geochronological and Geochemical Characteristics of Fractionated I-type Granites Associated with the Skarn Mineralization in the Sangan Mining Region, NE Iran
Abstract The Sangan mining region, which has a proven reserve of approximately 1000 Mt of 53% iron ore, is located in the Khaf-Kashmar-Bardeskan volcano-plutonic belt in northeastern Iran. The geological units in the eastern zone of the Sangan region consist of Precambrian schists, Jurassic sedimentary rocks and Tertiary subvolcanic granitoid intrusions. Iron skarn mineralization consists of stratiform and massive bodies in the carbonate rocks that are adjacent to the granitoid intrusions. Detailed field mapping revealed that 39-Ma syenitic intrusive bodies in the western and central zones of the Sangan region were the main sources of heat and fluid for the iron mineralization. A Mid-Cenozoic biotite granite pluton is associated with the eastern anomalies. However, field relationships suggested that this pluton is not the source of the metals, heat or fluids that were responsible for the iron mineralization. This pluton is rich in silica (SiO 2 contents from 66.4 to 79.1 wt%) and is characterized by high-K series with metaluminous to slightly peraluminous affinity. Geochronological (U-Pb zircon method) and geochemical data, including major and trace elements and Sr-Nd-Pb isotopes, define the complex origin of these plutons, which consist of alkaline granitoids that appear to be A-type in character but also show I-type affinity. New geochemical and isotopic data from plutons in the eastern anomalies and data from previous studies of the western and central anomalies and the southeastern intrusive rocks in the Sangan region show that these plutons have close affiliation with lower to upper crust-derived melts and were largely modified into highly fractionated I-type granite. These rocks were derived from and emplaced by varying degrees of partial melting during the Middle Eocene (Bartonian to Lutetian, 38.3–43.9 Ma) from a crustal protolith in a normal to mature volcanic continental arc setting. The Sangan granitoids were produced from crustal assimilation by a heat source from mantle melts, which are associated with the Kashmar-Neotethyan slab that subducted under Eurasia. The Khaf-Kashmar-Bardeskan igneous rocks reflect an active Cenozoic plate margin that was related to the closure of the Kashmar-Neotethyan Sea between the Lut and Eurasia blocks because of the continuous convergence between the Arabian and Iran plates during the Late Cretaceous–Early Paleocene.