| چکیده انگلیسی مقاله |
Zircon is a significant mineral due to its ubiquitous occurrence, chemically resistant and refractory, that can survive both weathering and transport processes as well as high-temperature metamorphism and anatexis (Ballard et al., 2002). It can, therefore, be found in many igneous, metamorphic, and sedimentary rocks and is particularly common in plutonic rocks. Zircon acts as a valuable archive of geochemical information regarding geochronology studies (Hoskin and Schaltegger, 2003), a record of the parent rock oxygen isotopic ratio (Hawkesworth and Kemp, 2006), provide a proxy for processes such as crustal recycling by Hf isotopic composition (Scherer et al., 2007), reflect the oxidation state of parent magma by Ce and Eu anomalies (Trail et al., 2012), and temperature estimation by Ti content (Hofmann et al., 2014).
The Sangan mining district, the largest skarn iron ore district in Iran, is located in the northeastern part of the Alborz Magmatic Arc. Fourteen skarn anomalies occur along the contact of the syenite to the syenogranite Sarnowsar pluton in the north and the Sarkhar and the Bermani plutons in the southeast (Mehrabi et al., 2021).
Previous works have used the zircon U–Pb geochronology, whole rock geochemistry, and zircon chemistry to constrain the emplacement age, fertility of magmatism, and petrogenesis of these granites (Malekzadeh Shafaroudi et al., 2013; Golmohammadi et al., 2015; Mazhari et al., 2017; Mehrabi et al., 2021; Ghasemi Siani et al., 2022), but neglected the importance of zircon trace element concentrations when interpreting the parental magma evolution. Here, we examine the trace elements of zircons from Sarnowsar and Sarkhar-Bermani intrusions, to verify the origin of these zircons and the evolution of the parent magma.
Regional Geology
The oldest rocks in the Sangan mining district include weakly metamorphosed Precambrian slates and metasiltstones. The Lower Jurassic Shemshak Formation consists of chert, weakly metamorphosed and metasomatized shale, siltstone, and red sandstone. The Middle Jurassic rocks are characterized by limestones and marls of the Dalichay Formation. The overlying Upper Jurassic Lar Formation composed of limestone, dolostone, and dolomitic limestone. Cretaceous formations are dominated by massive limestone, conglomerate, and intercalated crystal tuff. These metasedimentary formations are uncomfortably covered by the intermediate to felsic volcanic rocks crosscutting by plutonic rocks. Intermediate to felsic volcanic rocks cover an area of 10 km2 in the southwestern part of the Sangan mining district and extended within central ore bodies. Volcanic rocks include dacite, andesite, rhyolite, latite, and their pyroclastic equivalents.
Analytical methods
Zircon from Sarkhar and Bermani granitoids were analyzed at the State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Wuhan, using a laser ablation system, ICP-MS instrument (Agilent 7700a ICP-MS instrument). Also, samples from Sarnowsar granitoids were analyzed at the Nanjing Hongchuang Geological Exploration Technology Service Co. Ltd., China. Zircons were analyzed for trace elements using a laser energy density of 3.6 J/cm2, a spot size of 30 μm, and a repetition rate of 5 Hz.
Results
Analytical data of zircon trace element concentrate are presented in the supplementary Table. Results are plotted against the 206Pb/238U date for each zircon grain. Zircons from the intrusions have scattered geochemical signatures that show no correlation with U–Pb dates. Conversely, some geochemical parameters of zircons from the intrusive rocks show distinct temporal trends. For example, zircon Yb/Dy and Ce/Nd values broadly increase with age younging. It should be noted that the Th/U and Ce/Nd of the Sarnowsar zircons are higher than those of Sarkhar and Bermani intrusions.
Discussion
Correlations between rock type and the trace element compositions of zircon from a wide range of igneous rocks can be illustrated with a series of discriminant plots. For example, plots of Nb vs. Ta, Y versus Yb/Sm, and Y vs. Ce/Ce* and similar plots (Belousova et al., 2002) indicate that the studied zircons are classified as granitoid igneous type as a parental magma. The uniformly high Hf contents of zircons in this study point to their crystallization derived from a more evolved felsic magma, particularly Sarkhar and Bermani intrusions. The accompanying low Eu/Eu* ratios are indicative of plagioclase crystallization. The U/Yb ratio of zircons can be used to distinguish their origin (Grimes et al., 2015). Continental-arc zircons have U/Yb ratios mostly between 0.1 and 4, and low U/Yb ratios (<0.1) are characteristic of zircons derived from a mantle source. In the discrimination diagrams of U/Yb vs. Hf and U vs. Yb, all the obtained data are plotted in the continental-series area and are distinguishable from ocean crust zircons. In the U/Yb versus Nb/Yb diagram, both the whole-rock and zircon compositions show the characteristics of a magmatic-arc array. Overall, a continental-crust source for the zircons, mirroring the origin of the parent magma. The disparate geochemical behaviors of Hf, Th, and Nb within zircon provide a potential method for establishing the tectonic setting of host magma. The Nb content of arc magmas is depleted relative to magmas formed in within-plate settings (Pearce and Peat 1995), and as such, arc zircons possess lower Nb/Hf and higher Th/Nb ratios at a comparable degree of magmatic fractionation. Accordingly, bivariate discrimination diagrams of Th/U vs. Nb/Hf and Th/Nb vs. Hf/Th are meaningful tools for distinguishing within-plate (anorogenic) from arc-related (orogenic) settings (Hawkesworth and Kemp, 2006). The majority of zircons are plotted in the orogenic field, signifying a magmatic-arc or orogenic setting and a calc-alkaline parent magma.
Conclusion
Based on the trace-element composition of zircon grains, whole-rock trace-element contents, and patterns of two granitoid intrusions in the Sangan mining district, the following conclusions can be drawn:
- The disparate geochemical behaviors of U, Hf, Th, and Nb indicate a continental-crust source in a magmatic-arc tectonic setting.
- All of the studied zircons are located in the granitoid igneous rocks fields with a series of discriminant plots.
- The studied zircon grains of the Sarnowsar show relatively high Ce4+/Ce3+ ratios, pointing to their formation in an oxidized magmatic medium.
|