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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.

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.

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.

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.

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.

The Iranian plateau is located in one of the most complex geodynamic settings within the Alpine-Himalayan belt. The Sangan mining area is the largest Cenozoic skarn iron ore district in the far eastern part of the Alborz magmatic arc in Iran. Granitoid intrusions in the Sangan mining area are subdivided into the fertile Sarnowsar intrusion composed of syenite, syenogranite, and granite; and the barren Sarkhar and Bermani intrusions composed of monzogranite and syenogranite. In this study, we used the zircon geochemical composition from eight samples of Sarnowsar, Sarkhar and Bermani granitoids as an important exploration tool for reconstructing the crustal thickness and its evolution in the Sangan magmatic arc. Zircon Eu/Eu* [chondrite normalized] correlates with whole rock La/Yb, has been used to estimate the crustal thickness. Our results reveal that fertile Sarnowsar granitoids formed in the thicker crust than barren Sarkhar and Bermani granitoids.

In this research, to contribute to the understanding of the geochemistry of trace elements in zircon, we determined the REEs, Y, Nb, Ta, Th, and U contents in zircon grains in three granitoids (Sarnowsar, Sarkhar and Bermani) by laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) in order to determining the magma sourcee and magmatism fertility. The zircon/rock partitioning coefficients of REE, Y, Nb, Ta, Th, and U contents indicate that the patterns of the trace elements of the studied granitoids are controlled by the liquid composition at the magmatic crystallization. Compared to Sarkhar and Bermani granitoids, the Sarnowsar granite has a higher temperature (736° to 915 °C), higher zircon/rock partitioning coefficient of REEs, Y and Th (up to 2770 for Zr) and it was formed in higher oxidant conditions (△FMQ values between -0.06 to 17.01). The results of this study show that oxidized and I-type magmas in subduction-related tectonic environments are more favorable for porphyry and skarn mineralization.

Zircon is the most commonly analyzed accessory mineral and is routinely employed in U–Th–Pb geochronology, (U–Th)/He and fission track thermochronology, radiogenic (Hf) and stable (O) isotopic studies, crystallization thermometry, and trace element geochemistry (Belousova et al., 2002). A critical presumption in zircon chemistry studies is that data obtained from zircon is a proxy for the parent igneous rock. This is evidently true for most types of analyses, however, relating trace element and rare earth element (REE) concentrations in zircon compared to bulk rock or melt concentrations has been verified for causing some difficulties. The incentive to establish more accurate estimates of parental bulk rock concentrations using in-situ zircon measurements is significant as it would link zircon to a large body of whole rock geochemical literature with numerous possible applications including studies of magma source, crustal thickness, mineral exploration, crustal evolution, metamorphism, and petrogenesis (Chapman et al., 2016 and references therein). In this research, we determined the contents of REEs, Y, Nb, Ta, Hf, U, and Th in the zircon grains of eight granitoid samples from the Sangan mining district, NE Iran, to calculate zircon/rock partitioning coefficients of REEs applicable to magma source studies and mineral exploration.

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 laser ablation system, ICP-MS instrument (Agilent 7700a ICP-MS). 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.
The estimations of melt composition and the measured trace element concentrations in zircon, values of DCezircon/rock was calculated. Estimates for DCe3+zircon/rock and DCe4+zircon/rock were implemented after Ballard et al. (2002) method. Partition coefficients of the trivalent REEs and the quadrivalent series Hf, Th, and U are used to constrain DCe3+zircon/rock and DCe4+zircon/rock, respectively. Blundy and Wood (1994) showed that the mineral melt partition coefficient for a cation i can be related to the lattice strain energy created by substituting a cation, whose ionic radius (ri) differs from the optimal value for that site (r0) (Equation 1).
lnDi=lnD0-4π ENA/RT (ri/3+r0/6) (ri- r0)2                                                                                     (1)
Plotting lnDi against the (ri/3 +r0/6)(ri−r0)2 yields a linear relation for an isovalent series of cations. With knowing the ionic radii of Ce3+ and Ce4+, partition coefficients of these species can be determined by interpolation. Since Ce will be a mixture of Ce3+ and Ce4+, the value of DCezircon/rock will lie between these two-partition coefficient end-members, and by combining Equations (1) and (2) oxygen fugacity fO2 of crystallization can be estimated.
ln[xmelt Ce4+/ xmelt Ce3+]= 1/4 ln fO2 + 13136 (±591)/T − 2.064 (±0.011) NBO/T −8.878(±0.112).xH2O −8.955 (±0.091)                                                                                                                                              (2)
Temperatures were calculated using the Ti content of zircon, by using the Equation (3) (Ferry and Watson, 2007):log(Tizircon) = (5.711 ± 0.072) – 4800 ± 86/T −logaSiO2 + logaTiO2                                                     (3)
where Tizircon is the concentration of Ti in zircon in ppm, T is temperature degrees in Kelvin, and ai (aSiO2 and aTiO2) is the ratio of component i concentration in the melt over the concentration of component i in the rock at saturation.

