In total, 18 local U-Pb isotope analyses were conducted on 15 zircon grains. The results are presented in Table S5 and illustrated in Fig. 10. The studied zircons were divided into two populations based on isotopic parameters.
The first population included eight zircon grains of different morphologies. Five of the zircon grains exhibit a short prismatic shape and crystal fragments ranging in size from 90 × 60 to 230 × 140 µm, with length-to-width ratio varying from 1.5:1 to 2.1:1 (Fig. 10a). The two zircon grains exhibit oval and rounded shapes with dimensions of 90 × 45 µm and 90 × 90 µm, respectively. One zircon grain exhibited an elongated and resorbed shape with dimensions of 80 × 20 µm and a length-to-width ratio of 4:1 (Fig. 10a). According to CL characteristics, most of zircon grains in this population exhibit a heterogeneous internal structure comprising two to three domains that differ in brightness within the CL image. In some instances, the internal domains exhibit a combination of small dark and light areas, as observed in grain 17 (Fig. 10a). Additionally, in one zircon grain with a homogeneous CL-dark image, small irregularly shaped areas with a CL-gray image can be observed (19.1 and 20.1, Fig. 10a). One zircon from this population exhibits oscillatory zoning, which is defined as fine, periodic variations in the mineral composition, in the absence of a visible core and with a thin, discontinuous rim (grain 13, Fig. 10a).
Zircons from this population exhibit considerable variability in U and Th contents, ranging from 138 to 1303 ppm U and 8 to 4610 ppm Th. The average contents of these elements are (n = 9): U = 537 ± 352 ppm, Th = 1234 ± 1331 ppm, and Th/U = 1.91 ± 1.68 (Table S5). The variations in U and Th contents observed in different magmatic zircon domains were not correlated with changes in the brightness of their CL images. For instance, the U content in the seven CL-dark grey domains variesd from 158 to 1303 ppm, whereas the Th content varied from 8 to 1609 ppm. In contrast, in the two CL-light grey domains, the U content was 226 and 289 ppm, and the Th content was 64 and 282 ppm (Table S5, Fig. 10a). There was no correlation between U and Th contents in zircons from this population (Fig. 10b). All zircons from the first population exhibited similar U and Pb isotopic ratios (Table S5), indicating the absence of a significant impact of secondary processes and contamination by a substance of a different composition on the U-Pb isotope system. All obtained age values were concordant or subconcordant.
A concordant U-Pb age was obtained for this zircon population, equal to 2492.5 ± 4.1 Ma and, MSWD = 0.68 (n = 9, 2σ) (Fig. 10c). Age is considered concordant when the ellipses of analytical errors intersect the concordia. This result is corroborated by the low degree of discordance, which ranges from − 0.16 to 3.62 (Table S5). The concordant age is equivalent to the weighted average 207Pb/206Pb age of zircons from this population, which is 2492.6 ± 4.2 Ma, MSWD = 0.66 (n = 9, 2σ) (Fig. 10d). The resulting concordant age can be interpreted as the time zircon magmatic crystallization and accordingly the age of the OH330 rocks.
The second zircon population comprises seven zircon grains represented by long-prismatic crystals and their fragments. These grains are sometimes well-preserved with crystallographic shapes ranging in size from 90 × 45 to 200 × 80 µm and a length-to-width ratios ranging from 2:1 to 4:1 (Fig. 10a). The majority of zircons in this population exhibit oscillatory zoning in CL images (grains 7, 9, 10, and 14–15, Fig. 10a). In some cases, they contain a core in the absence of rims. The remaining zircons display a more homogeneous internal structure, typically comprising two to three domains that differ in brightness in the CL image. The U and Th contents of this zircon population, as well as those of the zircons of the first population, exhibit significant variability. They range from 212 to 1829 ppm U and from 78 to 1090 ppm Th, with Th/U ratios ranging from 0.17 to 0.62 (Table S5). The average values of these components were as follows (n = 9): 674 ± 458 ppm U, 314 ± 269 ppm Th, and Th/U = 0.43 ± 0.10. Furthermore, there is a strong positive correlation between the U and Th contents (Fig. 10b).
The concordant U-Pb age of this zircon population is 2818.0 ± 3.1 Ma, with a MSWD = 1.8 (n = 9, 2σ) (Fig. 10c). A weighted average 207Pb/206Pb age of 2818 ± 3.2 Ma was determined with an MSWD = 1.7 (n = 9, 2σ) (Fig. 10d). This concordant age provides a rationale for considering the zircon of the second population xenocryst, as extracted from the rocks of the Archean granitoid basement. This is because it falls within the age range of the Archean granitoid of the Kola Block, which is estimated to be between 2835 and 2736 Ma (Chen et al. 1998; Pripachkin et al. 2020).
It is important to note that the CL-light grey domain 4.1 of xenocryst zircon differs from other zircons of this population in terms of its lowest contents of U = 212 ppm and 206Pb* = 89 ppm, as well as its lower values of Pb-U isotopic parameters: 207Pb*/235U = 13.55 ± 0.16 and 206Pb*/238U = 0.4894 ± 0.0054 (Table S5). In addition, this domain exhibits a high degree of discordance (D = 10.32, Table S5). These results suggest that this zicron was subjected to later metamorphic-metasomatic processes.