The composition of zircon trace elements in the various Cadomian granitoid types of Taknar zone, south of Sabzevar

Article Type:
Research/Original Article (دارای رتبه معتبر)

The composition of minerals in igneous rocks is influenced by the type and nature of their parent magma, and for this reason, minerals can provide valuable information regarding the original magma. Zircon (with the general formula ZrSiO4), due to its widespread distribution in various types of rocks,  is considered a valuable tool in many geological studies. For this study, the composition of trace elements in zircon crystals from Neoproterozoic to Lower Cambrian granitoids in the Kaboodan area Sabzevar is geochemically investigated. As the previous studies have documented the granitoids in this area are classified into two distinct groups, I- and S-type (Mazhari et al., 2020).

Regional Geology:

The study area lies in the northeast of Iran and belongs to the Central Iran Zone on the northern margin of the Lut Block. The granitoids from the southern part of the Sabzevar region were investigated for this research. The main fault of the Taknar separates the northern Sabzevar Zone and the southern Taknar Zone occurrences in this area.A small part of the intrusive complex appearing in the Kaboodan area includes both felsic and mafic rocks with the Cretaceous age. The geochemical and isotopic characteristics of these rocks point to their formation in an active arc setting during the Late Cretaceous (Mazhari et al., 2019). However, the main volume of the rock formations in the Kaboodan area consists of various magmatic rocks related to Cadomian events. The mafic intrusive rocks, with an age of Ma 552-545, consist of a gabbro-diorite composition derived from the partial melting of a mantle source enriched in spinel peridotite at shallow depths (Mazhari et al., 2020a). The granitoid rocks in the area are of two different types, I (Ma 549-547) and S. (Ma 531-528) types (Mazhari et al., 2020a).

Analytical methods

Two samples of S-type (Ka18 and Ka39) and two samples of I-type granitoids (Ka6 and Ka33) were selected. The trace element analysis of zircons was carried out using the Agilent 7700x ICP–MS, which was equipped with a Resonetics Resolution M-50 series 193nm excimer laser ablation system. The laser ablation beams had a diameter of approximately 31 μm with a laser energy density of 8.5 J/cm2. All spot analyses were conducted with a repetition rate of 10 Hz. External standards NIST SRM610 and TEMORA 2 (TEM) were utilized for the analysis. The detailed operating conditions for the laser ablation system and the ICP-MS instrument, as well as data reduction procedures, were the same as those described by Liang et al. (2018). Petrography, nature, and formation setting of zircon crystals The separated zircon samples from the two types of granitoids have a similar appearance. They are all transparent and come in various colors ranging from colorless to pale yellow and light brown. The length ofthe zircon crystals is approximately 100 to 300 microns, andthe length-to-width ratio is about 3:1. The zircon grains in the studied granitoids are mostly euhedral to subhedral and often exhibit zoning. The observed zoning in the zircon samples is mostly of the oscillatory or sector type, and some crystals also display banding textures. These textures are prominent features of magmatic zircon crystals. All the analyzed crystals have high Th/U ratios (>0.1), which is consistent with zircons of magmatic origin. Moreover, the Th/U ratio is approximately 1.5-1 in these zircons. All zircon crystals in the studied samples exhibit a rare earth element (REE) pattern similar tomagmatic zircons. The U/Yb ratios in the zircon crystals indicate that the studied zircons fall within the range of continental zircons. Trace element distinction between I- and S- types of granitoids With careful examination of the results of trace element analysis in the zircon crystals from different types of granitoids in the studied region, significant differences are observable. While some elements like Sr, Nb, and Ta show consistent ranges of variation in zircon crystals from different granitoid types, the composition of other trace elements varies significantly. Zircon crystals in S-type granitoids are enriched in elements (i.e. Ti, P, Hf) compared to I-type granitoids, whereas in I type samples, zircons are relatively enriched in elements like Y, Th, U, and REE. These compositional differences in magmatic zircon crystals indicate different sources and distinct evolutionary processes that gave rise to the formation of the primary magmas.Crystallization temperature and oxygen partial pressure (fO2) in zircon crystals The calculation of the Titanium-in-Zircon Thermometry (TZT) documents that zircon crystals in S-type granitoids crystallized at higher temperatures (877-910°C) compared to I-type granitoids(808-860°C). The concentrations of elements (i.e. Nb, Hf, Th, U) in zircon crystals decrease with increasing crystallization temperature in both I- and S-type granitoids. The trend of variation in trace elements with increasing crystallization temperature in zircon crystals indicates systematic changes in the melt composition simultaneous with the cooling of the system. This behavior of trace elements is in line with magmatic differentiation processes such as fractional crystallization.I-type granitoids were formed at higher oxygen partial pressures compared to S-type samples. The average values of ∆FMQ for zircon crystals in S-type granitoids are -18.8, while the zircon crystals in I-type granitoids have an average of -16.3. The relationship between temperature and oxygen partial pressure indicates that all samples were formed under conditions lower than FMQ. The calculation of temperature and fO2 using the trace element composition of zircon crystals in the studied region suggests higher temperatures and lower oxygen partial pressures for S-type granitoids compared to I-type ones.

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