Concentrations of trace elements in the zircon grains are given in the Supplementary Table and are summarized in Table 1, where the geometric mean (G), the variation coefficient CV (which corresponds to the ratio of standard deviation by the arithmetic mean), and the number of determinations (n) are listed. Heavy rare earth elements (HREEs) show a relative enrichment. (Lu)N in the Sarnowsar granitoids range from 1522.40 to 8869.00 (DA sample), 1477.00 to 6442.00 (TP sample) and 1655.37 to 53.00 7523 (CSK sample). These values for Sarkhar and Bermani granitoids are in the range of 835.43 to 4077.95 (BR-01 sample), 1233.46 to 4337.80 (SK-01 sample), 1984.37 to 5681.81 (SK-1-1 sample), 2281.71 to 4955.97 (SK-1-2 sample) and 2268.68 to 6422.57 (SK-2-2 sample). Furthermore, the LREEs show lower concentrations, resulting in an (La/Yb)N that is generally less than 0.1. A negative Eu anomaly and a positive Ce anomaly are observed in studied granitoids. Contents and patterns of REEs in the studied granite samples are similar to those reported by other authors with higher HREEs contents than LREEs (e.g., Hoskin and Schaltegger, 2003).
Zircon Ti concentrations used to constrain its crystallization temperatures, and T is calculated by the revised Ti-in-zircon thermometry of Ferry and Watson (2007) (Supplementary Table). Calculated Ti-in-zircon temperatures for Sarnowsar intrusions (736° to 915 °C) are higher than those of Sarkhar (646° to 819 °C) and Bermani granitoids (653° to 861 °C). The △FMQ values (calculated by equation of Trail et al., 2011; Trail et al., 2012) for Sarnowsar range from -0.06 to 17.01. For Sarkhar and Bermani intrusions, the △FMQ values are in the range -4.43 to 4 and -8.53 to 0.07, respectively. These data indicating different magmatic conditions for productive (Sarnowsar) and barren (Sarkhar and Bermani) granitoids.

Defining the magma source and fertility of acidic to intermediate magmatism within the Cenozoic volcano-plutonic magmatic belts of Iran has immense scientific and mineral exploration significance. By using the zircon/whole rock partition coefficient in the Sangan mining area (Sarnowsar, Sarkhar and Bermani granitoids), we discuss the magma source and fertility of magmatism. The magmatism of Sangan mining district area is alkaline and classified as I-type granitoids, which have a direct genetic association with iron skarn mineralization in the area. The current results show that Sarnowsar granitoid was formed at a higher temperature (736° to 915 °C), lower negative Eu anomaly (with geometric mean 0.35 to 0.4), higher values ​​of Ce4+/Ce3+ (25.56 to 718.62) ratios compared to Sarkhar and Bermani granitoids. Also, the Sarnowsar granitoid has lower Hf values ​​(700 to 1199 ppm) and higher Zr/Hf (65 to 87) values, which shows that it formed in an earlier magmatic stage compared to Sarkhar and Bermani granitoids. The results of this research can be used for identifying productive Cenozoic intrusions in Iran that are related to porphyry and skarn mineralization systems.